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Message-Id: <1518845990-20733-6-git-send-email-changbin.du@intel.com>
Date: Sat, 17 Feb 2018 13:39:38 +0800
From: changbin.du@...el.com
To: corbet@....net, rostedt@...dmis.org
Cc: mingo@...nel.org, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, Changbin Du <changbin.du@...el.com>
Subject: [PATCH 05/17] trace doc: convert trace/ftrace.txt to rst format
From: Changbin Du <changbin.du@...el.com>
This converts the plain text documentation to reStructuredText format and
add it into Sphinx TOC tree. No essential content change.
Cc: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Changbin Du <changbin.du@...el.com>
---
Documentation/trace/ftrace.rst | 3332 ++++++++++++++++++++++++++++++++++++++++
Documentation/trace/ftrace.txt | 3220 --------------------------------------
Documentation/trace/index.rst | 1 +
3 files changed, 3333 insertions(+), 3220 deletions(-)
create mode 100644 Documentation/trace/ftrace.rst
delete mode 100644 Documentation/trace/ftrace.txt
diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
new file mode 100644
index 0000000..636aa9bf
--- /dev/null
+++ b/Documentation/trace/ftrace.rst
@@ -0,0 +1,3332 @@
+========================
+ftrace - Function Tracer
+========================
+
+Copyright 2008 Red Hat Inc.
+
+:Author: Steven Rostedt <srostedt@...hat.com>
+:License: The GNU Free Documentation License, Version 1.2
+ (dual licensed under the GPL v2)
+:Original Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton,
+ John Kacur, and David Teigland.
+
+- Written for: 2.6.28-rc2
+- Updated for: 3.10
+- Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt
+- Converted to rst format - Changbin Du <changbin.du@...el.com>
+
+Introduction
+------------
+
+Ftrace is an internal tracer designed to help out developers and
+designers of systems to find what is going on inside the kernel.
+It can be used for debugging or analyzing latencies and
+performance issues that take place outside of user-space.
+
+Although ftrace is typically considered the function tracer, it
+is really a frame work of several assorted tracing utilities.
+There's latency tracing to examine what occurs between interrupts
+disabled and enabled, as well as for preemption and from a time
+a task is woken to the task is actually scheduled in.
+
+One of the most common uses of ftrace is the event tracing.
+Through out the kernel is hundreds of static event points that
+can be enabled via the tracefs file system to see what is
+going on in certain parts of the kernel.
+
+See events.txt for more information.
+
+
+Implementation Details
+----------------------
+
+See :doc:`ftrace-design` for details for arch porters and such.
+
+
+The File System
+---------------
+
+Ftrace uses the tracefs file system to hold the control files as
+well as the files to display output.
+
+When tracefs is configured into the kernel (which selecting any ftrace
+option will do) the directory /sys/kernel/tracing will be created. To mount
+this directory, you can add to your /etc/fstab file::
+
+ tracefs /sys/kernel/tracing tracefs defaults 0 0
+
+Or you can mount it at run time with::
+
+ mount -t tracefs nodev /sys/kernel/tracing
+
+For quicker access to that directory you may want to make a soft link to
+it::
+
+ ln -s /sys/kernel/tracing /tracing
+
+.. attention::
+
+ Before 4.1, all ftrace tracing control files were within the debugfs
+ file system, which is typically located at /sys/kernel/debug/tracing.
+ For backward compatibility, when mounting the debugfs file system,
+ the tracefs file system will be automatically mounted at:
+
+ /sys/kernel/debug/tracing
+
+ All files located in the tracefs file system will be located in that
+ debugfs file system directory as well.
+
+.. attention::
+
+ Any selected ftrace option will also create the tracefs file system.
+ The rest of the document will assume that you are in the ftrace directory
+ (cd /sys/kernel/tracing) and will only concentrate on the files within that
+ directory and not distract from the content with the extended
+ "/sys/kernel/tracing" path name.
+
+That's it! (assuming that you have ftrace configured into your kernel)
+
+After mounting tracefs you will have access to the control and output files
+of ftrace. Here is a list of some of the key files:
+
+
+ Note: all time values are in microseconds.
+
+ current_tracer:
+
+ This is used to set or display the current tracer
+ that is configured.
+
+ available_tracers:
+
+ This holds the different types of tracers that
+ have been compiled into the kernel. The
+ tracers listed here can be configured by
+ echoing their name into current_tracer.
+
+ tracing_on:
+
+ This sets or displays whether writing to the trace
+ ring buffer is enabled. Echo 0 into this file to disable
+ the tracer or 1 to enable it. Note, this only disables
+ writing to the ring buffer, the tracing overhead may
+ still be occurring.
+
+ The kernel function tracing_off() can be used within the
+ kernel to disable writing to the ring buffer, which will
+ set this file to "0". User space can re-enable tracing by
+ echoing "1" into the file.
+
+ Note, the function and event trigger "traceoff" will also
+ set this file to zero and stop tracing. Which can also
+ be re-enabled by user space using this file.
+
+ trace:
+
+ This file holds the output of the trace in a human
+ readable format (described below). Note, tracing is temporarily
+ disabled while this file is being read (opened).
+
+ trace_pipe:
+
+ The output is the same as the "trace" file but this
+ file is meant to be streamed with live tracing.
+ Reads from this file will block until new data is
+ retrieved. Unlike the "trace" file, this file is a
+ consumer. This means reading from this file causes
+ sequential reads to display more current data. Once
+ data is read from this file, it is consumed, and
+ will not be read again with a sequential read. The
+ "trace" file is static, and if the tracer is not
+ adding more data, it will display the same
+ information every time it is read. This file will not
+ disable tracing while being read.
+
+ trace_options:
+
+ This file lets the user control the amount of data
+ that is displayed in one of the above output
+ files. Options also exist to modify how a tracer
+ or events work (stack traces, timestamps, etc).
+
+ options:
+
+ This is a directory that has a file for every available
+ trace option (also in trace_options). Options may also be set
+ or cleared by writing a "1" or "0" respectively into the
+ corresponding file with the option name.
+
+ tracing_max_latency:
+
+ Some of the tracers record the max latency.
+ For example, the maximum time that interrupts are disabled.
+ The maximum time is saved in this file. The max trace will also be
+ stored, and displayed by "trace". A new max trace will only be
+ recorded if the latency is greater than the value in this file
+ (in microseconds).
+
+ By echoing in a time into this file, no latency will be recorded
+ unless it is greater than the time in this file.
+
+ tracing_thresh:
+
+ Some latency tracers will record a trace whenever the
+ latency is greater than the number in this file.
+ Only active when the file contains a number greater than 0.
+ (in microseconds)
+
+ buffer_size_kb:
+
+ This sets or displays the number of kilobytes each CPU
+ buffer holds. By default, the trace buffers are the same size
+ for each CPU. The displayed number is the size of the
+ CPU buffer and not total size of all buffers. The
+ trace buffers are allocated in pages (blocks of memory
+ that the kernel uses for allocation, usually 4 KB in size).
+ If the last page allocated has room for more bytes
+ than requested, the rest of the page will be used,
+ making the actual allocation bigger than requested or shown.
+ ( Note, the size may not be a multiple of the page size
+ due to buffer management meta-data. )
+
+ Buffer sizes for individual CPUs may vary
+ (see "per_cpu/cpu0/buffer_size_kb" below), and if they do
+ this file will show "X".
+
+ buffer_total_size_kb:
+
+ This displays the total combined size of all the trace buffers.
+
+ free_buffer:
+
+ If a process is performing tracing, and the ring buffer should be
+ shrunk "freed" when the process is finished, even if it were to be
+ killed by a signal, this file can be used for that purpose. On close
+ of this file, the ring buffer will be resized to its minimum size.
+ Having a process that is tracing also open this file, when the process
+ exits its file descriptor for this file will be closed, and in doing so,
+ the ring buffer will be "freed".
+
+ It may also stop tracing if disable_on_free option is set.
+
+ tracing_cpumask:
+
+ This is a mask that lets the user only trace on specified CPUs.
+ The format is a hex string representing the CPUs.
+
+ set_ftrace_filter:
+
+ When dynamic ftrace is configured in (see the
+ section below "dynamic ftrace"), the code is dynamically
+ modified (code text rewrite) to disable calling of the
+ function profiler (mcount). This lets tracing be configured
+ in with practically no overhead in performance. This also
+ has a side effect of enabling or disabling specific functions
+ to be traced. Echoing names of functions into this file
+ will limit the trace to only those functions.
+
+ The functions listed in "available_filter_functions" are what
+ can be written into this file.
+
+ This interface also allows for commands to be used. See the
+ "Filter commands" section for more details.
+
+ set_ftrace_notrace:
+
+ This has an effect opposite to that of
+ set_ftrace_filter. Any function that is added here will not
+ be traced. If a function exists in both set_ftrace_filter
+ and set_ftrace_notrace, the function will _not_ be traced.
+
+ set_ftrace_pid:
+
+ Have the function tracer only trace the threads whose PID are
+ listed in this file.
+
+ If the "function-fork" option is set, then when a task whose
+ PID is listed in this file forks, the child's PID will
+ automatically be added to this file, and the child will be
+ traced by the function tracer as well. This option will also
+ cause PIDs of tasks that exit to be removed from the file.
+
+ set_event_pid:
+
+ Have the events only trace a task with a PID listed in this file.
+ Note, sched_switch and sched_wake_up will also trace events
+ listed in this file.
+
+ To have the PIDs of children of tasks with their PID in this file
+ added on fork, enable the "event-fork" option. That option will also
+ cause the PIDs of tasks to be removed from this file when the task
+ exits.
+
+ set_graph_function:
+
+ Functions listed in this file will cause the function graph
+ tracer to only trace these functions and the functions that
+ they call. (See the section "dynamic ftrace" for more details).
+
+ set_graph_notrace:
+
+ Similar to set_graph_function, but will disable function graph
+ tracing when the function is hit until it exits the function.
+ This makes it possible to ignore tracing functions that are called
+ by a specific function.
+
+ available_filter_functions:
+
+ This lists the functions that ftrace has processed and can trace.
+ These are the function names that you can pass to
+ "set_ftrace_filter" or "set_ftrace_notrace".
+ (See the section "dynamic ftrace" below for more details.)
+
+ dyn_ftrace_total_info:
+
+ This file is for debugging purposes. The number of functions that
+ have been converted to nops and are available to be traced.
+
+ enabled_functions:
+
+ This file is more for debugging ftrace, but can also be useful
+ in seeing if any function has a callback attached to it.
+ Not only does the trace infrastructure use ftrace function
+ trace utility, but other subsystems might too. This file
+ displays all functions that have a callback attached to them
+ as well as the number of callbacks that have been attached.
+ Note, a callback may also call multiple functions which will
+ not be listed in this count.
+
+ If the callback registered to be traced by a function with
+ the "save regs" attribute (thus even more overhead), a 'R'
+ will be displayed on the same line as the function that
+ is returning registers.
+
+ If the callback registered to be traced by a function with
+ the "ip modify" attribute (thus the regs->ip can be changed),
+ an 'I' will be displayed on the same line as the function that
+ can be overridden.
+
+ If the architecture supports it, it will also show what callback
+ is being directly called by the function. If the count is greater
+ than 1 it most likely will be ftrace_ops_list_func().
+
+ If the callback of the function jumps to a trampoline that is
+ specific to a the callback and not the standard trampoline,
+ its address will be printed as well as the function that the
+ trampoline calls.
+
+ function_profile_enabled:
+
+ When set it will enable all functions with either the function
+ tracer, or if configured, the function graph tracer. It will
+ keep a histogram of the number of functions that were called
+ and if the function graph tracer was configured, it will also keep
+ track of the time spent in those functions. The histogram
+ content can be displayed in the files:
+
+ trace_stats/function<cpu> ( function0, function1, etc).
+
+ trace_stats:
+
+ A directory that holds different tracing stats.
+
+ kprobe_events:
+
+ Enable dynamic trace points. See kprobetrace.txt.
+
+ kprobe_profile:
+
+ Dynamic trace points stats. See kprobetrace.txt.
+
+ max_graph_depth:
+
+ Used with the function graph tracer. This is the max depth
+ it will trace into a function. Setting this to a value of
+ one will show only the first kernel function that is called
+ from user space.
+
+ printk_formats:
+
+ This is for tools that read the raw format files. If an event in
+ the ring buffer references a string, only a pointer to the string
+ is recorded into the buffer and not the string itself. This prevents
+ tools from knowing what that string was. This file displays the string
+ and address for the string allowing tools to map the pointers to what
+ the strings were.
+
+ saved_cmdlines:
+
+ Only the pid of the task is recorded in a trace event unless
+ the event specifically saves the task comm as well. Ftrace
+ makes a cache of pid mappings to comms to try to display
+ comms for events. If a pid for a comm is not listed, then
+ "<...>" is displayed in the output.
+
+ If the option "record-cmd" is set to "0", then comms of tasks
+ will not be saved during recording. By default, it is enabled.
+
+ saved_cmdlines_size:
+
+ By default, 128 comms are saved (see "saved_cmdlines" above). To
+ increase or decrease the amount of comms that are cached, echo
+ in a the number of comms to cache, into this file.
+
+ saved_tgids:
+
+ If the option "record-tgid" is set, on each scheduling context switch
+ the Task Group ID of a task is saved in a table mapping the PID of
+ the thread to its TGID. By default, the "record-tgid" option is
+ disabled.
+
+ snapshot:
+
+ This displays the "snapshot" buffer and also lets the user
+ take a snapshot of the current running trace.
+ See the "Snapshot" section below for more details.
+
+ stack_max_size:
+
+ When the stack tracer is activated, this will display the
+ maximum stack size it has encountered.
+ See the "Stack Trace" section below.
+
+ stack_trace:
+
+ This displays the stack back trace of the largest stack
+ that was encountered when the stack tracer is activated.
+ See the "Stack Trace" section below.
+
+ stack_trace_filter:
+
+ This is similar to "set_ftrace_filter" but it limits what
+ functions the stack tracer will check.
+
+ trace_clock:
+
+ Whenever an event is recorded into the ring buffer, a
+ "timestamp" is added. This stamp comes from a specified
+ clock. By default, ftrace uses the "local" clock. This
+ clock is very fast and strictly per cpu, but on some
+ systems it may not be monotonic with respect to other
+ CPUs. In other words, the local clocks may not be in sync
+ with local clocks on other CPUs.
+
+ Usual clocks for tracing::
+
+ # cat trace_clock
+ [local] global counter x86-tsc
+
+ The clock with the square brackets around it is the one in effect.
+
+ local:
+ Default clock, but may not be in sync across CPUs
+
+ global:
+ This clock is in sync with all CPUs but may
+ be a bit slower than the local clock.
+
+ counter:
+ This is not a clock at all, but literally an atomic
+ counter. It counts up one by one, but is in sync
+ with all CPUs. This is useful when you need to
+ know exactly the order events occurred with respect to
+ each other on different CPUs.
+
+ uptime:
+ This uses the jiffies counter and the time stamp
+ is relative to the time since boot up.
+
+ perf:
+ This makes ftrace use the same clock that perf uses.
+ Eventually perf will be able to read ftrace buffers
+ and this will help out in interleaving the data.
+
+ x86-tsc:
+ Architectures may define their own clocks. For
+ example, x86 uses its own TSC cycle clock here.
+
+ ppc-tb:
+ This uses the powerpc timebase register value.
+ This is in sync across CPUs and can also be used
+ to correlate events across hypervisor/guest if
+ tb_offset is known.
+
+ mono:
+ This uses the fast monotonic clock (CLOCK_MONOTONIC)
+ which is monotonic and is subject to NTP rate adjustments.
+
+ mono_raw:
+ This is the raw monotonic clock (CLOCK_MONOTONIC_RAW)
+ which is montonic but is not subject to any rate adjustments
+ and ticks at the same rate as the hardware clocksource.
+
+ boot:
+ This is the boot clock (CLOCK_BOOTTIME) and is based on the
+ fast monotonic clock, but also accounts for time spent in
+ suspend. Since the clock access is designed for use in
+ tracing in the suspend path, some side effects are possible
+ if clock is accessed after the suspend time is accounted before
+ the fast mono clock is updated. In this case, the clock update
+ appears to happen slightly sooner than it normally would have.
+ Also on 32-bit systems, it's possible that the 64-bit boot offset
+ sees a partial update. These effects are rare and post
+ processing should be able to handle them. See comments in the
+ ktime_get_boot_fast_ns() function for more information.
+
+ To set a clock, simply echo the clock name into this file::
+
+ echo global > trace_clock
+
+ trace_marker:
+
+ This is a very useful file for synchronizing user space
+ with events happening in the kernel. Writing strings into
+ this file will be written into the ftrace buffer.
+
+ It is useful in applications to open this file at the start
+ of the application and just reference the file descriptor
+ for the file::
+
+ void trace_write(const char *fmt, ...)
+ {
+ va_list ap;
+ char buf[256];
+ int n;
+
+ if (trace_fd < 0)
+ return;
+
+ va_start(ap, fmt);
+ n = vsnprintf(buf, 256, fmt, ap);
+ va_end(ap);
+
+ write(trace_fd, buf, n);
+ }
+
+ start::
+
+ trace_fd = open("trace_marker", WR_ONLY);
+
+ trace_marker_raw:
+
+ This is similar to trace_marker above, but is meant for for binary data
+ to be written to it, where a tool can be used to parse the data
+ from trace_pipe_raw.
+
+ uprobe_events:
+
+ Add dynamic tracepoints in programs.
+ See uprobetracer.txt
+
+ uprobe_profile:
+
+ Uprobe statistics. See uprobetrace.txt
+
+ instances:
+
+ This is a way to make multiple trace buffers where different
+ events can be recorded in different buffers.
+ See "Instances" section below.
+
+ events:
+
+ This is the trace event directory. It holds event tracepoints
+ (also known as static tracepoints) that have been compiled
+ into the kernel. It shows what event tracepoints exist
+ and how they are grouped by system. There are "enable"
+ files at various levels that can enable the tracepoints
+ when a "1" is written to them.
+
+ See events.txt for more information.
+
+ set_event:
+
+ By echoing in the event into this file, will enable that event.
+
+ See events.txt for more information.
+
+ available_events:
+
+ A list of events that can be enabled in tracing.
+
+ See events.txt for more information.
+
+ hwlat_detector:
+
+ Directory for the Hardware Latency Detector.
+ See "Hardware Latency Detector" section below.
+
+ per_cpu:
+
+ This is a directory that contains the trace per_cpu information.
+
+ per_cpu/cpu0/buffer_size_kb:
+
+ The ftrace buffer is defined per_cpu. That is, there's a separate
+ buffer for each CPU to allow writes to be done atomically,
+ and free from cache bouncing. These buffers may have different
+ size buffers. This file is similar to the buffer_size_kb
+ file, but it only displays or sets the buffer size for the
+ specific CPU. (here cpu0).
+
+ per_cpu/cpu0/trace:
+
+ This is similar to the "trace" file, but it will only display
+ the data specific for the CPU. If written to, it only clears
+ the specific CPU buffer.
+
+ per_cpu/cpu0/trace_pipe
+
+ This is similar to the "trace_pipe" file, and is a consuming
+ read, but it will only display (and consume) the data specific
+ for the CPU.
+
+ per_cpu/cpu0/trace_pipe_raw
+
+ For tools that can parse the ftrace ring buffer binary format,
+ the trace_pipe_raw file can be used to extract the data
+ from the ring buffer directly. With the use of the splice()
+ system call, the buffer data can be quickly transferred to
+ a file or to the network where a server is collecting the
+ data.
+
+ Like trace_pipe, this is a consuming reader, where multiple
+ reads will always produce different data.
+
+ per_cpu/cpu0/snapshot:
+
+ This is similar to the main "snapshot" file, but will only
+ snapshot the current CPU (if supported). It only displays
+ the content of the snapshot for a given CPU, and if
+ written to, only clears this CPU buffer.
+
+ per_cpu/cpu0/snapshot_raw:
+
+ Similar to the trace_pipe_raw, but will read the binary format
+ from the snapshot buffer for the given CPU.
+
+ per_cpu/cpu0/stats:
+
+ This displays certain stats about the ring buffer:
+
+ entries:
+ The number of events that are still in the buffer.
+
+ overrun:
+ The number of lost events due to overwriting when
+ the buffer was full.
+
+ commit overrun:
+ Should always be zero.
+ This gets set if so many events happened within a nested
+ event (ring buffer is re-entrant), that it fills the
+ buffer and starts dropping events.
+
+ bytes:
+ Bytes actually read (not overwritten).
+
+ oldest event ts:
+ The oldest timestamp in the buffer
+
+ now ts:
+ The current timestamp
+
+ dropped events:
+ Events lost due to overwrite option being off.
+
+ read events:
+ The number of events read.
+
+The Tracers
+-----------
+
+Here is the list of current tracers that may be configured.
+
+ "function"
+
+ Function call tracer to trace all kernel functions.
+
+ "function_graph"
+
+ Similar to the function tracer except that the
+ function tracer probes the functions on their entry
+ whereas the function graph tracer traces on both entry
+ and exit of the functions. It then provides the ability
+ to draw a graph of function calls similar to C code
+ source.
+
+ "blk"
+
+ The block tracer. The tracer used by the blktrace user
+ application.
+
+ "hwlat"
+
+ The Hardware Latency tracer is used to detect if the hardware
+ produces any latency. See "Hardware Latency Detector" section
+ below.
+
+ "irqsoff"
+
+ Traces the areas that disable interrupts and saves
+ the trace with the longest max latency.
+ See tracing_max_latency. When a new max is recorded,
+ it replaces the old trace. It is best to view this
+ trace with the latency-format option enabled, which
+ happens automatically when the tracer is selected.
+
+ "preemptoff"
+
+ Similar to irqsoff but traces and records the amount of
+ time for which preemption is disabled.
+
+ "preemptirqsoff"
+
+ Similar to irqsoff and preemptoff, but traces and
+ records the largest time for which irqs and/or preemption
+ is disabled.
+
+ "wakeup"
+
+ Traces and records the max latency that it takes for
+ the highest priority task to get scheduled after
+ it has been woken up.
+ Traces all tasks as an average developer would expect.
+
+ "wakeup_rt"
+
+ Traces and records the max latency that it takes for just
+ RT tasks (as the current "wakeup" does). This is useful
+ for those interested in wake up timings of RT tasks.
+
+ "wakeup_dl"
+
+ Traces and records the max latency that it takes for
+ a SCHED_DEADLINE task to be woken (as the "wakeup" and
+ "wakeup_rt" does).
+
+ "mmiotrace"
+
+ A special tracer that is used to trace binary module.
+ It will trace all the calls that a module makes to the
+ hardware. Everything it writes and reads from the I/O
+ as well.
+
+ "branch"
+
+ This tracer can be configured when tracing likely/unlikely
+ calls within the kernel. It will trace when a likely and
+ unlikely branch is hit and if it was correct in its prediction
+ of being correct.
+
+ "nop"
+
+ This is the "trace nothing" tracer. To remove all
+ tracers from tracing simply echo "nop" into
+ current_tracer.
+
+
+Examples of using the tracer
+----------------------------
+
+Here are typical examples of using the tracers when controlling
+them only with the tracefs interface (without using any
+user-land utilities).
+
+Output format:
+--------------
+
+Here is an example of the output format of the file "trace"::
+
+ # tracer: function
+ #
+ # entries-in-buffer/entries-written: 140080/250280 #P:4
+ #
+ # _-----=> irqs-off
+ # / _----=> need-resched
+ # | / _---=> hardirq/softirq
+ # || / _--=> preempt-depth
+ # ||| / delay
+ # TASK-PID CPU# |||| TIMESTAMP FUNCTION
+ # | | | |||| | |
+ bash-1977 [000] .... 17284.993652: sys_close <-system_call_fastpath
+ bash-1977 [000] .... 17284.993653: __close_fd <-sys_close
+ bash-1977 [000] .... 17284.993653: _raw_spin_lock <-__close_fd
+ sshd-1974 [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
+ bash-1977 [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
+ bash-1977 [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
+ bash-1977 [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
+ bash-1977 [000] .... 17284.993657: filp_close <-__close_fd
+ bash-1977 [000] .... 17284.993657: dnotify_flush <-filp_close
+ sshd-1974 [003] .... 17284.993658: sys_select <-system_call_fastpath
+ ....
+
+A header is printed with the tracer name that is represented by
+the trace. In this case the tracer is "function". Then it shows the
+number of events in the buffer as well as the total number of entries
+that were written. The difference is the number of entries that were
+lost due to the buffer filling up (250280 - 140080 = 110200 events
+lost).
+
+The header explains the content of the events. Task name "bash", the task
+PID "1977", the CPU that it was running on "000", the latency format
+(explained below), the timestamp in <secs>.<usecs> format, the
+function name that was traced "sys_close" and the parent function that
+called this function "system_call_fastpath". The timestamp is the time
+at which the function was entered.
+
+Latency trace format
+--------------------
+
+When the latency-format option is enabled or when one of the latency
+tracers is set, the trace file gives somewhat more information to see
+why a latency happened. Here is a typical trace::
+
+ # tracer: irqsoff
+ #
+ # irqsoff latency trace v1.1.5 on 3.8.0-test+
+ # --------------------------------------------------------------------
+ # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+ # -----------------
+ # | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
+ # -----------------
+ # => started at: __lock_task_sighand
+ # => ended at: _raw_spin_unlock_irqrestore
+ #
+ #
+ # _------=> CPU#
+ # / _-----=> irqs-off
+ # | / _----=> need-resched
+ # || / _---=> hardirq/softirq
+ # ||| / _--=> preempt-depth
+ # |||| / delay
+ # cmd pid ||||| time | caller
+ # \ / ||||| \ | /
+ ps-6143 2d... 0us!: trace_hardirqs_off <-__lock_task_sighand
+ ps-6143 2d..1 259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
+ ps-6143 2d..1 263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
+ ps-6143 2d..1 306us : <stack trace>
+ => trace_hardirqs_on_caller
+ => trace_hardirqs_on
+ => _raw_spin_unlock_irqrestore
+ => do_task_stat
+ => proc_tgid_stat
+ => proc_single_show
+ => seq_read
+ => vfs_read
+ => sys_read
+ => system_call_fastpath
+
+
+This shows that the current tracer is "irqsoff" tracing the time
+for which interrupts were disabled. It gives the trace version (which
+never changes) and the version of the kernel upon which this was executed on
+(3.8). Then it displays the max latency in microseconds (259 us). The number
+of trace entries displayed and the total number (both are four: #4/4).
+VP, KP, SP, and HP are always zero and are reserved for later use.
+#P is the number of online CPUs (#P:4).
+
+The task is the process that was running when the latency
+occurred. (ps pid: 6143).
+
+The start and stop (the functions in which the interrupts were
+disabled and enabled respectively) that caused the latencies:
+
+ - __lock_task_sighand is where the interrupts were disabled.
+ - _raw_spin_unlock_irqrestore is where they were enabled again.
+
+The next lines after the header are the trace itself. The header
+explains which is which.
+
+ cmd: The name of the process in the trace.
+
+ pid: The PID of that process.
+
+ CPU#: The CPU which the process was running on.
+
+ irqs-off: 'd' interrupts are disabled. '.' otherwise.
+ .. caution:: If the architecture does not support a way to
+ read the irq flags variable, an 'X' will always
+ be printed here.
+
+ need-resched:
+ - 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
+ - 'n' only TIF_NEED_RESCHED is set,
+ - 'p' only PREEMPT_NEED_RESCHED is set,
+ - '.' otherwise.
+
+ hardirq/softirq:
+ - 'Z' - NMI occurred inside a hardirq
+ - 'z' - NMI is running
+ - 'H' - hard irq occurred inside a softirq.
+ - 'h' - hard irq is running
+ - 's' - soft irq is running
+ - '.' - normal context.
+
+ preempt-depth: The level of preempt_disabled
+
+The above is mostly meaningful for kernel developers.
+
+ time:
+ When the latency-format option is enabled, the trace file
+ output includes a timestamp relative to the start of the
+ trace. This differs from the output when latency-format
+ is disabled, which includes an absolute timestamp.
+
+ delay:
+ This is just to help catch your eye a bit better. And
+ needs to be fixed to be only relative to the same CPU.
+ The marks are determined by the difference between this
+ current trace and the next trace.
+
+ - '$' - greater than 1 second
+ - '@' - greater than 100 milisecond
+ - '*' - greater than 10 milisecond
+ - '#' - greater than 1000 microsecond
+ - '!' - greater than 100 microsecond
+ - '+' - greater than 10 microsecond
+ - ' ' - less than or equal to 10 microsecond.
+
+ The rest is the same as the 'trace' file.
+
+ Note, the latency tracers will usually end with a back trace
+ to easily find where the latency occurred.
+
+trace_options
+-------------
+
+The trace_options file (or the options directory) is used to control
+what gets printed in the trace output, or manipulate the tracers.
+To see what is available, simply cat the file::
+
+ cat trace_options
+ print-parent
+ nosym-offset
+ nosym-addr
+ noverbose
+ noraw
+ nohex
+ nobin
+ noblock
+ trace_printk
+ annotate
+ nouserstacktrace
+ nosym-userobj
+ noprintk-msg-only
+ context-info
+ nolatency-format
+ record-cmd
+ norecord-tgid
+ overwrite
+ nodisable_on_free
+ irq-info
+ markers
+ noevent-fork
+ function-trace
+ nofunction-fork
+ nodisplay-graph
+ nostacktrace
+ nobranch
+
+To disable one of the options, echo in the option prepended with
+"no"::
+
+ echo noprint-parent > trace_options
+
+To enable an option, leave off the "no"::
+
+ echo sym-offset > trace_options
+
+Here are the available options:
+
+ print-parent
+ On function traces, display the calling (parent)
+ function as well as the function being traced.
+ ::
+
+ print-parent:
+ bash-4000 [01] 1477.606694: simple_strtoul <-kstrtoul
+
+ noprint-parent:
+ bash-4000 [01] 1477.606694: simple_strtoul
+
+
+ sym-offset
+ Display not only the function name, but also the
+ offset in the function. For example, instead of
+ seeing just "ktime_get", you will see
+ "ktime_get+0xb/0x20".
+ ::
+
+ sym-offset:
+ bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0
+
+ sym-addr
+ This will also display the function address as well
+ as the function name.
+ ::
+
+ sym-addr:
+ bash-4000 [01] 1477.606694: simple_strtoul <c0339346>
+
+ verbose
+ This deals with the trace file when the
+ latency-format option is enabled.
+ ::
+
+ bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
+ (+0.000ms): simple_strtoul (kstrtoul)
+
+ raw
+ This will display raw numbers. This option is best for
+ use with user applications that can translate the raw
+ numbers better than having it done in the kernel.
+
+ hex
+ Similar to raw, but the numbers will be in a hexadecimal format.
+
+ bin
+ This will print out the formats in raw binary.
+
+ block
+ When set, reading trace_pipe will not block when polled.
+
+ trace_printk
+ Can disable trace_printk() from writing into the buffer.
+
+ annotate
+ It is sometimes confusing when the CPU buffers are full
+ and one CPU buffer had a lot of events recently, thus
+ a shorter time frame, were another CPU may have only had
+ a few events, which lets it have older events. When
+ the trace is reported, it shows the oldest events first,
+ and it may look like only one CPU ran (the one with the
+ oldest events). When the annotate option is set, it will
+ display when a new CPU buffer started::
+
+ <idle>-0 [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
+ <idle>-0 [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
+ <idle>-0 [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
+ ##### CPU 2 buffer started ####
+ <idle>-0 [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
+ <idle>-0 [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
+ <idle>-0 [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
+
+ userstacktrace
+ This option changes the trace. It records a
+ stacktrace of the current user space thread after
+ each trace event.
+
+ sym-userobj
+ when user stacktrace are enabled, look up which
+ object the address belongs to, and print a
+ relative address. This is especially useful when
+ ASLR is on, otherwise you don't get a chance to
+ resolve the address to object/file/line after
+ the app is no longer running
+
+ The lookup is performed when you read
+ trace,trace_pipe. Example::
+
+ a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
+ x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
+
+
+ printk-msg-only
+ When set, trace_printk()s will only show the format
+ and not their parameters (if trace_bprintk() or
+ trace_bputs() was used to save the trace_printk()).
+
+ context-info
+ Show only the event data. Hides the comm, PID,
+ timestamp, CPU, and other useful data.
+
+ latency-format
+ This option changes the trace output. When it is enabled,
+ the trace displays additional information about the
+ latency, as described in "Latency trace format".
+
+ record-cmd
+ When any event or tracer is enabled, a hook is enabled
+ in the sched_switch trace point to fill comm cache
+ with mapped pids and comms. But this may cause some
+ overhead, and if you only care about pids, and not the
+ name of the task, disabling this option can lower the
+ impact of tracing. See "saved_cmdlines".
+
+ record-tgid
+ When any event or tracer is enabled, a hook is enabled
+ in the sched_switch trace point to fill the cache of
+ mapped Thread Group IDs (TGID) mapping to pids. See
+ "saved_tgids".
+
+ overwrite
+ This controls what happens when the trace buffer is
+ full. If "1" (default), the oldest events are
+ discarded and overwritten. If "0", then the newest
+ events are discarded.
+ (see per_cpu/cpu0/stats for overrun and dropped)
+
+ disable_on_free
+ When the free_buffer is closed, tracing will
+ stop (tracing_on set to 0).
+
+ irq-info
+ Shows the interrupt, preempt count, need resched data.
+ When disabled, the trace looks like::
+
+ # tracer: function
+ #
+ # entries-in-buffer/entries-written: 144405/9452052 #P:4
+ #
+ # TASK-PID CPU# TIMESTAMP FUNCTION
+ # | | | | |
+ <idle>-0 [002] 23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
+ <idle>-0 [002] 23636.756054: activate_task <-ttwu_do_activate.constprop.89
+ <idle>-0 [002] 23636.756055: enqueue_task <-activate_task
+
+
+ markers
+ When set, the trace_marker is writable (only by root).
+ When disabled, the trace_marker will error with EINVAL
+ on write.
+
+ event-fork
+ When set, tasks with PIDs listed in set_event_pid will have
+ the PIDs of their children added to set_event_pid when those
+ tasks fork. Also, when tasks with PIDs in set_event_pid exit,
+ their PIDs will be removed from the file.
+
+ function-trace
+ The latency tracers will enable function tracing
+ if this option is enabled (default it is). When
+ it is disabled, the latency tracers do not trace
+ functions. This keeps the overhead of the tracer down
+ when performing latency tests.
+
+ function-fork
+ When set, tasks with PIDs listed in set_ftrace_pid will
+ have the PIDs of their children added to set_ftrace_pid
+ when those tasks fork. Also, when tasks with PIDs in
+ set_ftrace_pid exit, their PIDs will be removed from the
+ file.
+
+ display-graph
+ When set, the latency tracers (irqsoff, wakeup, etc) will
+ use function graph tracing instead of function tracing.
+
+ stacktrace
+ When set, a stack trace is recorded after any trace event
+ is recorded.
+
+ branch
+ Enable branch tracing with the tracer. This enables branch
+ tracer along with the currently set tracer. Enabling this
+ with the "nop" tracer is the same as just enabling the
+ "branch" tracer.
+
+.. tip:: Some tracers have their own options. They only appear in this
+ file when the tracer is active. They always appear in the
+ options directory.
+
+
+Here are the per tracer options:
+
+Options for function tracer:
+
+ func_stack_trace
+ When set, a stack trace is recorded after every
+ function that is recorded. NOTE! Limit the functions
+ that are recorded before enabling this, with
+ "set_ftrace_filter" otherwise the system performance
+ will be critically degraded. Remember to disable
+ this option before clearing the function filter.
+
+Options for function_graph tracer:
+
+ Since the function_graph tracer has a slightly different output
+ it has its own options to control what is displayed.
+
+ funcgraph-overrun
+ When set, the "overrun" of the graph stack is
+ displayed after each function traced. The
+ overrun, is when the stack depth of the calls
+ is greater than what is reserved for each task.
+ Each task has a fixed array of functions to
+ trace in the call graph. If the depth of the
+ calls exceeds that, the function is not traced.
+ The overrun is the number of functions missed
+ due to exceeding this array.
+
+ funcgraph-cpu
+ When set, the CPU number of the CPU where the trace
+ occurred is displayed.
+
+ funcgraph-overhead
+ When set, if the function takes longer than
+ A certain amount, then a delay marker is
+ displayed. See "delay" above, under the
+ header description.
+
+ funcgraph-proc
+ Unlike other tracers, the process' command line
+ is not displayed by default, but instead only
+ when a task is traced in and out during a context
+ switch. Enabling this options has the command
+ of each process displayed at every line.
+
+ funcgraph-duration
+ At the end of each function (the return)
+ the duration of the amount of time in the
+ function is displayed in microseconds.
+
+ funcgraph-abstime
+ When set, the timestamp is displayed at each line.
+
+ funcgraph-irqs
+ When disabled, functions that happen inside an
+ interrupt will not be traced.
+
+ funcgraph-tail
+ When set, the return event will include the function
+ that it represents. By default this is off, and
+ only a closing curly bracket "}" is displayed for
+ the return of a function.
+
+ sleep-time
+ When running function graph tracer, to include
+ the time a task schedules out in its function.
+ When enabled, it will account time the task has been
+ scheduled out as part of the function call.
+
+ graph-time
+ When running function profiler with function graph tracer,
+ to include the time to call nested functions. When this is
+ not set, the time reported for the function will only
+ include the time the function itself executed for, not the
+ time for functions that it called.
+
+Options for blk tracer:
+
+ blk_classic
+ Shows a more minimalistic output.
+
+
+irqsoff
+-------
+
+When interrupts are disabled, the CPU can not react to any other
+external event (besides NMIs and SMIs). This prevents the timer
+interrupt from triggering or the mouse interrupt from letting
+the kernel know of a new mouse event. The result is a latency
+with the reaction time.
+
+The irqsoff tracer tracks the time for which interrupts are
+disabled. When a new maximum latency is hit, the tracer saves
+the trace leading up to that latency point so that every time a
+new maximum is reached, the old saved trace is discarded and the
+new trace is saved.
+
+To reset the maximum, echo 0 into tracing_max_latency. Here is
+an example::
+
+ # echo 0 > options/function-trace
+ # echo irqsoff > current_tracer
+ # echo 1 > tracing_on
+ # echo 0 > tracing_max_latency
+ # ls -ltr
+ [...]
+ # echo 0 > tracing_on
+ # cat trace
+ # tracer: irqsoff
+ #
+ # irqsoff latency trace v1.1.5 on 3.8.0-test+
+ # --------------------------------------------------------------------
+ # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+ # -----------------
+ # | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
+ # -----------------
+ # => started at: run_timer_softirq
+ # => ended at: run_timer_softirq
+ #
+ #
+ # _------=> CPU#
+ # / _-----=> irqs-off
+ # | / _----=> need-resched
+ # || / _---=> hardirq/softirq
+ # ||| / _--=> preempt-depth
+ # |||| / delay
+ # cmd pid ||||| time | caller
+ # \ / ||||| \ | /
+ <idle>-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq
+ <idle>-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq
+ <idle>-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq
+ <idle>-0 0dNs3 25us : <stack trace>
+ => _raw_spin_unlock_irq
+ => run_timer_softirq
+ => __do_softirq
+ => call_softirq
+ => do_softirq
+ => irq_exit
+ => smp_apic_timer_interrupt
+ => apic_timer_interrupt
+ => rcu_idle_exit
+ => cpu_idle
+ => rest_init
+ => start_kernel
+ => x86_64_start_reservations
+ => x86_64_start_kernel
+
+Here we see that that we had a latency of 16 microseconds (which is
+very good). The _raw_spin_lock_irq in run_timer_softirq disabled
+interrupts. The difference between the 16 and the displayed
+timestamp 25us occurred because the clock was incremented
+between the time of recording the max latency and the time of
+recording the function that had that latency.
+
+Note the above example had function-trace not set. If we set
+function-trace, we get a much larger output::
+
+ with echo 1 > options/function-trace
+
+ # tracer: irqsoff
+ #
+ # irqsoff latency trace v1.1.5 on 3.8.0-test+
+ # --------------------------------------------------------------------
+ # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+ # -----------------
+ # | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
+ # -----------------
+ # => started at: ata_scsi_queuecmd
+ # => ended at: ata_scsi_queuecmd
+ #
+ #
+ # _------=> CPU#
+ # / _-----=> irqs-off
+ # | / _----=> need-resched
+ # || / _---=> hardirq/softirq
+ # ||| / _--=> preempt-depth
+ # |||| / delay
+ # cmd pid ||||| time | caller
+ # \ / ||||| \ | /
+ bash-2042 3d... 0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
+ bash-2042 3d... 0us : add_preempt_count <-_raw_spin_lock_irqsave
+ bash-2042 3d..1 1us : ata_scsi_find_dev <-ata_scsi_queuecmd
+ bash-2042 3d..1 1us : __ata_scsi_find_dev <-ata_scsi_find_dev
+ bash-2042 3d..1 2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
+ bash-2042 3d..1 2us : ata_qc_new_init <-__ata_scsi_queuecmd
+ bash-2042 3d..1 3us : ata_sg_init <-__ata_scsi_queuecmd
+ bash-2042 3d..1 4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
+ bash-2042 3d..1 4us : ata_build_rw_tf <-ata_scsi_rw_xlat
+ [...]
+ bash-2042 3d..1 67us : delay_tsc <-__delay
+ bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
+ bash-2042 3d..2 67us : sub_preempt_count <-delay_tsc
+ bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
+ bash-2042 3d..2 68us : sub_preempt_count <-delay_tsc
+ bash-2042 3d..1 68us+: ata_bmdma_start <-ata_bmdma_qc_issue
+ bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
+ bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
+ bash-2042 3d..1 72us+: trace_hardirqs_on <-ata_scsi_queuecmd
+ bash-2042 3d..1 120us : <stack trace>
+ => _raw_spin_unlock_irqrestore
+ => ata_scsi_queuecmd
+ => scsi_dispatch_cmd
+ => scsi_request_fn
+ => __blk_run_queue_uncond
+ => __blk_run_queue
+ => blk_queue_bio
+ => generic_make_request
+ => submit_bio
+ => submit_bh
+ => __ext3_get_inode_loc
+ => ext3_iget
+ => ext3_lookup
+ => lookup_real
+ => __lookup_hash
+ => walk_component
+ => lookup_last
+ => path_lookupat
+ => filename_lookup
+ => user_path_at_empty
+ => user_path_at
+ => vfs_fstatat
+ => vfs_stat
+ => sys_newstat
+ => system_call_fastpath
+
+
+Here we traced a 71 microsecond latency. But we also see all the
+functions that were called during that time. Note that by
+enabling function tracing, we incur an added overhead. This
+overhead may extend the latency times. But nevertheless, this
+trace has provided some very helpful debugging information.
+
+
+preemptoff
+----------
+
+When preemption is disabled, we may be able to receive
+interrupts but the task cannot be preempted and a higher
+priority task must wait for preemption to be enabled again
+before it can preempt a lower priority task.
+
+The preemptoff tracer traces the places that disable preemption.
+Like the irqsoff tracer, it records the maximum latency for
+which preemption was disabled. The control of preemptoff tracer
+is much like the irqsoff tracer.
+::
+
+ # echo 0 > options/function-trace
+ # echo preemptoff > current_tracer
+ # echo 1 > tracing_on
+ # echo 0 > tracing_max_latency
+ # ls -ltr
+ [...]
+ # echo 0 > tracing_on
+ # cat trace
+ # tracer: preemptoff
+ #
+ # preemptoff latency trace v1.1.5 on 3.8.0-test+
+ # --------------------------------------------------------------------
+ # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+ # -----------------
+ # | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
+ # -----------------
+ # => started at: do_IRQ
+ # => ended at: do_IRQ
+ #
+ #
+ # _------=> CPU#
+ # / _-----=> irqs-off
+ # | / _----=> need-resched
+ # || / _---=> hardirq/softirq
+ # ||| / _--=> preempt-depth
+ # |||| / delay
+ # cmd pid ||||| time | caller
+ # \ / ||||| \ | /
+ sshd-1991 1d.h. 0us+: irq_enter <-do_IRQ
+ sshd-1991 1d..1 46us : irq_exit <-do_IRQ
+ sshd-1991 1d..1 47us+: trace_preempt_on <-do_IRQ
+ sshd-1991 1d..1 52us : <stack trace>
+ => sub_preempt_count
+ => irq_exit
+ => do_IRQ
+ => ret_from_intr
+
+
+This has some more changes. Preemption was disabled when an
+interrupt came in (notice the 'h'), and was enabled on exit.
+But we also see that interrupts have been disabled when entering
+the preempt off section and leaving it (the 'd'). We do not know if
+interrupts were enabled in the mean time or shortly after this
+was over.
+::
+
+ # tracer: preemptoff
+ #
+ # preemptoff latency trace v1.1.5 on 3.8.0-test+
+ # --------------------------------------------------------------------
+ # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+ # -----------------
+ # | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
+ # -----------------
+ # => started at: wake_up_new_task
+ # => ended at: task_rq_unlock
+ #
+ #
+ # _------=> CPU#
+ # / _-----=> irqs-off
+ # | / _----=> need-resched
+ # || / _---=> hardirq/softirq
+ # ||| / _--=> preempt-depth
+ # |||| / delay
+ # cmd pid ||||| time | caller
+ # \ / ||||| \ | /
+ bash-1994 1d..1 0us : _raw_spin_lock_irqsave <-wake_up_new_task
+ bash-1994 1d..1 0us : select_task_rq_fair <-select_task_rq
+ bash-1994 1d..1 1us : __rcu_read_lock <-select_task_rq_fair
+ bash-1994 1d..1 1us : source_load <-select_task_rq_fair
+ bash-1994 1d..1 1us : source_load <-select_task_rq_fair
+ [...]
+ bash-1994 1d..1 12us : irq_enter <-smp_apic_timer_interrupt
+ bash-1994 1d..1 12us : rcu_irq_enter <-irq_enter
+ bash-1994 1d..1 13us : add_preempt_count <-irq_enter
+ bash-1994 1d.h1 13us : exit_idle <-smp_apic_timer_interrupt
+ bash-1994 1d.h1 13us : hrtimer_interrupt <-smp_apic_timer_interrupt
+ bash-1994 1d.h1 13us : _raw_spin_lock <-hrtimer_interrupt
+ bash-1994 1d.h1 14us : add_preempt_count <-_raw_spin_lock
+ bash-1994 1d.h2 14us : ktime_get_update_offsets <-hrtimer_interrupt
+ [...]
+ bash-1994 1d.h1 35us : lapic_next_event <-clockevents_program_event
+ bash-1994 1d.h1 35us : irq_exit <-smp_apic_timer_interrupt
+ bash-1994 1d.h1 36us : sub_preempt_count <-irq_exit
+ bash-1994 1d..2 36us : do_softirq <-irq_exit
+ bash-1994 1d..2 36us : __do_softirq <-call_softirq
+ bash-1994 1d..2 36us : __local_bh_disable <-__do_softirq
+ bash-1994 1d.s2 37us : add_preempt_count <-_raw_spin_lock_irq
+ bash-1994 1d.s3 38us : _raw_spin_unlock <-run_timer_softirq
+ bash-1994 1d.s3 39us : sub_preempt_count <-_raw_spin_unlock
+ bash-1994 1d.s2 39us : call_timer_fn <-run_timer_softirq
+ [...]
+ bash-1994 1dNs2 81us : cpu_needs_another_gp <-rcu_process_callbacks
+ bash-1994 1dNs2 82us : __local_bh_enable <-__do_softirq
+ bash-1994 1dNs2 82us : sub_preempt_count <-__local_bh_enable
+ bash-1994 1dN.2 82us : idle_cpu <-irq_exit
+ bash-1994 1dN.2 83us : rcu_irq_exit <-irq_exit
+ bash-1994 1dN.2 83us : sub_preempt_count <-irq_exit
+ bash-1994 1.N.1 84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
+ bash-1994 1.N.1 84us+: trace_preempt_on <-task_rq_unlock
+ bash-1994 1.N.1 104us : <stack trace>
+ => sub_preempt_count
+ => _raw_spin_unlock_irqrestore
+ => task_rq_unlock
+ => wake_up_new_task
+ => do_fork
+ => sys_clone
+ => stub_clone
+
+
+The above is an example of the preemptoff trace with
+function-trace set. Here we see that interrupts were not disabled
+the entire time. The irq_enter code lets us know that we entered
+an interrupt 'h'. Before that, the functions being traced still
+show that it is not in an interrupt, but we can see from the
+functions themselves that this is not the case.
+
+preemptirqsoff
+--------------
+
+Knowing the locations that have interrupts disabled or
+preemption disabled for the longest times is helpful. But
+sometimes we would like to know when either preemption and/or
+interrupts are disabled.
+
+Consider the following code::
+
+ local_irq_disable();
+ call_function_with_irqs_off();
+ preempt_disable();
+ call_function_with_irqs_and_preemption_off();
+ local_irq_enable();
+ call_function_with_preemption_off();
+ preempt_enable();
+
+The irqsoff tracer will record the total length of
+call_function_with_irqs_off() and
+call_function_with_irqs_and_preemption_off().
+
+The preemptoff tracer will record the total length of
+call_function_with_irqs_and_preemption_off() and
+call_function_with_preemption_off().
+
+But neither will trace the time that interrupts and/or
+preemption is disabled. This total time is the time that we can
+not schedule. To record this time, use the preemptirqsoff
+tracer.
+
+Again, using this trace is much like the irqsoff and preemptoff
+tracers.
+::
+
+ # echo 0 > options/function-trace
+ # echo preemptirqsoff > current_tracer
+ # echo 1 > tracing_on
+ # echo 0 > tracing_max_latency
+ # ls -ltr
+ [...]
+ # echo 0 > tracing_on
+ # cat trace
+ # tracer: preemptirqsoff
+ #
+ # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
+ # --------------------------------------------------------------------
+ # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+ # -----------------
+ # | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
+ # -----------------
+ # => started at: ata_scsi_queuecmd
+ # => ended at: ata_scsi_queuecmd
+ #
+ #
+ # _------=> CPU#
+ # / _-----=> irqs-off
+ # | / _----=> need-resched
+ # || / _---=> hardirq/softirq
+ # ||| / _--=> preempt-depth
+ # |||| / delay
+ # cmd pid ||||| time | caller
+ # \ / ||||| \ | /
+ ls-2230 3d... 0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
+ ls-2230 3...1 100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
+ ls-2230 3...1 101us+: trace_preempt_on <-ata_scsi_queuecmd
+ ls-2230 3...1 111us : <stack trace>
+ => sub_preempt_count
+ => _raw_spin_unlock_irqrestore
+ => ata_scsi_queuecmd
+ => scsi_dispatch_cmd
+ => scsi_request_fn
+ => __blk_run_queue_uncond
+ => __blk_run_queue
+ => blk_queue_bio
+ => generic_make_request
+ => submit_bio
+ => submit_bh
+ => ext3_bread
+ => ext3_dir_bread
+ => htree_dirblock_to_tree
+ => ext3_htree_fill_tree
+ => ext3_readdir
+ => vfs_readdir
+ => sys_getdents
+ => system_call_fastpath
+
+
+The trace_hardirqs_off_thunk is called from assembly on x86 when
+interrupts are disabled in the assembly code. Without the
+function tracing, we do not know if interrupts were enabled
+within the preemption points. We do see that it started with
+preemption enabled.
+
+Here is a trace with function-trace set::
+
+ # tracer: preemptirqsoff
+ #
+ # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
+ # --------------------------------------------------------------------
+ # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+ # -----------------
+ # | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
+ # -----------------
+ # => started at: schedule
+ # => ended at: mutex_unlock
+ #
+ #
+ # _------=> CPU#
+ # / _-----=> irqs-off
+ # | / _----=> need-resched
+ # || / _---=> hardirq/softirq
+ # ||| / _--=> preempt-depth
+ # |||| / delay
+ # cmd pid ||||| time | caller
+ # \ / ||||| \ | /
+ kworker/-59 3...1 0us : __schedule <-schedule
+ kworker/-59 3d..1 0us : rcu_preempt_qs <-rcu_note_context_switch
+ kworker/-59 3d..1 1us : add_preempt_count <-_raw_spin_lock_irq
+ kworker/-59 3d..2 1us : deactivate_task <-__schedule
+ kworker/-59 3d..2 1us : dequeue_task <-deactivate_task
+ kworker/-59 3d..2 2us : update_rq_clock <-dequeue_task
+ kworker/-59 3d..2 2us : dequeue_task_fair <-dequeue_task
+ kworker/-59 3d..2 2us : update_curr <-dequeue_task_fair
+ kworker/-59 3d..2 2us : update_min_vruntime <-update_curr
+ kworker/-59 3d..2 3us : cpuacct_charge <-update_curr
+ kworker/-59 3d..2 3us : __rcu_read_lock <-cpuacct_charge
+ kworker/-59 3d..2 3us : __rcu_read_unlock <-cpuacct_charge
+ kworker/-59 3d..2 3us : update_cfs_rq_blocked_load <-dequeue_task_fair
+ kworker/-59 3d..2 4us : clear_buddies <-dequeue_task_fair
+ kworker/-59 3d..2 4us : account_entity_dequeue <-dequeue_task_fair
+ kworker/-59 3d..2 4us : update_min_vruntime <-dequeue_task_fair
+ kworker/-59 3d..2 4us : update_cfs_shares <-dequeue_task_fair
+ kworker/-59 3d..2 5us : hrtick_update <-dequeue_task_fair
+ kworker/-59 3d..2 5us : wq_worker_sleeping <-__schedule
+ kworker/-59 3d..2 5us : kthread_data <-wq_worker_sleeping
+ kworker/-59 3d..2 5us : put_prev_task_fair <-__schedule
+ kworker/-59 3d..2 6us : pick_next_task_fair <-pick_next_task
+ kworker/-59 3d..2 6us : clear_buddies <-pick_next_task_fair
+ kworker/-59 3d..2 6us : set_next_entity <-pick_next_task_fair
+ kworker/-59 3d..2 6us : update_stats_wait_end <-set_next_entity
+ ls-2269 3d..2 7us : finish_task_switch <-__schedule
+ ls-2269 3d..2 7us : _raw_spin_unlock_irq <-finish_task_switch
+ ls-2269 3d..2 8us : do_IRQ <-ret_from_intr
+ ls-2269 3d..2 8us : irq_enter <-do_IRQ
+ ls-2269 3d..2 8us : rcu_irq_enter <-irq_enter
+ ls-2269 3d..2 9us : add_preempt_count <-irq_enter
+ ls-2269 3d.h2 9us : exit_idle <-do_IRQ
+ [...]
+ ls-2269 3d.h3 20us : sub_preempt_count <-_raw_spin_unlock
+ ls-2269 3d.h2 20us : irq_exit <-do_IRQ
+ ls-2269 3d.h2 21us : sub_preempt_count <-irq_exit
+ ls-2269 3d..3 21us : do_softirq <-irq_exit
+ ls-2269 3d..3 21us : __do_softirq <-call_softirq
+ ls-2269 3d..3 21us+: __local_bh_disable <-__do_softirq
+ ls-2269 3d.s4 29us : sub_preempt_count <-_local_bh_enable_ip
+ ls-2269 3d.s5 29us : sub_preempt_count <-_local_bh_enable_ip
+ ls-2269 3d.s5 31us : do_IRQ <-ret_from_intr
+ ls-2269 3d.s5 31us : irq_enter <-do_IRQ
+ ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
+ [...]
+ ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
+ ls-2269 3d.s5 32us : add_preempt_count <-irq_enter
+ ls-2269 3d.H5 32us : exit_idle <-do_IRQ
+ ls-2269 3d.H5 32us : handle_irq <-do_IRQ
+ ls-2269 3d.H5 32us : irq_to_desc <-handle_irq
+ ls-2269 3d.H5 33us : handle_fasteoi_irq <-handle_irq
+ [...]
+ ls-2269 3d.s5 158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
+ ls-2269 3d.s3 158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
+ ls-2269 3d.s3 159us : __local_bh_enable <-__do_softirq
+ ls-2269 3d.s3 159us : sub_preempt_count <-__local_bh_enable
+ ls-2269 3d..3 159us : idle_cpu <-irq_exit
+ ls-2269 3d..3 159us : rcu_irq_exit <-irq_exit
+ ls-2269 3d..3 160us : sub_preempt_count <-irq_exit
+ ls-2269 3d... 161us : __mutex_unlock_slowpath <-mutex_unlock
+ ls-2269 3d... 162us+: trace_hardirqs_on <-mutex_unlock
+ ls-2269 3d... 186us : <stack trace>
+ => __mutex_unlock_slowpath
+ => mutex_unlock
+ => process_output
+ => n_tty_write
+ => tty_write
+ => vfs_write
+ => sys_write
+ => system_call_fastpath
+
+This is an interesting trace. It started with kworker running and
+scheduling out and ls taking over. But as soon as ls released the
+rq lock and enabled interrupts (but not preemption) an interrupt
+triggered. When the interrupt finished, it started running softirqs.
+But while the softirq was running, another interrupt triggered.
+When an interrupt is running inside a softirq, the annotation is 'H'.
+
+
+wakeup
+------
+
+One common case that people are interested in tracing is the
+time it takes for a task that is woken to actually wake up.
+Now for non Real-Time tasks, this can be arbitrary. But tracing
+it none the less can be interesting.
+
+Without function tracing::
+
+ # echo 0 > options/function-trace
+ # echo wakeup > current_tracer
+ # echo 1 > tracing_on
+ # echo 0 > tracing_max_latency
+ # chrt -f 5 sleep 1
+ # echo 0 > tracing_on
+ # cat trace
+ # tracer: wakeup
+ #
+ # wakeup latency trace v1.1.5 on 3.8.0-test+
+ # --------------------------------------------------------------------
+ # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+ # -----------------
+ # | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
+ # -----------------
+ #
+ # _------=> CPU#
+ # / _-----=> irqs-off
+ # | / _----=> need-resched
+ # || / _---=> hardirq/softirq
+ # ||| / _--=> preempt-depth
+ # |||| / delay
+ # cmd pid ||||| time | caller
+ # \ / ||||| \ | /
+ <idle>-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H
+ <idle>-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
+ <idle>-0 3d..3 15us : __schedule <-schedule
+ <idle>-0 3d..3 15us : 0:120:R ==> [003] 312:100:R kworker/3:1H
+
+The tracer only traces the highest priority task in the system
+to avoid tracing the normal circumstances. Here we see that
+the kworker with a nice priority of -20 (not very nice), took
+just 15 microseconds from the time it woke up, to the time it
+ran.
+
+Non Real-Time tasks are not that interesting. A more interesting
+trace is to concentrate only on Real-Time tasks.
+
+wakeup_rt
+---------
+
+In a Real-Time environment it is very important to know the
+wakeup time it takes for the highest priority task that is woken
+up to the time that it executes. This is also known as "schedule
+latency". I stress the point that this is about RT tasks. It is
+also important to know the scheduling latency of non-RT tasks,
+but the average schedule latency is better for non-RT tasks.
+Tools like LatencyTop are more appropriate for such
+measurements.
+
+Real-Time environments are interested in the worst case latency.
+That is the longest latency it takes for something to happen,
+and not the average. We can have a very fast scheduler that may
+only have a large latency once in a while, but that would not
+work well with Real-Time tasks. The wakeup_rt tracer was designed
+to record the worst case wakeups of RT tasks. Non-RT tasks are
+not recorded because the tracer only records one worst case and
+tracing non-RT tasks that are unpredictable will overwrite the
+worst case latency of RT tasks (just run the normal wakeup
+tracer for a while to see that effect).
+
+Since this tracer only deals with RT tasks, we will run this
+slightly differently than we did with the previous tracers.
+Instead of performing an 'ls', we will run 'sleep 1' under
+'chrt' which changes the priority of the task.
+::
+
+ # echo 0 > options/function-trace
+ # echo wakeup_rt > current_tracer
+ # echo 1 > tracing_on
+ # echo 0 > tracing_max_latency
+ # chrt -f 5 sleep 1
+ # echo 0 > tracing_on
+ # cat trace
+ # tracer: wakeup
+ #
+ # tracer: wakeup_rt
+ #
+ # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
+ # --------------------------------------------------------------------
+ # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+ # -----------------
+ # | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
+ # -----------------
+ #
+ # _------=> CPU#
+ # / _-----=> irqs-off
+ # | / _----=> need-resched
+ # || / _---=> hardirq/softirq
+ # ||| / _--=> preempt-depth
+ # |||| / delay
+ # cmd pid ||||| time | caller
+ # \ / ||||| \ | /
+ <idle>-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep
+ <idle>-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
+ <idle>-0 3d..3 5us : __schedule <-schedule
+ <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep
+
+
+Running this on an idle system, we see that it only took 5 microseconds
+to perform the task switch. Note, since the trace point in the schedule
+is before the actual "switch", we stop the tracing when the recorded task
+is about to schedule in. This may change if we add a new marker at the
+end of the scheduler.
+
+Notice that the recorded task is 'sleep' with the PID of 2389
+and it has an rt_prio of 5. This priority is user-space priority
+and not the internal kernel priority. The policy is 1 for
+SCHED_FIFO and 2 for SCHED_RR.
+
+Note, that the trace data shows the internal priority (99 - rtprio).
+::
+
+ <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep
+
+The 0:120:R means idle was running with a nice priority of 0 (120 - 120)
+and in the running state 'R'. The sleep task was scheduled in with
+2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
+and it too is in the running state.
+
+Doing the same with chrt -r 5 and function-trace set.
+::
+
+ echo 1 > options/function-trace
+
+ # tracer: wakeup_rt
+ #
+ # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
+ # --------------------------------------------------------------------
+ # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+ # -----------------
+ # | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
+ # -----------------
+ #
+ # _------=> CPU#
+ # / _-----=> irqs-off
+ # | / _----=> need-resched
+ # || / _---=> hardirq/softirq
+ # ||| / _--=> preempt-depth
+ # |||| / delay
+ # cmd pid ||||| time | caller
+ # \ / ||||| \ | /
+ <idle>-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep
+ <idle>-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
+ <idle>-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup
+ <idle>-0 3d.h3 3us : resched_curr <-check_preempt_curr
+ <idle>-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup
+ <idle>-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up
+ <idle>-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock
+ <idle>-0 3dNh2 5us : ttwu_stat <-try_to_wake_up
+ <idle>-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
+ <idle>-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
+ <idle>-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer
+ <idle>-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock
+ <idle>-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt
+ <idle>-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock
+ <idle>-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt
+ <idle>-0 3dNh1 7us : clockevents_program_event <-tick_program_event
+ <idle>-0 3dNh1 8us : ktime_get <-clockevents_program_event
+ <idle>-0 3dNh1 8us : lapic_next_event <-clockevents_program_event
+ <idle>-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt
+ <idle>-0 3dNh1 9us : sub_preempt_count <-irq_exit
+ <idle>-0 3dN.2 9us : idle_cpu <-irq_exit
+ <idle>-0 3dN.2 9us : rcu_irq_exit <-irq_exit
+ <idle>-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
+ <idle>-0 3dN.2 10us : sub_preempt_count <-irq_exit
+ <idle>-0 3.N.1 11us : rcu_idle_exit <-cpu_idle
+ <idle>-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
+ <idle>-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle
+ <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 13us : cpu_load_update_nohz <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 13us : _raw_spin_lock <-cpu_load_update_nohz
+ <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock
+ <idle>-0 3dN.2 13us : __cpu_load_update <-cpu_load_update_nohz
+ <idle>-0 3dN.2 14us : sched_avg_update <-__cpu_load_update
+ <idle>-0 3dN.2 14us : _raw_spin_unlock <-cpu_load_update_nohz
+ <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock
+ <idle>-0 3dN.1 15us : calc_load_nohz_stop <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel
+ <idle>-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
+ <idle>-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
+ <idle>-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave
+ <idle>-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16
+ <idle>-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer
+ <idle>-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram
+ <idle>-0 3dN.2 18us : clockevents_program_event <-tick_program_event
+ <idle>-0 3dN.2 18us : ktime_get <-clockevents_program_event
+ <idle>-0 3dN.2 18us : lapic_next_event <-clockevents_program_event
+ <idle>-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
+ <idle>-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
+ <idle>-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
+ <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
+ <idle>-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
+ <idle>-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
+ <idle>-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
+ <idle>-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
+ <idle>-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave
+ <idle>-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns
+ <idle>-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns
+ <idle>-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns
+ <idle>-0 3dN.2 23us : clockevents_program_event <-tick_program_event
+ <idle>-0 3dN.2 23us : ktime_get <-clockevents_program_event
+ <idle>-0 3dN.2 23us : lapic_next_event <-clockevents_program_event
+ <idle>-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
+ <idle>-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
+ <idle>-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit
+ <idle>-0 3dN.1 24us : account_idle_time <-account_idle_ticks
+ <idle>-0 3.N.1 25us : sub_preempt_count <-cpu_idle
+ <idle>-0 3.N.. 25us : schedule <-cpu_idle
+ <idle>-0 3.N.. 25us : __schedule <-preempt_schedule
+ <idle>-0 3.N.. 26us : add_preempt_count <-__schedule
+ <idle>-0 3.N.1 26us : rcu_note_context_switch <-__schedule
+ <idle>-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch
+ <idle>-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch
+ <idle>-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule
+ <idle>-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq
+ <idle>-0 3dN.2 28us : put_prev_task_idle <-__schedule
+ <idle>-0 3dN.2 28us : pick_next_task_stop <-pick_next_task
+ <idle>-0 3dN.2 28us : pick_next_task_rt <-pick_next_task
+ <idle>-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt
+ <idle>-0 3d..3 29us : __schedule <-preempt_schedule
+ <idle>-0 3d..3 30us : 0:120:R ==> [003] 2448: 94:R sleep
+
+This isn't that big of a trace, even with function tracing enabled,
+so I included the entire trace.
+
+The interrupt went off while when the system was idle. Somewhere
+before task_woken_rt() was called, the NEED_RESCHED flag was set,
+this is indicated by the first occurrence of the 'N' flag.
+
+Latency tracing and events
+--------------------------
+As function tracing can induce a much larger latency, but without
+seeing what happens within the latency it is hard to know what
+caused it. There is a middle ground, and that is with enabling
+events.
+::
+
+ # echo 0 > options/function-trace
+ # echo wakeup_rt > current_tracer
+ # echo 1 > events/enable
+ # echo 1 > tracing_on
+ # echo 0 > tracing_max_latency
+ # chrt -f 5 sleep 1
+ # echo 0 > tracing_on
+ # cat trace
+ # tracer: wakeup_rt
+ #
+ # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
+ # --------------------------------------------------------------------
+ # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+ # -----------------
+ # | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
+ # -----------------
+ #
+ # _------=> CPU#
+ # / _-----=> irqs-off
+ # | / _----=> need-resched
+ # || / _---=> hardirq/softirq
+ # ||| / _--=> preempt-depth
+ # |||| / delay
+ # cmd pid ||||| time | caller
+ # \ / ||||| \ | /
+ <idle>-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep
+ <idle>-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
+ <idle>-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
+ <idle>-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
+ <idle>-0 2.N.2 2us : power_end: cpu_id=2
+ <idle>-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2
+ <idle>-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
+ <idle>-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
+ <idle>-0 2.N.2 5us : rcu_utilization: Start context switch
+ <idle>-0 2.N.2 5us : rcu_utilization: End context switch
+ <idle>-0 2d..3 6us : __schedule <-schedule
+ <idle>-0 2d..3 6us : 0:120:R ==> [002] 5882: 94:R sleep
+
+
+Hardware Latency Detector
+-------------------------
+
+The hardware latency detector is executed by enabling the "hwlat" tracer.
+
+NOTE, this tracer will affect the performance of the system as it will
+periodically make a CPU constantly busy with interrupts disabled.
+::
+
+ # echo hwlat > current_tracer
+ # sleep 100
+ # cat trace
+ # tracer: hwlat
+ #
+ # _-----=> irqs-off
+ # / _----=> need-resched
+ # | / _---=> hardirq/softirq
+ # || / _--=> preempt-depth
+ # ||| / delay
+ # TASK-PID CPU# |||| TIMESTAMP FUNCTION
+ # | | | |||| | |
+ <...>-3638 [001] d... 19452.055471: #1 inner/outer(us): 12/14 ts:1499801089.066141940
+ <...>-3638 [003] d... 19454.071354: #2 inner/outer(us): 11/9 ts:1499801091.082164365
+ <...>-3638 [002] dn.. 19461.126852: #3 inner/outer(us): 12/9 ts:1499801098.138150062
+ <...>-3638 [001] d... 19488.340960: #4 inner/outer(us): 8/12 ts:1499801125.354139633
+ <...>-3638 [003] d... 19494.388553: #5 inner/outer(us): 8/12 ts:1499801131.402150961
+ <...>-3638 [003] d... 19501.283419: #6 inner/outer(us): 0/12 ts:1499801138.297435289 nmi-total:4 nmi-count:1
+
+
+The above output is somewhat the same in the header. All events will have
+interrupts disabled 'd'. Under the FUNCTION title there is:
+
+ #1
+ This is the count of events recorded that were greater than the
+ tracing_threshold (See below).
+
+ inner/outer(us): 12/14
+
+ This shows two numbers as "inner latency" and "outer latency". The test
+ runs in a loop checking a timestamp twice. The latency detected within
+ the two timestamps is the "inner latency" and the latency detected
+ after the previous timestamp and the next timestamp in the loop is
+ the "outer latency".
+
+ ts:1499801089.066141940
+
+ The absolute timestamp that the event happened.
+
+ nmi-total:4 nmi-count:1
+
+ On architectures that support it, if an NMI comes in during the
+ test, the time spent in NMI is reported in "nmi-total" (in
+ microseconds).
+
+ All architectures that have NMIs will show the "nmi-count" if an
+ NMI comes in during the test.
+
+hwlat files:
+
+ tracing_threshold
+ This gets automatically set to "10" to represent 10
+ microseconds. This is the threshold of latency that
+ needs to be detected before the trace will be recorded.
+
+ Note, when hwlat tracer is finished (another tracer is
+ written into "current_tracer"), the original value for
+ tracing_threshold is placed back into this file.
+
+ hwlat_detector/width
+ The length of time the test runs with interrupts disabled.
+
+ hwlat_detector/window
+ The length of time of the window which the test
+ runs. That is, the test will run for "width"
+ microseconds per "window" microseconds
+
+ tracing_cpumask
+ When the test is started. A kernel thread is created that
+ runs the test. This thread will alternate between CPUs
+ listed in the tracing_cpumask between each period
+ (one "window"). To limit the test to specific CPUs
+ set the mask in this file to only the CPUs that the test
+ should run on.
+
+function
+--------
+
+This tracer is the function tracer. Enabling the function tracer
+can be done from the debug file system. Make sure the
+ftrace_enabled is set; otherwise this tracer is a nop.
+See the "ftrace_enabled" section below.
+::
+
+ # sysctl kernel.ftrace_enabled=1
+ # echo function > current_tracer
+ # echo 1 > tracing_on
+ # usleep 1
+ # echo 0 > tracing_on
+ # cat trace
+ # tracer: function
+ #
+ # entries-in-buffer/entries-written: 24799/24799 #P:4
+ #
+ # _-----=> irqs-off
+ # / _----=> need-resched
+ # | / _---=> hardirq/softirq
+ # || / _--=> preempt-depth
+ # ||| / delay
+ # TASK-PID CPU# |||| TIMESTAMP FUNCTION
+ # | | | |||| | |
+ bash-1994 [002] .... 3082.063030: mutex_unlock <-rb_simple_write
+ bash-1994 [002] .... 3082.063031: __mutex_unlock_slowpath <-mutex_unlock
+ bash-1994 [002] .... 3082.063031: __fsnotify_parent <-fsnotify_modify
+ bash-1994 [002] .... 3082.063032: fsnotify <-fsnotify_modify
+ bash-1994 [002] .... 3082.063032: __srcu_read_lock <-fsnotify
+ bash-1994 [002] .... 3082.063032: add_preempt_count <-__srcu_read_lock
+ bash-1994 [002] ...1 3082.063032: sub_preempt_count <-__srcu_read_lock
+ bash-1994 [002] .... 3082.063033: __srcu_read_unlock <-fsnotify
+ [...]
+
+
+Note: function tracer uses ring buffers to store the above
+entries. The newest data may overwrite the oldest data.
+Sometimes using echo to stop the trace is not sufficient because
+the tracing could have overwritten the data that you wanted to
+record. For this reason, it is sometimes better to disable
+tracing directly from a program. This allows you to stop the
+tracing at the point that you hit the part that you are
+interested in. To disable the tracing directly from a C program,
+something like following code snippet can be used::
+
+ int trace_fd;
+ [...]
+ int main(int argc, char *argv[]) {
+ [...]
+ trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
+ [...]
+ if (condition_hit()) {
+ write(trace_fd, "0", 1);
+ }
+ [...]
+ }
+
+
+Single thread tracing
+---------------------
+
+By writing into set_ftrace_pid you can trace a
+single thread. For example::
+
+ # cat set_ftrace_pid
+ no pid
+ # echo 3111 > set_ftrace_pid
+ # cat set_ftrace_pid
+ 3111
+ # echo function > current_tracer
+ # cat trace | head
+ # tracer: function
+ #
+ # TASK-PID CPU# TIMESTAMP FUNCTION
+ # | | | | |
+ yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return
+ yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
+ yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
+ yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
+ yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll
+ yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll
+ # echo > set_ftrace_pid
+ # cat trace |head
+ # tracer: function
+ #
+ # TASK-PID CPU# TIMESTAMP FUNCTION
+ # | | | | |
+ ##### CPU 3 buffer started ####
+ yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait
+ yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry
+ yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry
+ yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit
+ yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit
+
+If you want to trace a function when executing, you could use
+something like this simple program.
+::
+
+ #include <stdio.h>
+ #include <stdlib.h>
+ #include <sys/types.h>
+ #include <sys/stat.h>
+ #include <fcntl.h>
+ #include <unistd.h>
+ #include <string.h>
+
+ #define _STR(x) #x
+ #define STR(x) _STR(x)
+ #define MAX_PATH 256
+
+ const char *find_tracefs(void)
+ {
+ static char tracefs[MAX_PATH+1];
+ static int tracefs_found;
+ char type[100];
+ FILE *fp;
+
+ if (tracefs_found)
+ return tracefs;
+
+ if ((fp = fopen("/proc/mounts","r")) == NULL) {
+ perror("/proc/mounts");
+ return NULL;
+ }
+
+ while (fscanf(fp, "%*s %"
+ STR(MAX_PATH)
+ "s %99s %*s %*d %*d\n",
+ tracefs, type) == 2) {
+ if (strcmp(type, "tracefs") == 0)
+ break;
+ }
+ fclose(fp);
+
+ if (strcmp(type, "tracefs") != 0) {
+ fprintf(stderr, "tracefs not mounted");
+ return NULL;
+ }
+
+ strcat(tracefs, "/tracing/");
+ tracefs_found = 1;
+
+ return tracefs;
+ }
+
+ const char *tracing_file(const char *file_name)
+ {
+ static char trace_file[MAX_PATH+1];
+ snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name);
+ return trace_file;
+ }
+
+ int main (int argc, char **argv)
+ {
+ if (argc < 1)
+ exit(-1);
+
+ if (fork() > 0) {
+ int fd, ffd;
+ char line[64];
+ int s;
+
+ ffd = open(tracing_file("current_tracer"), O_WRONLY);
+ if (ffd < 0)
+ exit(-1);
+ write(ffd, "nop", 3);
+
+ fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
+ s = sprintf(line, "%d\n", getpid());
+ write(fd, line, s);
+
+ write(ffd, "function", 8);
+
+ close(fd);
+ close(ffd);
+
+ execvp(argv[1], argv+1);
+ }
+
+ return 0;
+ }
+
+Or this simple script!
+::
+
+ #!/bin/bash
+
+ tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts`
+ echo nop > $tracefs/tracing/current_tracer
+ echo 0 > $tracefs/tracing/tracing_on
+ echo $$ > $tracefs/tracing/set_ftrace_pid
+ echo function > $tracefs/tracing/current_tracer
+ echo 1 > $tracefs/tracing/tracing_on
+ exec "$@"
+
+
+function graph tracer
+---------------------------
+
+This tracer is similar to the function tracer except that it
+probes a function on its entry and its exit. This is done by
+using a dynamically allocated stack of return addresses in each
+task_struct. On function entry the tracer overwrites the return
+address of each function traced to set a custom probe. Thus the
+original return address is stored on the stack of return address
+in the task_struct.
+
+Probing on both ends of a function leads to special features
+such as:
+
+- measure of a function's time execution
+- having a reliable call stack to draw function calls graph
+
+This tracer is useful in several situations:
+
+- you want to find the reason of a strange kernel behavior and
+ need to see what happens in detail on any areas (or specific
+ ones).
+
+- you are experiencing weird latencies but it's difficult to
+ find its origin.
+
+- you want to find quickly which path is taken by a specific
+ function
+
+- you just want to peek inside a working kernel and want to see
+ what happens there.
+
+::
+
+ # tracer: function_graph
+ #
+ # CPU DURATION FUNCTION CALLS
+ # | | | | | | |
+
+ 0) | sys_open() {
+ 0) | do_sys_open() {
+ 0) | getname() {
+ 0) | kmem_cache_alloc() {
+ 0) 1.382 us | __might_sleep();
+ 0) 2.478 us | }
+ 0) | strncpy_from_user() {
+ 0) | might_fault() {
+ 0) 1.389 us | __might_sleep();
+ 0) 2.553 us | }
+ 0) 3.807 us | }
+ 0) 7.876 us | }
+ 0) | alloc_fd() {
+ 0) 0.668 us | _spin_lock();
+ 0) 0.570 us | expand_files();
+ 0) 0.586 us | _spin_unlock();
+
+
+There are several columns that can be dynamically
+enabled/disabled. You can use every combination of options you
+want, depending on your needs.
+
+- The cpu number on which the function executed is default
+ enabled. It is sometimes better to only trace one cpu (see
+ tracing_cpu_mask file) or you might sometimes see unordered
+ function calls while cpu tracing switch.
+
+ - hide: echo nofuncgraph-cpu > trace_options
+ - show: echo funcgraph-cpu > trace_options
+
+- The duration (function's time of execution) is displayed on
+ the closing bracket line of a function or on the same line
+ than the current function in case of a leaf one. It is default
+ enabled.
+
+ - hide: echo nofuncgraph-duration > trace_options
+ - show: echo funcgraph-duration > trace_options
+
+- The overhead field precedes the duration field in case of
+ reached duration thresholds.
+
+ - hide: echo nofuncgraph-overhead > trace_options
+ - show: echo funcgraph-overhead > trace_options
+ - depends on: funcgraph-duration
+
+ ie::
+
+ 3) # 1837.709 us | } /* __switch_to */
+ 3) | finish_task_switch() {
+ 3) 0.313 us | _raw_spin_unlock_irq();
+ 3) 3.177 us | }
+ 3) # 1889.063 us | } /* __schedule */
+ 3) ! 140.417 us | } /* __schedule */
+ 3) # 2034.948 us | } /* schedule */
+ 3) * 33998.59 us | } /* schedule_preempt_disabled */
+
+ [...]
+
+ 1) 0.260 us | msecs_to_jiffies();
+ 1) 0.313 us | __rcu_read_unlock();
+ 1) + 61.770 us | }
+ 1) + 64.479 us | }
+ 1) 0.313 us | rcu_bh_qs();
+ 1) 0.313 us | __local_bh_enable();
+ 1) ! 217.240 us | }
+ 1) 0.365 us | idle_cpu();
+ 1) | rcu_irq_exit() {
+ 1) 0.417 us | rcu_eqs_enter_common.isra.47();
+ 1) 3.125 us | }
+ 1) ! 227.812 us | }
+ 1) ! 457.395 us | }
+ 1) @ 119760.2 us | }
+
+ [...]
+
+ 2) | handle_IPI() {
+ 1) 6.979 us | }
+ 2) 0.417 us | scheduler_ipi();
+ 1) 9.791 us | }
+ 1) + 12.917 us | }
+ 2) 3.490 us | }
+ 1) + 15.729 us | }
+ 1) + 18.542 us | }
+ 2) $ 3594274 us | }
+
+Flags::
+
+ + means that the function exceeded 10 usecs.
+ ! means that the function exceeded 100 usecs.
+ # means that the function exceeded 1000 usecs.
+ * means that the function exceeded 10 msecs.
+ @ means that the function exceeded 100 msecs.
+ $ means that the function exceeded 1 sec.
+
+
+- The task/pid field displays the thread cmdline and pid which
+ executed the function. It is default disabled.
+
+ - hide: echo nofuncgraph-proc > trace_options
+ - show: echo funcgraph-proc > trace_options
+
+ ie::
+
+ # tracer: function_graph
+ #
+ # CPU TASK/PID DURATION FUNCTION CALLS
+ # | | | | | | | | |
+ 0) sh-4802 | | d_free() {
+ 0) sh-4802 | | call_rcu() {
+ 0) sh-4802 | | __call_rcu() {
+ 0) sh-4802 | 0.616 us | rcu_process_gp_end();
+ 0) sh-4802 | 0.586 us | check_for_new_grace_period();
+ 0) sh-4802 | 2.899 us | }
+ 0) sh-4802 | 4.040 us | }
+ 0) sh-4802 | 5.151 us | }
+ 0) sh-4802 | + 49.370 us | }
+
+
+- The absolute time field is an absolute timestamp given by the
+ system clock since it started. A snapshot of this time is
+ given on each entry/exit of functions
+
+ - hide: echo nofuncgraph-abstime > trace_options
+ - show: echo funcgraph-abstime > trace_options
+
+ ie::
+
+ #
+ # TIME CPU DURATION FUNCTION CALLS
+ # | | | | | | | |
+ 360.774522 | 1) 0.541 us | }
+ 360.774522 | 1) 4.663 us | }
+ 360.774523 | 1) 0.541 us | __wake_up_bit();
+ 360.774524 | 1) 6.796 us | }
+ 360.774524 | 1) 7.952 us | }
+ 360.774525 | 1) 9.063 us | }
+ 360.774525 | 1) 0.615 us | journal_mark_dirty();
+ 360.774527 | 1) 0.578 us | __brelse();
+ 360.774528 | 1) | reiserfs_prepare_for_journal() {
+ 360.774528 | 1) | unlock_buffer() {
+ 360.774529 | 1) | wake_up_bit() {
+ 360.774529 | 1) | bit_waitqueue() {
+ 360.774530 | 1) 0.594 us | __phys_addr();
+
+
+The function name is always displayed after the closing bracket
+for a function if the start of that function is not in the
+trace buffer.
+
+Display of the function name after the closing bracket may be
+enabled for functions whose start is in the trace buffer,
+allowing easier searching with grep for function durations.
+It is default disabled.
+
+ - hide: echo nofuncgraph-tail > trace_options
+ - show: echo funcgraph-tail > trace_options
+
+ Example with nofuncgraph-tail (default)::
+
+ 0) | putname() {
+ 0) | kmem_cache_free() {
+ 0) 0.518 us | __phys_addr();
+ 0) 1.757 us | }
+ 0) 2.861 us | }
+
+ Example with funcgraph-tail::
+
+ 0) | putname() {
+ 0) | kmem_cache_free() {
+ 0) 0.518 us | __phys_addr();
+ 0) 1.757 us | } /* kmem_cache_free() */
+ 0) 2.861 us | } /* putname() */
+
+You can put some comments on specific functions by using
+trace_printk() For example, if you want to put a comment inside
+the __might_sleep() function, you just have to include
+<linux/ftrace.h> and call trace_printk() inside __might_sleep()::
+
+ trace_printk("I'm a comment!\n")
+
+will produce::
+
+ 1) | __might_sleep() {
+ 1) | /* I'm a comment! */
+ 1) 1.449 us | }
+
+
+You might find other useful features for this tracer in the
+following "dynamic ftrace" section such as tracing only specific
+functions or tasks.
+
+dynamic ftrace
+--------------
+
+If CONFIG_DYNAMIC_FTRACE is set, the system will run with
+virtually no overhead when function tracing is disabled. The way
+this works is the mcount function call (placed at the start of
+every kernel function, produced by the -pg switch in gcc),
+starts of pointing to a simple return. (Enabling FTRACE will
+include the -pg switch in the compiling of the kernel.)
+
+At compile time every C file object is run through the
+recordmcount program (located in the scripts directory). This
+program will parse the ELF headers in the C object to find all
+the locations in the .text section that call mcount. Starting
+with gcc verson 4.6, the -mfentry has been added for x86, which
+calls "__fentry__" instead of "mcount". Which is called before
+the creation of the stack frame.
+
+Note, not all sections are traced. They may be prevented by either
+a notrace, or blocked another way and all inline functions are not
+traced. Check the "available_filter_functions" file to see what functions
+can be traced.
+
+A section called "__mcount_loc" is created that holds
+references to all the mcount/fentry call sites in the .text section.
+The recordmcount program re-links this section back into the
+original object. The final linking stage of the kernel will add all these
+references into a single table.
+
+On boot up, before SMP is initialized, the dynamic ftrace code
+scans this table and updates all the locations into nops. It
+also records the locations, which are added to the
+available_filter_functions list. Modules are processed as they
+are loaded and before they are executed. When a module is
+unloaded, it also removes its functions from the ftrace function
+list. This is automatic in the module unload code, and the
+module author does not need to worry about it.
+
+When tracing is enabled, the process of modifying the function
+tracepoints is dependent on architecture. The old method is to use
+kstop_machine to prevent races with the CPUs executing code being
+modified (which can cause the CPU to do undesirable things, especially
+if the modified code crosses cache (or page) boundaries), and the nops are
+patched back to calls. But this time, they do not call mcount
+(which is just a function stub). They now call into the ftrace
+infrastructure.
+
+The new method of modifying the function tracepoints is to place
+a breakpoint at the location to be modified, sync all CPUs, modify
+the rest of the instruction not covered by the breakpoint. Sync
+all CPUs again, and then remove the breakpoint with the finished
+version to the ftrace call site.
+
+Some archs do not even need to monkey around with the synchronization,
+and can just slap the new code on top of the old without any
+problems with other CPUs executing it at the same time.
+
+One special side-effect to the recording of the functions being
+traced is that we can now selectively choose which functions we
+wish to trace and which ones we want the mcount calls to remain
+as nops.
+
+Two files are used, one for enabling and one for disabling the
+tracing of specified functions. They are:
+
+ set_ftrace_filter
+
+and
+
+ set_ftrace_notrace
+
+A list of available functions that you can add to these files is
+listed in:
+
+ available_filter_functions
+
+::
+
+ # cat available_filter_functions
+ put_prev_task_idle
+ kmem_cache_create
+ pick_next_task_rt
+ get_online_cpus
+ pick_next_task_fair
+ mutex_lock
+ [...]
+
+If I am only interested in sys_nanosleep and hrtimer_interrupt::
+
+ # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
+ # echo function > current_tracer
+ # echo 1 > tracing_on
+ # usleep 1
+ # echo 0 > tracing_on
+ # cat trace
+ # tracer: function
+ #
+ # entries-in-buffer/entries-written: 5/5 #P:4
+ #
+ # _-----=> irqs-off
+ # / _----=> need-resched
+ # | / _---=> hardirq/softirq
+ # || / _--=> preempt-depth
+ # ||| / delay
+ # TASK-PID CPU# |||| TIMESTAMP FUNCTION
+ # | | | |||| | |
+ usleep-2665 [001] .... 4186.475355: sys_nanosleep <-system_call_fastpath
+ <idle>-0 [001] d.h1 4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
+ usleep-2665 [001] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
+ <idle>-0 [003] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
+ <idle>-0 [002] d.h1 4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
+
+To see which functions are being traced, you can cat the file:
+::
+
+ # cat set_ftrace_filter
+ hrtimer_interrupt
+ sys_nanosleep
+
+
+Perhaps this is not enough. The filters also allow glob(7) matching.
+
+ <match>*
+ will match functions that begin with <match>
+ *<match>
+ will match functions that end with <match>
+ *<match>*
+ will match functions that have <match> in it
+ <match1>*<match2>
+ will match functions that begin with <match1> and end with <match2>
+
+.. note::
+ It is better to use quotes to enclose the wild cards,
+ otherwise the shell may expand the parameters into names
+ of files in the local directory.
+
+::
+
+ # echo 'hrtimer_*' > set_ftrace_filter
+
+Produces::
+
+ # tracer: function
+ #
+ # entries-in-buffer/entries-written: 897/897 #P:4
+ #
+ # _-----=> irqs-off
+ # / _----=> need-resched
+ # | / _---=> hardirq/softirq
+ # || / _--=> preempt-depth
+ # ||| / delay
+ # TASK-PID CPU# |||| TIMESTAMP FUNCTION
+ # | | | |||| | |
+ <idle>-0 [003] dN.1 4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
+ <idle>-0 [003] dN.1 4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
+ <idle>-0 [003] dN.2 4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
+ <idle>-0 [003] dN.1 4228.547805: hrtimer_forward <-tick_nohz_idle_exit
+ <idle>-0 [003] dN.1 4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
+ <idle>-0 [003] d..1 4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
+ <idle>-0 [003] d..1 4228.547859: hrtimer_start <-__tick_nohz_idle_enter
+ <idle>-0 [003] d..2 4228.547860: hrtimer_force_reprogram <-__rem
+
+Notice that we lost the sys_nanosleep.
+::
+
+ # cat set_ftrace_filter
+ hrtimer_run_queues
+ hrtimer_run_pending
+ hrtimer_init
+ hrtimer_cancel
+ hrtimer_try_to_cancel
+ hrtimer_forward
+ hrtimer_start
+ hrtimer_reprogram
+ hrtimer_force_reprogram
+ hrtimer_get_next_event
+ hrtimer_interrupt
+ hrtimer_nanosleep
+ hrtimer_wakeup
+ hrtimer_get_remaining
+ hrtimer_get_res
+ hrtimer_init_sleeper
+
+
+This is because the '>' and '>>' act just like they do in bash.
+To rewrite the filters, use '>'
+To append to the filters, use '>>'
+
+To clear out a filter so that all functions will be recorded
+again::
+
+ # echo > set_ftrace_filter
+ # cat set_ftrace_filter
+ #
+
+Again, now we want to append.
+
+::
+
+ # echo sys_nanosleep > set_ftrace_filter
+ # cat set_ftrace_filter
+ sys_nanosleep
+ # echo 'hrtimer_*' >> set_ftrace_filter
+ # cat set_ftrace_filter
+ hrtimer_run_queues
+ hrtimer_run_pending
+ hrtimer_init
+ hrtimer_cancel
+ hrtimer_try_to_cancel
+ hrtimer_forward
+ hrtimer_start
+ hrtimer_reprogram
+ hrtimer_force_reprogram
+ hrtimer_get_next_event
+ hrtimer_interrupt
+ sys_nanosleep
+ hrtimer_nanosleep
+ hrtimer_wakeup
+ hrtimer_get_remaining
+ hrtimer_get_res
+ hrtimer_init_sleeper
+
+
+The set_ftrace_notrace prevents those functions from being
+traced.
+::
+
+ # echo '*preempt*' '*lock*' > set_ftrace_notrace
+
+Produces::
+
+ # tracer: function
+ #
+ # entries-in-buffer/entries-written: 39608/39608 #P:4
+ #
+ # _-----=> irqs-off
+ # / _----=> need-resched
+ # | / _---=> hardirq/softirq
+ # || / _--=> preempt-depth
+ # ||| / delay
+ # TASK-PID CPU# |||| TIMESTAMP FUNCTION
+ # | | | |||| | |
+ bash-1994 [000] .... 4342.324896: file_ra_state_init <-do_dentry_open
+ bash-1994 [000] .... 4342.324897: open_check_o_direct <-do_last
+ bash-1994 [000] .... 4342.324897: ima_file_check <-do_last
+ bash-1994 [000] .... 4342.324898: process_measurement <-ima_file_check
+ bash-1994 [000] .... 4342.324898: ima_get_action <-process_measurement
+ bash-1994 [000] .... 4342.324898: ima_match_policy <-ima_get_action
+ bash-1994 [000] .... 4342.324899: do_truncate <-do_last
+ bash-1994 [000] .... 4342.324899: should_remove_suid <-do_truncate
+ bash-1994 [000] .... 4342.324899: notify_change <-do_truncate
+ bash-1994 [000] .... 4342.324900: current_fs_time <-notify_change
+ bash-1994 [000] .... 4342.324900: current_kernel_time <-current_fs_time
+ bash-1994 [000] .... 4342.324900: timespec_trunc <-current_fs_time
+
+We can see that there's no more lock or preempt tracing.
+
+
+Dynamic ftrace with the function graph tracer
+---------------------------------------------
+
+Although what has been explained above concerns both the
+function tracer and the function-graph-tracer, there are some
+special features only available in the function-graph tracer.
+
+If you want to trace only one function and all of its children,
+you just have to echo its name into set_graph_function::
+
+ echo __do_fault > set_graph_function
+
+will produce the following "expanded" trace of the __do_fault()
+function::
+
+ 0) | __do_fault() {
+ 0) | filemap_fault() {
+ 0) | find_lock_page() {
+ 0) 0.804 us | find_get_page();
+ 0) | __might_sleep() {
+ 0) 1.329 us | }
+ 0) 3.904 us | }
+ 0) 4.979 us | }
+ 0) 0.653 us | _spin_lock();
+ 0) 0.578 us | page_add_file_rmap();
+ 0) 0.525 us | native_set_pte_at();
+ 0) 0.585 us | _spin_unlock();
+ 0) | unlock_page() {
+ 0) 0.541 us | page_waitqueue();
+ 0) 0.639 us | __wake_up_bit();
+ 0) 2.786 us | }
+ 0) + 14.237 us | }
+ 0) | __do_fault() {
+ 0) | filemap_fault() {
+ 0) | find_lock_page() {
+ 0) 0.698 us | find_get_page();
+ 0) | __might_sleep() {
+ 0) 1.412 us | }
+ 0) 3.950 us | }
+ 0) 5.098 us | }
+ 0) 0.631 us | _spin_lock();
+ 0) 0.571 us | page_add_file_rmap();
+ 0) 0.526 us | native_set_pte_at();
+ 0) 0.586 us | _spin_unlock();
+ 0) | unlock_page() {
+ 0) 0.533 us | page_waitqueue();
+ 0) 0.638 us | __wake_up_bit();
+ 0) 2.793 us | }
+ 0) + 14.012 us | }
+
+You can also expand several functions at once::
+
+ echo sys_open > set_graph_function
+ echo sys_close >> set_graph_function
+
+Now if you want to go back to trace all functions you can clear
+this special filter via::
+
+ echo > set_graph_function
+
+
+ftrace_enabled
+--------------
+
+Note, the proc sysctl ftrace_enable is a big on/off switch for the
+function tracer. By default it is enabled (when function tracing is
+enabled in the kernel). If it is disabled, all function tracing is
+disabled. This includes not only the function tracers for ftrace, but
+also for any other uses (perf, kprobes, stack tracing, profiling, etc).
+
+Please disable this with care.
+
+This can be disable (and enabled) with::
+
+ sysctl kernel.ftrace_enabled=0
+ sysctl kernel.ftrace_enabled=1
+
+ or
+
+ echo 0 > /proc/sys/kernel/ftrace_enabled
+ echo 1 > /proc/sys/kernel/ftrace_enabled
+
+
+Filter commands
+---------------
+
+A few commands are supported by the set_ftrace_filter interface.
+Trace commands have the following format::
+
+ <function>:<command>:<parameter>
+
+The following commands are supported:
+
+- mod:
+ This command enables function filtering per module. The
+ parameter defines the module. For example, if only the write*
+ functions in the ext3 module are desired, run:
+
+ echo 'write*:mod:ext3' > set_ftrace_filter
+
+ This command interacts with the filter in the same way as
+ filtering based on function names. Thus, adding more functions
+ in a different module is accomplished by appending (>>) to the
+ filter file. Remove specific module functions by prepending
+ '!'::
+
+ echo '!writeback*:mod:ext3' >> set_ftrace_filter
+
+ Mod command supports module globbing. Disable tracing for all
+ functions except a specific module::
+
+ echo '!*:mod:!ext3' >> set_ftrace_filter
+
+ Disable tracing for all modules, but still trace kernel::
+
+ echo '!*:mod:*' >> set_ftrace_filter
+
+ Enable filter only for kernel::
+
+ echo '*write*:mod:!*' >> set_ftrace_filter
+
+ Enable filter for module globbing::
+
+ echo '*write*:mod:*snd*' >> set_ftrace_filter
+
+- traceon/traceoff:
+ These commands turn tracing on and off when the specified
+ functions are hit. The parameter determines how many times the
+ tracing system is turned on and off. If unspecified, there is
+ no limit. For example, to disable tracing when a schedule bug
+ is hit the first 5 times, run::
+
+ echo '__schedule_bug:traceoff:5' > set_ftrace_filter
+
+ To always disable tracing when __schedule_bug is hit::
+
+ echo '__schedule_bug:traceoff' > set_ftrace_filter
+
+ These commands are cumulative whether or not they are appended
+ to set_ftrace_filter. To remove a command, prepend it by '!'
+ and drop the parameter::
+
+ echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
+
+ The above removes the traceoff command for __schedule_bug
+ that have a counter. To remove commands without counters::
+
+ echo '!__schedule_bug:traceoff' > set_ftrace_filter
+
+- snapshot:
+ Will cause a snapshot to be triggered when the function is hit.
+ ::
+
+ echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
+
+ To only snapshot once:
+ ::
+
+ echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
+
+ To remove the above commands::
+
+ echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
+ echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
+
+- enable_event/disable_event:
+ These commands can enable or disable a trace event. Note, because
+ function tracing callbacks are very sensitive, when these commands
+ are registered, the trace point is activated, but disabled in
+ a "soft" mode. That is, the tracepoint will be called, but
+ just will not be traced. The event tracepoint stays in this mode
+ as long as there's a command that triggers it.
+ ::
+
+ echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
+ set_ftrace_filter
+
+ The format is::
+
+ <function>:enable_event:<system>:<event>[:count]
+ <function>:disable_event:<system>:<event>[:count]
+
+ To remove the events commands::
+
+ echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
+ set_ftrace_filter
+ echo '!schedule:disable_event:sched:sched_switch' > \
+ set_ftrace_filter
+
+- dump:
+ When the function is hit, it will dump the contents of the ftrace
+ ring buffer to the console. This is useful if you need to debug
+ something, and want to dump the trace when a certain function
+ is hit. Perhaps its a function that is called before a tripple
+ fault happens and does not allow you to get a regular dump.
+
+- cpudump:
+ When the function is hit, it will dump the contents of the ftrace
+ ring buffer for the current CPU to the console. Unlike the "dump"
+ command, it only prints out the contents of the ring buffer for the
+ CPU that executed the function that triggered the dump.
+
+trace_pipe
+----------
+
+The trace_pipe outputs the same content as the trace file, but
+the effect on the tracing is different. Every read from
+trace_pipe is consumed. This means that subsequent reads will be
+different. The trace is live.
+::
+
+ # echo function > current_tracer
+ # cat trace_pipe > /tmp/trace.out &
+ [1] 4153
+ # echo 1 > tracing_on
+ # usleep 1
+ # echo 0 > tracing_on
+ # cat trace
+ # tracer: function
+ #
+ # entries-in-buffer/entries-written: 0/0 #P:4
+ #
+ # _-----=> irqs-off
+ # / _----=> need-resched
+ # | / _---=> hardirq/softirq
+ # || / _--=> preempt-depth
+ # ||| / delay
+ # TASK-PID CPU# |||| TIMESTAMP FUNCTION
+ # | | | |||| | |
+
+ #
+ # cat /tmp/trace.out
+ bash-1994 [000] .... 5281.568961: mutex_unlock <-rb_simple_write
+ bash-1994 [000] .... 5281.568963: __mutex_unlock_slowpath <-mutex_unlock
+ bash-1994 [000] .... 5281.568963: __fsnotify_parent <-fsnotify_modify
+ bash-1994 [000] .... 5281.568964: fsnotify <-fsnotify_modify
+ bash-1994 [000] .... 5281.568964: __srcu_read_lock <-fsnotify
+ bash-1994 [000] .... 5281.568964: add_preempt_count <-__srcu_read_lock
+ bash-1994 [000] ...1 5281.568965: sub_preempt_count <-__srcu_read_lock
+ bash-1994 [000] .... 5281.568965: __srcu_read_unlock <-fsnotify
+ bash-1994 [000] .... 5281.568967: sys_dup2 <-system_call_fastpath
+
+
+Note, reading the trace_pipe file will block until more input is
+added.
+
+trace entries
+-------------
+
+Having too much or not enough data can be troublesome in
+diagnosing an issue in the kernel. The file buffer_size_kb is
+used to modify the size of the internal trace buffers. The
+number listed is the number of entries that can be recorded per
+CPU. To know the full size, multiply the number of possible CPUs
+with the number of entries.
+::
+
+ # cat buffer_size_kb
+ 1408 (units kilobytes)
+
+Or simply read buffer_total_size_kb
+::
+
+ # cat buffer_total_size_kb
+ 5632
+
+To modify the buffer, simple echo in a number (in 1024 byte segments).
+::
+
+ # echo 10000 > buffer_size_kb
+ # cat buffer_size_kb
+ 10000 (units kilobytes)
+
+It will try to allocate as much as possible. If you allocate too
+much, it can cause Out-Of-Memory to trigger.
+::
+
+ # echo 1000000000000 > buffer_size_kb
+ -bash: echo: write error: Cannot allocate memory
+ # cat buffer_size_kb
+ 85
+
+The per_cpu buffers can be changed individually as well:
+::
+
+ # echo 10000 > per_cpu/cpu0/buffer_size_kb
+ # echo 100 > per_cpu/cpu1/buffer_size_kb
+
+When the per_cpu buffers are not the same, the buffer_size_kb
+at the top level will just show an X
+::
+
+ # cat buffer_size_kb
+ X
+
+This is where the buffer_total_size_kb is useful:
+::
+
+ # cat buffer_total_size_kb
+ 12916
+
+Writing to the top level buffer_size_kb will reset all the buffers
+to be the same again.
+
+Snapshot
+--------
+CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
+available to all non latency tracers. (Latency tracers which
+record max latency, such as "irqsoff" or "wakeup", can't use
+this feature, since those are already using the snapshot
+mechanism internally.)
+
+Snapshot preserves a current trace buffer at a particular point
+in time without stopping tracing. Ftrace swaps the current
+buffer with a spare buffer, and tracing continues in the new
+current (=previous spare) buffer.
+
+The following tracefs files in "tracing" are related to this
+feature:
+
+ snapshot:
+
+ This is used to take a snapshot and to read the output
+ of the snapshot. Echo 1 into this file to allocate a
+ spare buffer and to take a snapshot (swap), then read
+ the snapshot from this file in the same format as
+ "trace" (described above in the section "The File
+ System"). Both reads snapshot and tracing are executable
+ in parallel. When the spare buffer is allocated, echoing
+ 0 frees it, and echoing else (positive) values clear the
+ snapshot contents.
+ More details are shown in the table below.
+
+ +--------------+------------+------------+------------+
+ |status\\input | 0 | 1 | else |
+ +==============+============+============+============+
+ |not allocated |(do nothing)| alloc+swap |(do nothing)|
+ +--------------+------------+------------+------------+
+ |allocated | free | swap | clear |
+ +--------------+------------+------------+------------+
+
+Here is an example of using the snapshot feature.
+::
+
+ # echo 1 > events/sched/enable
+ # echo 1 > snapshot
+ # cat snapshot
+ # tracer: nop
+ #
+ # entries-in-buffer/entries-written: 71/71 #P:8
+ #
+ # _-----=> irqs-off
+ # / _----=> need-resched
+ # | / _---=> hardirq/softirq
+ # || / _--=> preempt-depth
+ # ||| / delay
+ # TASK-PID CPU# |||| TIMESTAMP FUNCTION
+ # | | | |||| | |
+ <idle>-0 [005] d... 2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120
+ sleep-2242 [005] d... 2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120
+ [...]
+ <idle>-0 [002] d... 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120
+
+ # cat trace
+ # tracer: nop
+ #
+ # entries-in-buffer/entries-written: 77/77 #P:8
+ #
+ # _-----=> irqs-off
+ # / _----=> need-resched
+ # | / _---=> hardirq/softirq
+ # || / _--=> preempt-depth
+ # ||| / delay
+ # TASK-PID CPU# |||| TIMESTAMP FUNCTION
+ # | | | |||| | |
+ <idle>-0 [007] d... 2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120
+ snapshot-test-2-2229 [002] d... 2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120
+ [...]
+
+
+If you try to use this snapshot feature when current tracer is
+one of the latency tracers, you will get the following results.
+::
+
+ # echo wakeup > current_tracer
+ # echo 1 > snapshot
+ bash: echo: write error: Device or resource busy
+ # cat snapshot
+ cat: snapshot: Device or resource busy
+
+
+Instances
+---------
+In the tracefs tracing directory is a directory called "instances".
+This directory can have new directories created inside of it using
+mkdir, and removing directories with rmdir. The directory created
+with mkdir in this directory will already contain files and other
+directories after it is created.
+::
+
+ # mkdir instances/foo
+ # ls instances/foo
+ buffer_size_kb buffer_total_size_kb events free_buffer per_cpu
+ set_event snapshot trace trace_clock trace_marker trace_options
+ trace_pipe tracing_on
+
+As you can see, the new directory looks similar to the tracing directory
+itself. In fact, it is very similar, except that the buffer and
+events are agnostic from the main director, or from any other
+instances that are created.
+
+The files in the new directory work just like the files with the
+same name in the tracing directory except the buffer that is used
+is a separate and new buffer. The files affect that buffer but do not
+affect the main buffer with the exception of trace_options. Currently,
+the trace_options affect all instances and the top level buffer
+the same, but this may change in future releases. That is, options
+may become specific to the instance they reside in.
+
+Notice that none of the function tracer files are there, nor is
+current_tracer and available_tracers. This is because the buffers
+can currently only have events enabled for them.
+::
+
+ # mkdir instances/foo
+ # mkdir instances/bar
+ # mkdir instances/zoot
+ # echo 100000 > buffer_size_kb
+ # echo 1000 > instances/foo/buffer_size_kb
+ # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
+ # echo function > current_trace
+ # echo 1 > instances/foo/events/sched/sched_wakeup/enable
+ # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
+ # echo 1 > instances/foo/events/sched/sched_switch/enable
+ # echo 1 > instances/bar/events/irq/enable
+ # echo 1 > instances/zoot/events/syscalls/enable
+ # cat trace_pipe
+ CPU:2 [LOST 11745 EVENTS]
+ bash-2044 [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
+ bash-2044 [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
+ bash-2044 [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
+ bash-2044 [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
+ bash-2044 [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
+ bash-2044 [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
+ bash-2044 [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
+ bash-2044 [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
+ bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics
+ bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics
+ bash-2044 [002] .... 10594.481035: arch_dup_task_struct <-copy_process
+ [...]
+
+ # cat instances/foo/trace_pipe
+ bash-1998 [000] d..4 136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
+ bash-1998 [000] dN.4 136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
+ <idle>-0 [003] d.h3 136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
+ <idle>-0 [003] d..3 136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120
+ rcu_preempt-9 [003] d..3 136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120
+ bash-1998 [000] d..4 136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
+ bash-1998 [000] dN.4 136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
+ bash-1998 [000] d..3 136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120
+ kworker/0:1-59 [000] d..4 136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
+ kworker/0:1-59 [000] d..3 136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120
+ [...]
+
+ # cat instances/bar/trace_pipe
+ migration/1-14 [001] d.h3 138.732674: softirq_raise: vec=3 [action=NET_RX]
+ <idle>-0 [001] dNh3 138.732725: softirq_raise: vec=3 [action=NET_RX]
+ bash-1998 [000] d.h1 138.733101: softirq_raise: vec=1 [action=TIMER]
+ bash-1998 [000] d.h1 138.733102: softirq_raise: vec=9 [action=RCU]
+ bash-1998 [000] ..s2 138.733105: softirq_entry: vec=1 [action=TIMER]
+ bash-1998 [000] ..s2 138.733106: softirq_exit: vec=1 [action=TIMER]
+ bash-1998 [000] ..s2 138.733106: softirq_entry: vec=9 [action=RCU]
+ bash-1998 [000] ..s2 138.733109: softirq_exit: vec=9 [action=RCU]
+ sshd-1995 [001] d.h1 138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
+ sshd-1995 [001] d.h1 138.733280: irq_handler_exit: irq=21 ret=unhandled
+ sshd-1995 [001] d.h1 138.733281: irq_handler_entry: irq=21 name=eth0
+ sshd-1995 [001] d.h1 138.733283: irq_handler_exit: irq=21 ret=handled
+ [...]
+
+ # cat instances/zoot/trace
+ # tracer: nop
+ #
+ # entries-in-buffer/entries-written: 18996/18996 #P:4
+ #
+ # _-----=> irqs-off
+ # / _----=> need-resched
+ # | / _---=> hardirq/softirq
+ # || / _--=> preempt-depth
+ # ||| / delay
+ # TASK-PID CPU# |||| TIMESTAMP FUNCTION
+ # | | | |||| | |
+ bash-1998 [000] d... 140.733501: sys_write -> 0x2
+ bash-1998 [000] d... 140.733504: sys_dup2(oldfd: a, newfd: 1)
+ bash-1998 [000] d... 140.733506: sys_dup2 -> 0x1
+ bash-1998 [000] d... 140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
+ bash-1998 [000] d... 140.733509: sys_fcntl -> 0x1
+ bash-1998 [000] d... 140.733510: sys_close(fd: a)
+ bash-1998 [000] d... 140.733510: sys_close -> 0x0
+ bash-1998 [000] d... 140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
+ bash-1998 [000] d... 140.733515: sys_rt_sigprocmask -> 0x0
+ bash-1998 [000] d... 140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
+ bash-1998 [000] d... 140.733516: sys_rt_sigaction -> 0x0
+
+You can see that the trace of the top most trace buffer shows only
+the function tracing. The foo instance displays wakeups and task
+switches.
+
+To remove the instances, simply delete their directories:
+::
+
+ # rmdir instances/foo
+ # rmdir instances/bar
+ # rmdir instances/zoot
+
+Note, if a process has a trace file open in one of the instance
+directories, the rmdir will fail with EBUSY.
+
+
+Stack trace
+-----------
+Since the kernel has a fixed sized stack, it is important not to
+waste it in functions. A kernel developer must be conscience of
+what they allocate on the stack. If they add too much, the system
+can be in danger of a stack overflow, and corruption will occur,
+usually leading to a system panic.
+
+There are some tools that check this, usually with interrupts
+periodically checking usage. But if you can perform a check
+at every function call that will become very useful. As ftrace provides
+a function tracer, it makes it convenient to check the stack size
+at every function call. This is enabled via the stack tracer.
+
+CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
+To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
+::
+
+ # echo 1 > /proc/sys/kernel/stack_tracer_enabled
+
+You can also enable it from the kernel command line to trace
+the stack size of the kernel during boot up, by adding "stacktrace"
+to the kernel command line parameter.
+
+After running it for a few minutes, the output looks like:
+::
+
+ # cat stack_max_size
+ 2928
+
+ # cat stack_trace
+ Depth Size Location (18 entries)
+ ----- ---- --------
+ 0) 2928 224 update_sd_lb_stats+0xbc/0x4ac
+ 1) 2704 160 find_busiest_group+0x31/0x1f1
+ 2) 2544 256 load_balance+0xd9/0x662
+ 3) 2288 80 idle_balance+0xbb/0x130
+ 4) 2208 128 __schedule+0x26e/0x5b9
+ 5) 2080 16 schedule+0x64/0x66
+ 6) 2064 128 schedule_timeout+0x34/0xe0
+ 7) 1936 112 wait_for_common+0x97/0xf1
+ 8) 1824 16 wait_for_completion+0x1d/0x1f
+ 9) 1808 128 flush_work+0xfe/0x119
+ 10) 1680 16 tty_flush_to_ldisc+0x1e/0x20
+ 11) 1664 48 input_available_p+0x1d/0x5c
+ 12) 1616 48 n_tty_poll+0x6d/0x134
+ 13) 1568 64 tty_poll+0x64/0x7f
+ 14) 1504 880 do_select+0x31e/0x511
+ 15) 624 400 core_sys_select+0x177/0x216
+ 16) 224 96 sys_select+0x91/0xb9
+ 17) 128 128 system_call_fastpath+0x16/0x1b
+
+Note, if -mfentry is being used by gcc, functions get traced before
+they set up the stack frame. This means that leaf level functions
+are not tested by the stack tracer when -mfentry is used.
+
+Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
+
+More
+----
+More details can be found in the source code, in the `kernel/trace/*.c` files.
diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt
deleted file mode 100644
index d4601df..0000000
--- a/Documentation/trace/ftrace.txt
+++ /dev/null
@@ -1,3220 +0,0 @@
- ftrace - Function Tracer
- ========================
-
-Copyright 2008 Red Hat Inc.
- Author: Steven Rostedt <srostedt@...hat.com>
- License: The GNU Free Documentation License, Version 1.2
- (dual licensed under the GPL v2)
-Original Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton,
- John Kacur, and David Teigland.
-Written for: 2.6.28-rc2
-Updated for: 3.10
-Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt
-
-Introduction
-------------
-
-Ftrace is an internal tracer designed to help out developers and
-designers of systems to find what is going on inside the kernel.
-It can be used for debugging or analyzing latencies and
-performance issues that take place outside of user-space.
-
-Although ftrace is typically considered the function tracer, it
-is really a frame work of several assorted tracing utilities.
-There's latency tracing to examine what occurs between interrupts
-disabled and enabled, as well as for preemption and from a time
-a task is woken to the task is actually scheduled in.
-
-One of the most common uses of ftrace is the event tracing.
-Through out the kernel is hundreds of static event points that
-can be enabled via the tracefs file system to see what is
-going on in certain parts of the kernel.
-
-See events.txt for more information.
-
-
-Implementation Details
-----------------------
-
-See ftrace-design.txt for details for arch porters and such.
-
-
-The File System
----------------
-
-Ftrace uses the tracefs file system to hold the control files as
-well as the files to display output.
-
-When tracefs is configured into the kernel (which selecting any ftrace
-option will do) the directory /sys/kernel/tracing will be created. To mount
-this directory, you can add to your /etc/fstab file:
-
- tracefs /sys/kernel/tracing tracefs defaults 0 0
-
-Or you can mount it at run time with:
-
- mount -t tracefs nodev /sys/kernel/tracing
-
-For quicker access to that directory you may want to make a soft link to
-it:
-
- ln -s /sys/kernel/tracing /tracing
-
- *** NOTICE ***
-
-Before 4.1, all ftrace tracing control files were within the debugfs
-file system, which is typically located at /sys/kernel/debug/tracing.
-For backward compatibility, when mounting the debugfs file system,
-the tracefs file system will be automatically mounted at:
-
- /sys/kernel/debug/tracing
-
-All files located in the tracefs file system will be located in that
-debugfs file system directory as well.
-
- *** NOTICE ***
-
-Any selected ftrace option will also create the tracefs file system.
-The rest of the document will assume that you are in the ftrace directory
-(cd /sys/kernel/tracing) and will only concentrate on the files within that
-directory and not distract from the content with the extended
-"/sys/kernel/tracing" path name.
-
-That's it! (assuming that you have ftrace configured into your kernel)
-
-After mounting tracefs you will have access to the control and output files
-of ftrace. Here is a list of some of the key files:
-
-
- Note: all time values are in microseconds.
-
- current_tracer:
-
- This is used to set or display the current tracer
- that is configured.
-
- available_tracers:
-
- This holds the different types of tracers that
- have been compiled into the kernel. The
- tracers listed here can be configured by
- echoing their name into current_tracer.
-
- tracing_on:
-
- This sets or displays whether writing to the trace
- ring buffer is enabled. Echo 0 into this file to disable
- the tracer or 1 to enable it. Note, this only disables
- writing to the ring buffer, the tracing overhead may
- still be occurring.
-
- The kernel function tracing_off() can be used within the
- kernel to disable writing to the ring buffer, which will
- set this file to "0". User space can re-enable tracing by
- echoing "1" into the file.
-
- Note, the function and event trigger "traceoff" will also
- set this file to zero and stop tracing. Which can also
- be re-enabled by user space using this file.
-
- trace:
-
- This file holds the output of the trace in a human
- readable format (described below). Note, tracing is temporarily
- disabled while this file is being read (opened).
-
- trace_pipe:
-
- The output is the same as the "trace" file but this
- file is meant to be streamed with live tracing.
- Reads from this file will block until new data is
- retrieved. Unlike the "trace" file, this file is a
- consumer. This means reading from this file causes
- sequential reads to display more current data. Once
- data is read from this file, it is consumed, and
- will not be read again with a sequential read. The
- "trace" file is static, and if the tracer is not
- adding more data, it will display the same
- information every time it is read. This file will not
- disable tracing while being read.
-
- trace_options:
-
- This file lets the user control the amount of data
- that is displayed in one of the above output
- files. Options also exist to modify how a tracer
- or events work (stack traces, timestamps, etc).
-
- options:
-
- This is a directory that has a file for every available
- trace option (also in trace_options). Options may also be set
- or cleared by writing a "1" or "0" respectively into the
- corresponding file with the option name.
-
- tracing_max_latency:
-
- Some of the tracers record the max latency.
- For example, the maximum time that interrupts are disabled.
- The maximum time is saved in this file. The max trace will also be
- stored, and displayed by "trace". A new max trace will only be
- recorded if the latency is greater than the value in this file
- (in microseconds).
-
- By echoing in a time into this file, no latency will be recorded
- unless it is greater than the time in this file.
-
- tracing_thresh:
-
- Some latency tracers will record a trace whenever the
- latency is greater than the number in this file.
- Only active when the file contains a number greater than 0.
- (in microseconds)
-
- buffer_size_kb:
-
- This sets or displays the number of kilobytes each CPU
- buffer holds. By default, the trace buffers are the same size
- for each CPU. The displayed number is the size of the
- CPU buffer and not total size of all buffers. The
- trace buffers are allocated in pages (blocks of memory
- that the kernel uses for allocation, usually 4 KB in size).
- If the last page allocated has room for more bytes
- than requested, the rest of the page will be used,
- making the actual allocation bigger than requested or shown.
- ( Note, the size may not be a multiple of the page size
- due to buffer management meta-data. )
-
- Buffer sizes for individual CPUs may vary
- (see "per_cpu/cpu0/buffer_size_kb" below), and if they do
- this file will show "X".
-
- buffer_total_size_kb:
-
- This displays the total combined size of all the trace buffers.
-
- free_buffer:
-
- If a process is performing tracing, and the ring buffer should be
- shrunk "freed" when the process is finished, even if it were to be
- killed by a signal, this file can be used for that purpose. On close
- of this file, the ring buffer will be resized to its minimum size.
- Having a process that is tracing also open this file, when the process
- exits its file descriptor for this file will be closed, and in doing so,
- the ring buffer will be "freed".
-
- It may also stop tracing if disable_on_free option is set.
-
- tracing_cpumask:
-
- This is a mask that lets the user only trace on specified CPUs.
- The format is a hex string representing the CPUs.
-
- set_ftrace_filter:
-
- When dynamic ftrace is configured in (see the
- section below "dynamic ftrace"), the code is dynamically
- modified (code text rewrite) to disable calling of the
- function profiler (mcount). This lets tracing be configured
- in with practically no overhead in performance. This also
- has a side effect of enabling or disabling specific functions
- to be traced. Echoing names of functions into this file
- will limit the trace to only those functions.
-
- The functions listed in "available_filter_functions" are what
- can be written into this file.
-
- This interface also allows for commands to be used. See the
- "Filter commands" section for more details.
-
- set_ftrace_notrace:
-
- This has an effect opposite to that of
- set_ftrace_filter. Any function that is added here will not
- be traced. If a function exists in both set_ftrace_filter
- and set_ftrace_notrace, the function will _not_ be traced.
-
- set_ftrace_pid:
-
- Have the function tracer only trace the threads whose PID are
- listed in this file.
-
- If the "function-fork" option is set, then when a task whose
- PID is listed in this file forks, the child's PID will
- automatically be added to this file, and the child will be
- traced by the function tracer as well. This option will also
- cause PIDs of tasks that exit to be removed from the file.
-
- set_event_pid:
-
- Have the events only trace a task with a PID listed in this file.
- Note, sched_switch and sched_wake_up will also trace events
- listed in this file.
-
- To have the PIDs of children of tasks with their PID in this file
- added on fork, enable the "event-fork" option. That option will also
- cause the PIDs of tasks to be removed from this file when the task
- exits.
-
- set_graph_function:
-
- Functions listed in this file will cause the function graph
- tracer to only trace these functions and the functions that
- they call. (See the section "dynamic ftrace" for more details).
-
- set_graph_notrace:
-
- Similar to set_graph_function, but will disable function graph
- tracing when the function is hit until it exits the function.
- This makes it possible to ignore tracing functions that are called
- by a specific function.
-
- available_filter_functions:
-
- This lists the functions that ftrace has processed and can trace.
- These are the function names that you can pass to
- "set_ftrace_filter" or "set_ftrace_notrace".
- (See the section "dynamic ftrace" below for more details.)
-
- dyn_ftrace_total_info:
-
- This file is for debugging purposes. The number of functions that
- have been converted to nops and are available to be traced.
-
- enabled_functions:
-
- This file is more for debugging ftrace, but can also be useful
- in seeing if any function has a callback attached to it.
- Not only does the trace infrastructure use ftrace function
- trace utility, but other subsystems might too. This file
- displays all functions that have a callback attached to them
- as well as the number of callbacks that have been attached.
- Note, a callback may also call multiple functions which will
- not be listed in this count.
-
- If the callback registered to be traced by a function with
- the "save regs" attribute (thus even more overhead), a 'R'
- will be displayed on the same line as the function that
- is returning registers.
-
- If the callback registered to be traced by a function with
- the "ip modify" attribute (thus the regs->ip can be changed),
- an 'I' will be displayed on the same line as the function that
- can be overridden.
-
- If the architecture supports it, it will also show what callback
- is being directly called by the function. If the count is greater
- than 1 it most likely will be ftrace_ops_list_func().
-
- If the callback of the function jumps to a trampoline that is
- specific to a the callback and not the standard trampoline,
- its address will be printed as well as the function that the
- trampoline calls.
-
- function_profile_enabled:
-
- When set it will enable all functions with either the function
- tracer, or if configured, the function graph tracer. It will
- keep a histogram of the number of functions that were called
- and if the function graph tracer was configured, it will also keep
- track of the time spent in those functions. The histogram
- content can be displayed in the files:
-
- trace_stats/function<cpu> ( function0, function1, etc).
-
- trace_stats:
-
- A directory that holds different tracing stats.
-
- kprobe_events:
-
- Enable dynamic trace points. See kprobetrace.txt.
-
- kprobe_profile:
-
- Dynamic trace points stats. See kprobetrace.txt.
-
- max_graph_depth:
-
- Used with the function graph tracer. This is the max depth
- it will trace into a function. Setting this to a value of
- one will show only the first kernel function that is called
- from user space.
-
- printk_formats:
-
- This is for tools that read the raw format files. If an event in
- the ring buffer references a string, only a pointer to the string
- is recorded into the buffer and not the string itself. This prevents
- tools from knowing what that string was. This file displays the string
- and address for the string allowing tools to map the pointers to what
- the strings were.
-
- saved_cmdlines:
-
- Only the pid of the task is recorded in a trace event unless
- the event specifically saves the task comm as well. Ftrace
- makes a cache of pid mappings to comms to try to display
- comms for events. If a pid for a comm is not listed, then
- "<...>" is displayed in the output.
-
- If the option "record-cmd" is set to "0", then comms of tasks
- will not be saved during recording. By default, it is enabled.
-
- saved_cmdlines_size:
-
- By default, 128 comms are saved (see "saved_cmdlines" above). To
- increase or decrease the amount of comms that are cached, echo
- in a the number of comms to cache, into this file.
-
- saved_tgids:
-
- If the option "record-tgid" is set, on each scheduling context switch
- the Task Group ID of a task is saved in a table mapping the PID of
- the thread to its TGID. By default, the "record-tgid" option is
- disabled.
-
- snapshot:
-
- This displays the "snapshot" buffer and also lets the user
- take a snapshot of the current running trace.
- See the "Snapshot" section below for more details.
-
- stack_max_size:
-
- When the stack tracer is activated, this will display the
- maximum stack size it has encountered.
- See the "Stack Trace" section below.
-
- stack_trace:
-
- This displays the stack back trace of the largest stack
- that was encountered when the stack tracer is activated.
- See the "Stack Trace" section below.
-
- stack_trace_filter:
-
- This is similar to "set_ftrace_filter" but it limits what
- functions the stack tracer will check.
-
- trace_clock:
-
- Whenever an event is recorded into the ring buffer, a
- "timestamp" is added. This stamp comes from a specified
- clock. By default, ftrace uses the "local" clock. This
- clock is very fast and strictly per cpu, but on some
- systems it may not be monotonic with respect to other
- CPUs. In other words, the local clocks may not be in sync
- with local clocks on other CPUs.
-
- Usual clocks for tracing:
-
- # cat trace_clock
- [local] global counter x86-tsc
-
- The clock with the square brackets around it is the one
- in effect.
-
- local: Default clock, but may not be in sync across CPUs
-
- global: This clock is in sync with all CPUs but may
- be a bit slower than the local clock.
-
- counter: This is not a clock at all, but literally an atomic
- counter. It counts up one by one, but is in sync
- with all CPUs. This is useful when you need to
- know exactly the order events occurred with respect to
- each other on different CPUs.
-
- uptime: This uses the jiffies counter and the time stamp
- is relative to the time since boot up.
-
- perf: This makes ftrace use the same clock that perf uses.
- Eventually perf will be able to read ftrace buffers
- and this will help out in interleaving the data.
-
- x86-tsc: Architectures may define their own clocks. For
- example, x86 uses its own TSC cycle clock here.
-
- ppc-tb: This uses the powerpc timebase register value.
- This is in sync across CPUs and can also be used
- to correlate events across hypervisor/guest if
- tb_offset is known.
-
- mono: This uses the fast monotonic clock (CLOCK_MONOTONIC)
- which is monotonic and is subject to NTP rate adjustments.
-
- mono_raw:
- This is the raw monotonic clock (CLOCK_MONOTONIC_RAW)
- which is montonic but is not subject to any rate adjustments
- and ticks at the same rate as the hardware clocksource.
-
- boot: This is the boot clock (CLOCK_BOOTTIME) and is based on the
- fast monotonic clock, but also accounts for time spent in
- suspend. Since the clock access is designed for use in
- tracing in the suspend path, some side effects are possible
- if clock is accessed after the suspend time is accounted before
- the fast mono clock is updated. In this case, the clock update
- appears to happen slightly sooner than it normally would have.
- Also on 32-bit systems, it's possible that the 64-bit boot offset
- sees a partial update. These effects are rare and post
- processing should be able to handle them. See comments in the
- ktime_get_boot_fast_ns() function for more information.
-
- To set a clock, simply echo the clock name into this file.
-
- echo global > trace_clock
-
- trace_marker:
-
- This is a very useful file for synchronizing user space
- with events happening in the kernel. Writing strings into
- this file will be written into the ftrace buffer.
-
- It is useful in applications to open this file at the start
- of the application and just reference the file descriptor
- for the file.
-
- void trace_write(const char *fmt, ...)
- {
- va_list ap;
- char buf[256];
- int n;
-
- if (trace_fd < 0)
- return;
-
- va_start(ap, fmt);
- n = vsnprintf(buf, 256, fmt, ap);
- va_end(ap);
-
- write(trace_fd, buf, n);
- }
-
- start:
-
- trace_fd = open("trace_marker", WR_ONLY);
-
- trace_marker_raw:
-
- This is similar to trace_marker above, but is meant for for binary data
- to be written to it, where a tool can be used to parse the data
- from trace_pipe_raw.
-
- uprobe_events:
-
- Add dynamic tracepoints in programs.
- See uprobetracer.txt
-
- uprobe_profile:
-
- Uprobe statistics. See uprobetrace.txt
-
- instances:
-
- This is a way to make multiple trace buffers where different
- events can be recorded in different buffers.
- See "Instances" section below.
-
- events:
-
- This is the trace event directory. It holds event tracepoints
- (also known as static tracepoints) that have been compiled
- into the kernel. It shows what event tracepoints exist
- and how they are grouped by system. There are "enable"
- files at various levels that can enable the tracepoints
- when a "1" is written to them.
-
- See events.txt for more information.
-
- set_event:
-
- By echoing in the event into this file, will enable that event.
-
- See events.txt for more information.
-
- available_events:
-
- A list of events that can be enabled in tracing.
-
- See events.txt for more information.
-
- hwlat_detector:
-
- Directory for the Hardware Latency Detector.
- See "Hardware Latency Detector" section below.
-
- per_cpu:
-
- This is a directory that contains the trace per_cpu information.
-
- per_cpu/cpu0/buffer_size_kb:
-
- The ftrace buffer is defined per_cpu. That is, there's a separate
- buffer for each CPU to allow writes to be done atomically,
- and free from cache bouncing. These buffers may have different
- size buffers. This file is similar to the buffer_size_kb
- file, but it only displays or sets the buffer size for the
- specific CPU. (here cpu0).
-
- per_cpu/cpu0/trace:
-
- This is similar to the "trace" file, but it will only display
- the data specific for the CPU. If written to, it only clears
- the specific CPU buffer.
-
- per_cpu/cpu0/trace_pipe
-
- This is similar to the "trace_pipe" file, and is a consuming
- read, but it will only display (and consume) the data specific
- for the CPU.
-
- per_cpu/cpu0/trace_pipe_raw
-
- For tools that can parse the ftrace ring buffer binary format,
- the trace_pipe_raw file can be used to extract the data
- from the ring buffer directly. With the use of the splice()
- system call, the buffer data can be quickly transferred to
- a file or to the network where a server is collecting the
- data.
-
- Like trace_pipe, this is a consuming reader, where multiple
- reads will always produce different data.
-
- per_cpu/cpu0/snapshot:
-
- This is similar to the main "snapshot" file, but will only
- snapshot the current CPU (if supported). It only displays
- the content of the snapshot for a given CPU, and if
- written to, only clears this CPU buffer.
-
- per_cpu/cpu0/snapshot_raw:
-
- Similar to the trace_pipe_raw, but will read the binary format
- from the snapshot buffer for the given CPU.
-
- per_cpu/cpu0/stats:
-
- This displays certain stats about the ring buffer:
-
- entries: The number of events that are still in the buffer.
-
- overrun: The number of lost events due to overwriting when
- the buffer was full.
-
- commit overrun: Should always be zero.
- This gets set if so many events happened within a nested
- event (ring buffer is re-entrant), that it fills the
- buffer and starts dropping events.
-
- bytes: Bytes actually read (not overwritten).
-
- oldest event ts: The oldest timestamp in the buffer
-
- now ts: The current timestamp
-
- dropped events: Events lost due to overwrite option being off.
-
- read events: The number of events read.
-
-The Tracers
------------
-
-Here is the list of current tracers that may be configured.
-
- "function"
-
- Function call tracer to trace all kernel functions.
-
- "function_graph"
-
- Similar to the function tracer except that the
- function tracer probes the functions on their entry
- whereas the function graph tracer traces on both entry
- and exit of the functions. It then provides the ability
- to draw a graph of function calls similar to C code
- source.
-
- "blk"
-
- The block tracer. The tracer used by the blktrace user
- application.
-
- "hwlat"
-
- The Hardware Latency tracer is used to detect if the hardware
- produces any latency. See "Hardware Latency Detector" section
- below.
-
- "irqsoff"
-
- Traces the areas that disable interrupts and saves
- the trace with the longest max latency.
- See tracing_max_latency. When a new max is recorded,
- it replaces the old trace. It is best to view this
- trace with the latency-format option enabled, which
- happens automatically when the tracer is selected.
-
- "preemptoff"
-
- Similar to irqsoff but traces and records the amount of
- time for which preemption is disabled.
-
- "preemptirqsoff"
-
- Similar to irqsoff and preemptoff, but traces and
- records the largest time for which irqs and/or preemption
- is disabled.
-
- "wakeup"
-
- Traces and records the max latency that it takes for
- the highest priority task to get scheduled after
- it has been woken up.
- Traces all tasks as an average developer would expect.
-
- "wakeup_rt"
-
- Traces and records the max latency that it takes for just
- RT tasks (as the current "wakeup" does). This is useful
- for those interested in wake up timings of RT tasks.
-
- "wakeup_dl"
-
- Traces and records the max latency that it takes for
- a SCHED_DEADLINE task to be woken (as the "wakeup" and
- "wakeup_rt" does).
-
- "mmiotrace"
-
- A special tracer that is used to trace binary module.
- It will trace all the calls that a module makes to the
- hardware. Everything it writes and reads from the I/O
- as well.
-
- "branch"
-
- This tracer can be configured when tracing likely/unlikely
- calls within the kernel. It will trace when a likely and
- unlikely branch is hit and if it was correct in its prediction
- of being correct.
-
- "nop"
-
- This is the "trace nothing" tracer. To remove all
- tracers from tracing simply echo "nop" into
- current_tracer.
-
-
-Examples of using the tracer
-----------------------------
-
-Here are typical examples of using the tracers when controlling
-them only with the tracefs interface (without using any
-user-land utilities).
-
-Output format:
---------------
-
-Here is an example of the output format of the file "trace"
-
- --------
-# tracer: function
-#
-# entries-in-buffer/entries-written: 140080/250280 #P:4
-#
-# _-----=> irqs-off
-# / _----=> need-resched
-# | / _---=> hardirq/softirq
-# || / _--=> preempt-depth
-# ||| / delay
-# TASK-PID CPU# |||| TIMESTAMP FUNCTION
-# | | | |||| | |
- bash-1977 [000] .... 17284.993652: sys_close <-system_call_fastpath
- bash-1977 [000] .... 17284.993653: __close_fd <-sys_close
- bash-1977 [000] .... 17284.993653: _raw_spin_lock <-__close_fd
- sshd-1974 [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
- bash-1977 [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
- bash-1977 [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
- bash-1977 [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
- bash-1977 [000] .... 17284.993657: filp_close <-__close_fd
- bash-1977 [000] .... 17284.993657: dnotify_flush <-filp_close
- sshd-1974 [003] .... 17284.993658: sys_select <-system_call_fastpath
- --------
-
-A header is printed with the tracer name that is represented by
-the trace. In this case the tracer is "function". Then it shows the
-number of events in the buffer as well as the total number of entries
-that were written. The difference is the number of entries that were
-lost due to the buffer filling up (250280 - 140080 = 110200 events
-lost).
-
-The header explains the content of the events. Task name "bash", the task
-PID "1977", the CPU that it was running on "000", the latency format
-(explained below), the timestamp in <secs>.<usecs> format, the
-function name that was traced "sys_close" and the parent function that
-called this function "system_call_fastpath". The timestamp is the time
-at which the function was entered.
-
-Latency trace format
---------------------
-
-When the latency-format option is enabled or when one of the latency
-tracers is set, the trace file gives somewhat more information to see
-why a latency happened. Here is a typical trace.
-
-# tracer: irqsoff
-#
-# irqsoff latency trace v1.1.5 on 3.8.0-test+
-# --------------------------------------------------------------------
-# latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
-# -----------------
-# | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
-# -----------------
-# => started at: __lock_task_sighand
-# => ended at: _raw_spin_unlock_irqrestore
-#
-#
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| / delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- ps-6143 2d... 0us!: trace_hardirqs_off <-__lock_task_sighand
- ps-6143 2d..1 259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
- ps-6143 2d..1 263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
- ps-6143 2d..1 306us : <stack trace>
- => trace_hardirqs_on_caller
- => trace_hardirqs_on
- => _raw_spin_unlock_irqrestore
- => do_task_stat
- => proc_tgid_stat
- => proc_single_show
- => seq_read
- => vfs_read
- => sys_read
- => system_call_fastpath
-
-
-This shows that the current tracer is "irqsoff" tracing the time
-for which interrupts were disabled. It gives the trace version (which
-never changes) and the version of the kernel upon which this was executed on
-(3.8). Then it displays the max latency in microseconds (259 us). The number
-of trace entries displayed and the total number (both are four: #4/4).
-VP, KP, SP, and HP are always zero and are reserved for later use.
-#P is the number of online CPUs (#P:4).
-
-The task is the process that was running when the latency
-occurred. (ps pid: 6143).
-
-The start and stop (the functions in which the interrupts were
-disabled and enabled respectively) that caused the latencies:
-
- __lock_task_sighand is where the interrupts were disabled.
- _raw_spin_unlock_irqrestore is where they were enabled again.
-
-The next lines after the header are the trace itself. The header
-explains which is which.
-
- cmd: The name of the process in the trace.
-
- pid: The PID of that process.
-
- CPU#: The CPU which the process was running on.
-
- irqs-off: 'd' interrupts are disabled. '.' otherwise.
- Note: If the architecture does not support a way to
- read the irq flags variable, an 'X' will always
- be printed here.
-
- need-resched:
- 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
- 'n' only TIF_NEED_RESCHED is set,
- 'p' only PREEMPT_NEED_RESCHED is set,
- '.' otherwise.
-
- hardirq/softirq:
- 'Z' - NMI occurred inside a hardirq
- 'z' - NMI is running
- 'H' - hard irq occurred inside a softirq.
- 'h' - hard irq is running
- 's' - soft irq is running
- '.' - normal context.
-
- preempt-depth: The level of preempt_disabled
-
-The above is mostly meaningful for kernel developers.
-
- time: When the latency-format option is enabled, the trace file
- output includes a timestamp relative to the start of the
- trace. This differs from the output when latency-format
- is disabled, which includes an absolute timestamp.
-
- delay: This is just to help catch your eye a bit better. And
- needs to be fixed to be only relative to the same CPU.
- The marks are determined by the difference between this
- current trace and the next trace.
- '$' - greater than 1 second
- '@' - greater than 100 milisecond
- '*' - greater than 10 milisecond
- '#' - greater than 1000 microsecond
- '!' - greater than 100 microsecond
- '+' - greater than 10 microsecond
- ' ' - less than or equal to 10 microsecond.
-
- The rest is the same as the 'trace' file.
-
- Note, the latency tracers will usually end with a back trace
- to easily find where the latency occurred.
-
-trace_options
--------------
-
-The trace_options file (or the options directory) is used to control
-what gets printed in the trace output, or manipulate the tracers.
-To see what is available, simply cat the file:
-
- cat trace_options
-print-parent
-nosym-offset
-nosym-addr
-noverbose
-noraw
-nohex
-nobin
-noblock
-trace_printk
-annotate
-nouserstacktrace
-nosym-userobj
-noprintk-msg-only
-context-info
-nolatency-format
-record-cmd
-norecord-tgid
-overwrite
-nodisable_on_free
-irq-info
-markers
-noevent-fork
-function-trace
-nofunction-fork
-nodisplay-graph
-nostacktrace
-nobranch
-
-To disable one of the options, echo in the option prepended with
-"no".
-
- echo noprint-parent > trace_options
-
-To enable an option, leave off the "no".
-
- echo sym-offset > trace_options
-
-Here are the available options:
-
- print-parent - On function traces, display the calling (parent)
- function as well as the function being traced.
-
- print-parent:
- bash-4000 [01] 1477.606694: simple_strtoul <-kstrtoul
-
- noprint-parent:
- bash-4000 [01] 1477.606694: simple_strtoul
-
-
- sym-offset - Display not only the function name, but also the
- offset in the function. For example, instead of
- seeing just "ktime_get", you will see
- "ktime_get+0xb/0x20".
-
- sym-offset:
- bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0
-
- sym-addr - this will also display the function address as well
- as the function name.
-
- sym-addr:
- bash-4000 [01] 1477.606694: simple_strtoul <c0339346>
-
- verbose - This deals with the trace file when the
- latency-format option is enabled.
-
- bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
- (+0.000ms): simple_strtoul (kstrtoul)
-
- raw - This will display raw numbers. This option is best for
- use with user applications that can translate the raw
- numbers better than having it done in the kernel.
-
- hex - Similar to raw, but the numbers will be in a hexadecimal
- format.
-
- bin - This will print out the formats in raw binary.
-
- block - When set, reading trace_pipe will not block when polled.
-
- trace_printk - Can disable trace_printk() from writing into the buffer.
-
- annotate - It is sometimes confusing when the CPU buffers are full
- and one CPU buffer had a lot of events recently, thus
- a shorter time frame, were another CPU may have only had
- a few events, which lets it have older events. When
- the trace is reported, it shows the oldest events first,
- and it may look like only one CPU ran (the one with the
- oldest events). When the annotate option is set, it will
- display when a new CPU buffer started:
-
- <idle>-0 [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
- <idle>-0 [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
- <idle>-0 [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
-##### CPU 2 buffer started ####
- <idle>-0 [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
- <idle>-0 [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
- <idle>-0 [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
-
- userstacktrace - This option changes the trace. It records a
- stacktrace of the current user space thread after
- each trace event.
-
- sym-userobj - when user stacktrace are enabled, look up which
- object the address belongs to, and print a
- relative address. This is especially useful when
- ASLR is on, otherwise you don't get a chance to
- resolve the address to object/file/line after
- the app is no longer running
-
- The lookup is performed when you read
- trace,trace_pipe. Example:
-
- a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
-x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
-
-
- printk-msg-only - When set, trace_printk()s will only show the format
- and not their parameters (if trace_bprintk() or
- trace_bputs() was used to save the trace_printk()).
-
- context-info - Show only the event data. Hides the comm, PID,
- timestamp, CPU, and other useful data.
-
- latency-format - This option changes the trace output. When it is enabled,
- the trace displays additional information about the
- latency, as described in "Latency trace format".
-
- record-cmd - When any event or tracer is enabled, a hook is enabled
- in the sched_switch trace point to fill comm cache
- with mapped pids and comms. But this may cause some
- overhead, and if you only care about pids, and not the
- name of the task, disabling this option can lower the
- impact of tracing. See "saved_cmdlines".
-
- record-tgid - When any event or tracer is enabled, a hook is enabled
- in the sched_switch trace point to fill the cache of
- mapped Thread Group IDs (TGID) mapping to pids. See
- "saved_tgids".
-
- overwrite - This controls what happens when the trace buffer is
- full. If "1" (default), the oldest events are
- discarded and overwritten. If "0", then the newest
- events are discarded.
- (see per_cpu/cpu0/stats for overrun and dropped)
-
- disable_on_free - When the free_buffer is closed, tracing will
- stop (tracing_on set to 0).
-
- irq-info - Shows the interrupt, preempt count, need resched data.
- When disabled, the trace looks like:
-
-# tracer: function
-#
-# entries-in-buffer/entries-written: 144405/9452052 #P:4
-#
-# TASK-PID CPU# TIMESTAMP FUNCTION
-# | | | | |
- <idle>-0 [002] 23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
- <idle>-0 [002] 23636.756054: activate_task <-ttwu_do_activate.constprop.89
- <idle>-0 [002] 23636.756055: enqueue_task <-activate_task
-
-
- markers - When set, the trace_marker is writable (only by root).
- When disabled, the trace_marker will error with EINVAL
- on write.
-
- event-fork - When set, tasks with PIDs listed in set_event_pid will have
- the PIDs of their children added to set_event_pid when those
- tasks fork. Also, when tasks with PIDs in set_event_pid exit,
- their PIDs will be removed from the file.
-
- function-trace - The latency tracers will enable function tracing
- if this option is enabled (default it is). When
- it is disabled, the latency tracers do not trace
- functions. This keeps the overhead of the tracer down
- when performing latency tests.
-
- function-fork - When set, tasks with PIDs listed in set_ftrace_pid will
- have the PIDs of their children added to set_ftrace_pid
- when those tasks fork. Also, when tasks with PIDs in
- set_ftrace_pid exit, their PIDs will be removed from the
- file.
-
- display-graph - When set, the latency tracers (irqsoff, wakeup, etc) will
- use function graph tracing instead of function tracing.
-
- stacktrace - When set, a stack trace is recorded after any trace event
- is recorded.
-
- branch - Enable branch tracing with the tracer. This enables branch
- tracer along with the currently set tracer. Enabling this
- with the "nop" tracer is the same as just enabling the
- "branch" tracer.
-
- Note: Some tracers have their own options. They only appear in this
- file when the tracer is active. They always appear in the
- options directory.
-
-
-Here are the per tracer options:
-
-Options for function tracer:
-
- func_stack_trace - When set, a stack trace is recorded after every
- function that is recorded. NOTE! Limit the functions
- that are recorded before enabling this, with
- "set_ftrace_filter" otherwise the system performance
- will be critically degraded. Remember to disable
- this option before clearing the function filter.
-
-Options for function_graph tracer:
-
- Since the function_graph tracer has a slightly different output
- it has its own options to control what is displayed.
-
- funcgraph-overrun - When set, the "overrun" of the graph stack is
- displayed after each function traced. The
- overrun, is when the stack depth of the calls
- is greater than what is reserved for each task.
- Each task has a fixed array of functions to
- trace in the call graph. If the depth of the
- calls exceeds that, the function is not traced.
- The overrun is the number of functions missed
- due to exceeding this array.
-
- funcgraph-cpu - When set, the CPU number of the CPU where the trace
- occurred is displayed.
-
- funcgraph-overhead - When set, if the function takes longer than
- A certain amount, then a delay marker is
- displayed. See "delay" above, under the
- header description.
-
- funcgraph-proc - Unlike other tracers, the process' command line
- is not displayed by default, but instead only
- when a task is traced in and out during a context
- switch. Enabling this options has the command
- of each process displayed at every line.
-
- funcgraph-duration - At the end of each function (the return)
- the duration of the amount of time in the
- function is displayed in microseconds.
-
- funcgraph-abstime - When set, the timestamp is displayed at each
- line.
-
- funcgraph-irqs - When disabled, functions that happen inside an
- interrupt will not be traced.
-
- funcgraph-tail - When set, the return event will include the function
- that it represents. By default this is off, and
- only a closing curly bracket "}" is displayed for
- the return of a function.
-
- sleep-time - When running function graph tracer, to include
- the time a task schedules out in its function.
- When enabled, it will account time the task has been
- scheduled out as part of the function call.
-
- graph-time - When running function profiler with function graph tracer,
- to include the time to call nested functions. When this is
- not set, the time reported for the function will only
- include the time the function itself executed for, not the
- time for functions that it called.
-
-Options for blk tracer:
-
- blk_classic - Shows a more minimalistic output.
-
-
-irqsoff
--------
-
-When interrupts are disabled, the CPU can not react to any other
-external event (besides NMIs and SMIs). This prevents the timer
-interrupt from triggering or the mouse interrupt from letting
-the kernel know of a new mouse event. The result is a latency
-with the reaction time.
-
-The irqsoff tracer tracks the time for which interrupts are
-disabled. When a new maximum latency is hit, the tracer saves
-the trace leading up to that latency point so that every time a
-new maximum is reached, the old saved trace is discarded and the
-new trace is saved.
-
-To reset the maximum, echo 0 into tracing_max_latency. Here is
-an example:
-
- # echo 0 > options/function-trace
- # echo irqsoff > current_tracer
- # echo 1 > tracing_on
- # echo 0 > tracing_max_latency
- # ls -ltr
- [...]
- # echo 0 > tracing_on
- # cat trace
-# tracer: irqsoff
-#
-# irqsoff latency trace v1.1.5 on 3.8.0-test+
-# --------------------------------------------------------------------
-# latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
-# -----------------
-# | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
-# -----------------
-# => started at: run_timer_softirq
-# => ended at: run_timer_softirq
-#
-#
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| / delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- <idle>-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq
- <idle>-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq
- <idle>-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq
- <idle>-0 0dNs3 25us : <stack trace>
- => _raw_spin_unlock_irq
- => run_timer_softirq
- => __do_softirq
- => call_softirq
- => do_softirq
- => irq_exit
- => smp_apic_timer_interrupt
- => apic_timer_interrupt
- => rcu_idle_exit
- => cpu_idle
- => rest_init
- => start_kernel
- => x86_64_start_reservations
- => x86_64_start_kernel
-
-Here we see that that we had a latency of 16 microseconds (which is
-very good). The _raw_spin_lock_irq in run_timer_softirq disabled
-interrupts. The difference between the 16 and the displayed
-timestamp 25us occurred because the clock was incremented
-between the time of recording the max latency and the time of
-recording the function that had that latency.
-
-Note the above example had function-trace not set. If we set
-function-trace, we get a much larger output:
-
- with echo 1 > options/function-trace
-
-# tracer: irqsoff
-#
-# irqsoff latency trace v1.1.5 on 3.8.0-test+
-# --------------------------------------------------------------------
-# latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
-# -----------------
-# | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
-# -----------------
-# => started at: ata_scsi_queuecmd
-# => ended at: ata_scsi_queuecmd
-#
-#
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| / delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- bash-2042 3d... 0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
- bash-2042 3d... 0us : add_preempt_count <-_raw_spin_lock_irqsave
- bash-2042 3d..1 1us : ata_scsi_find_dev <-ata_scsi_queuecmd
- bash-2042 3d..1 1us : __ata_scsi_find_dev <-ata_scsi_find_dev
- bash-2042 3d..1 2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
- bash-2042 3d..1 2us : ata_qc_new_init <-__ata_scsi_queuecmd
- bash-2042 3d..1 3us : ata_sg_init <-__ata_scsi_queuecmd
- bash-2042 3d..1 4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
- bash-2042 3d..1 4us : ata_build_rw_tf <-ata_scsi_rw_xlat
-[...]
- bash-2042 3d..1 67us : delay_tsc <-__delay
- bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
- bash-2042 3d..2 67us : sub_preempt_count <-delay_tsc
- bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
- bash-2042 3d..2 68us : sub_preempt_count <-delay_tsc
- bash-2042 3d..1 68us+: ata_bmdma_start <-ata_bmdma_qc_issue
- bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
- bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
- bash-2042 3d..1 72us+: trace_hardirqs_on <-ata_scsi_queuecmd
- bash-2042 3d..1 120us : <stack trace>
- => _raw_spin_unlock_irqrestore
- => ata_scsi_queuecmd
- => scsi_dispatch_cmd
- => scsi_request_fn
- => __blk_run_queue_uncond
- => __blk_run_queue
- => blk_queue_bio
- => generic_make_request
- => submit_bio
- => submit_bh
- => __ext3_get_inode_loc
- => ext3_iget
- => ext3_lookup
- => lookup_real
- => __lookup_hash
- => walk_component
- => lookup_last
- => path_lookupat
- => filename_lookup
- => user_path_at_empty
- => user_path_at
- => vfs_fstatat
- => vfs_stat
- => sys_newstat
- => system_call_fastpath
-
-
-Here we traced a 71 microsecond latency. But we also see all the
-functions that were called during that time. Note that by
-enabling function tracing, we incur an added overhead. This
-overhead may extend the latency times. But nevertheless, this
-trace has provided some very helpful debugging information.
-
-
-preemptoff
-----------
-
-When preemption is disabled, we may be able to receive
-interrupts but the task cannot be preempted and a higher
-priority task must wait for preemption to be enabled again
-before it can preempt a lower priority task.
-
-The preemptoff tracer traces the places that disable preemption.
-Like the irqsoff tracer, it records the maximum latency for
-which preemption was disabled. The control of preemptoff tracer
-is much like the irqsoff tracer.
-
- # echo 0 > options/function-trace
- # echo preemptoff > current_tracer
- # echo 1 > tracing_on
- # echo 0 > tracing_max_latency
- # ls -ltr
- [...]
- # echo 0 > tracing_on
- # cat trace
-# tracer: preemptoff
-#
-# preemptoff latency trace v1.1.5 on 3.8.0-test+
-# --------------------------------------------------------------------
-# latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
-# -----------------
-# | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
-# -----------------
-# => started at: do_IRQ
-# => ended at: do_IRQ
-#
-#
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| / delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- sshd-1991 1d.h. 0us+: irq_enter <-do_IRQ
- sshd-1991 1d..1 46us : irq_exit <-do_IRQ
- sshd-1991 1d..1 47us+: trace_preempt_on <-do_IRQ
- sshd-1991 1d..1 52us : <stack trace>
- => sub_preempt_count
- => irq_exit
- => do_IRQ
- => ret_from_intr
-
-
-This has some more changes. Preemption was disabled when an
-interrupt came in (notice the 'h'), and was enabled on exit.
-But we also see that interrupts have been disabled when entering
-the preempt off section and leaving it (the 'd'). We do not know if
-interrupts were enabled in the mean time or shortly after this
-was over.
-
-# tracer: preemptoff
-#
-# preemptoff latency trace v1.1.5 on 3.8.0-test+
-# --------------------------------------------------------------------
-# latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
-# -----------------
-# | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
-# -----------------
-# => started at: wake_up_new_task
-# => ended at: task_rq_unlock
-#
-#
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| / delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- bash-1994 1d..1 0us : _raw_spin_lock_irqsave <-wake_up_new_task
- bash-1994 1d..1 0us : select_task_rq_fair <-select_task_rq
- bash-1994 1d..1 1us : __rcu_read_lock <-select_task_rq_fair
- bash-1994 1d..1 1us : source_load <-select_task_rq_fair
- bash-1994 1d..1 1us : source_load <-select_task_rq_fair
-[...]
- bash-1994 1d..1 12us : irq_enter <-smp_apic_timer_interrupt
- bash-1994 1d..1 12us : rcu_irq_enter <-irq_enter
- bash-1994 1d..1 13us : add_preempt_count <-irq_enter
- bash-1994 1d.h1 13us : exit_idle <-smp_apic_timer_interrupt
- bash-1994 1d.h1 13us : hrtimer_interrupt <-smp_apic_timer_interrupt
- bash-1994 1d.h1 13us : _raw_spin_lock <-hrtimer_interrupt
- bash-1994 1d.h1 14us : add_preempt_count <-_raw_spin_lock
- bash-1994 1d.h2 14us : ktime_get_update_offsets <-hrtimer_interrupt
-[...]
- bash-1994 1d.h1 35us : lapic_next_event <-clockevents_program_event
- bash-1994 1d.h1 35us : irq_exit <-smp_apic_timer_interrupt
- bash-1994 1d.h1 36us : sub_preempt_count <-irq_exit
- bash-1994 1d..2 36us : do_softirq <-irq_exit
- bash-1994 1d..2 36us : __do_softirq <-call_softirq
- bash-1994 1d..2 36us : __local_bh_disable <-__do_softirq
- bash-1994 1d.s2 37us : add_preempt_count <-_raw_spin_lock_irq
- bash-1994 1d.s3 38us : _raw_spin_unlock <-run_timer_softirq
- bash-1994 1d.s3 39us : sub_preempt_count <-_raw_spin_unlock
- bash-1994 1d.s2 39us : call_timer_fn <-run_timer_softirq
-[...]
- bash-1994 1dNs2 81us : cpu_needs_another_gp <-rcu_process_callbacks
- bash-1994 1dNs2 82us : __local_bh_enable <-__do_softirq
- bash-1994 1dNs2 82us : sub_preempt_count <-__local_bh_enable
- bash-1994 1dN.2 82us : idle_cpu <-irq_exit
- bash-1994 1dN.2 83us : rcu_irq_exit <-irq_exit
- bash-1994 1dN.2 83us : sub_preempt_count <-irq_exit
- bash-1994 1.N.1 84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
- bash-1994 1.N.1 84us+: trace_preempt_on <-task_rq_unlock
- bash-1994 1.N.1 104us : <stack trace>
- => sub_preempt_count
- => _raw_spin_unlock_irqrestore
- => task_rq_unlock
- => wake_up_new_task
- => do_fork
- => sys_clone
- => stub_clone
-
-
-The above is an example of the preemptoff trace with
-function-trace set. Here we see that interrupts were not disabled
-the entire time. The irq_enter code lets us know that we entered
-an interrupt 'h'. Before that, the functions being traced still
-show that it is not in an interrupt, but we can see from the
-functions themselves that this is not the case.
-
-preemptirqsoff
---------------
-
-Knowing the locations that have interrupts disabled or
-preemption disabled for the longest times is helpful. But
-sometimes we would like to know when either preemption and/or
-interrupts are disabled.
-
-Consider the following code:
-
- local_irq_disable();
- call_function_with_irqs_off();
- preempt_disable();
- call_function_with_irqs_and_preemption_off();
- local_irq_enable();
- call_function_with_preemption_off();
- preempt_enable();
-
-The irqsoff tracer will record the total length of
-call_function_with_irqs_off() and
-call_function_with_irqs_and_preemption_off().
-
-The preemptoff tracer will record the total length of
-call_function_with_irqs_and_preemption_off() and
-call_function_with_preemption_off().
-
-But neither will trace the time that interrupts and/or
-preemption is disabled. This total time is the time that we can
-not schedule. To record this time, use the preemptirqsoff
-tracer.
-
-Again, using this trace is much like the irqsoff and preemptoff
-tracers.
-
- # echo 0 > options/function-trace
- # echo preemptirqsoff > current_tracer
- # echo 1 > tracing_on
- # echo 0 > tracing_max_latency
- # ls -ltr
- [...]
- # echo 0 > tracing_on
- # cat trace
-# tracer: preemptirqsoff
-#
-# preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
-# --------------------------------------------------------------------
-# latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
-# -----------------
-# | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
-# -----------------
-# => started at: ata_scsi_queuecmd
-# => ended at: ata_scsi_queuecmd
-#
-#
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| / delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- ls-2230 3d... 0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
- ls-2230 3...1 100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
- ls-2230 3...1 101us+: trace_preempt_on <-ata_scsi_queuecmd
- ls-2230 3...1 111us : <stack trace>
- => sub_preempt_count
- => _raw_spin_unlock_irqrestore
- => ata_scsi_queuecmd
- => scsi_dispatch_cmd
- => scsi_request_fn
- => __blk_run_queue_uncond
- => __blk_run_queue
- => blk_queue_bio
- => generic_make_request
- => submit_bio
- => submit_bh
- => ext3_bread
- => ext3_dir_bread
- => htree_dirblock_to_tree
- => ext3_htree_fill_tree
- => ext3_readdir
- => vfs_readdir
- => sys_getdents
- => system_call_fastpath
-
-
-The trace_hardirqs_off_thunk is called from assembly on x86 when
-interrupts are disabled in the assembly code. Without the
-function tracing, we do not know if interrupts were enabled
-within the preemption points. We do see that it started with
-preemption enabled.
-
-Here is a trace with function-trace set:
-
-# tracer: preemptirqsoff
-#
-# preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
-# --------------------------------------------------------------------
-# latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
-# -----------------
-# | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
-# -----------------
-# => started at: schedule
-# => ended at: mutex_unlock
-#
-#
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| / delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
-kworker/-59 3...1 0us : __schedule <-schedule
-kworker/-59 3d..1 0us : rcu_preempt_qs <-rcu_note_context_switch
-kworker/-59 3d..1 1us : add_preempt_count <-_raw_spin_lock_irq
-kworker/-59 3d..2 1us : deactivate_task <-__schedule
-kworker/-59 3d..2 1us : dequeue_task <-deactivate_task
-kworker/-59 3d..2 2us : update_rq_clock <-dequeue_task
-kworker/-59 3d..2 2us : dequeue_task_fair <-dequeue_task
-kworker/-59 3d..2 2us : update_curr <-dequeue_task_fair
-kworker/-59 3d..2 2us : update_min_vruntime <-update_curr
-kworker/-59 3d..2 3us : cpuacct_charge <-update_curr
-kworker/-59 3d..2 3us : __rcu_read_lock <-cpuacct_charge
-kworker/-59 3d..2 3us : __rcu_read_unlock <-cpuacct_charge
-kworker/-59 3d..2 3us : update_cfs_rq_blocked_load <-dequeue_task_fair
-kworker/-59 3d..2 4us : clear_buddies <-dequeue_task_fair
-kworker/-59 3d..2 4us : account_entity_dequeue <-dequeue_task_fair
-kworker/-59 3d..2 4us : update_min_vruntime <-dequeue_task_fair
-kworker/-59 3d..2 4us : update_cfs_shares <-dequeue_task_fair
-kworker/-59 3d..2 5us : hrtick_update <-dequeue_task_fair
-kworker/-59 3d..2 5us : wq_worker_sleeping <-__schedule
-kworker/-59 3d..2 5us : kthread_data <-wq_worker_sleeping
-kworker/-59 3d..2 5us : put_prev_task_fair <-__schedule
-kworker/-59 3d..2 6us : pick_next_task_fair <-pick_next_task
-kworker/-59 3d..2 6us : clear_buddies <-pick_next_task_fair
-kworker/-59 3d..2 6us : set_next_entity <-pick_next_task_fair
-kworker/-59 3d..2 6us : update_stats_wait_end <-set_next_entity
- ls-2269 3d..2 7us : finish_task_switch <-__schedule
- ls-2269 3d..2 7us : _raw_spin_unlock_irq <-finish_task_switch
- ls-2269 3d..2 8us : do_IRQ <-ret_from_intr
- ls-2269 3d..2 8us : irq_enter <-do_IRQ
- ls-2269 3d..2 8us : rcu_irq_enter <-irq_enter
- ls-2269 3d..2 9us : add_preempt_count <-irq_enter
- ls-2269 3d.h2 9us : exit_idle <-do_IRQ
-[...]
- ls-2269 3d.h3 20us : sub_preempt_count <-_raw_spin_unlock
- ls-2269 3d.h2 20us : irq_exit <-do_IRQ
- ls-2269 3d.h2 21us : sub_preempt_count <-irq_exit
- ls-2269 3d..3 21us : do_softirq <-irq_exit
- ls-2269 3d..3 21us : __do_softirq <-call_softirq
- ls-2269 3d..3 21us+: __local_bh_disable <-__do_softirq
- ls-2269 3d.s4 29us : sub_preempt_count <-_local_bh_enable_ip
- ls-2269 3d.s5 29us : sub_preempt_count <-_local_bh_enable_ip
- ls-2269 3d.s5 31us : do_IRQ <-ret_from_intr
- ls-2269 3d.s5 31us : irq_enter <-do_IRQ
- ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
-[...]
- ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
- ls-2269 3d.s5 32us : add_preempt_count <-irq_enter
- ls-2269 3d.H5 32us : exit_idle <-do_IRQ
- ls-2269 3d.H5 32us : handle_irq <-do_IRQ
- ls-2269 3d.H5 32us : irq_to_desc <-handle_irq
- ls-2269 3d.H5 33us : handle_fasteoi_irq <-handle_irq
-[...]
- ls-2269 3d.s5 158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
- ls-2269 3d.s3 158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
- ls-2269 3d.s3 159us : __local_bh_enable <-__do_softirq
- ls-2269 3d.s3 159us : sub_preempt_count <-__local_bh_enable
- ls-2269 3d..3 159us : idle_cpu <-irq_exit
- ls-2269 3d..3 159us : rcu_irq_exit <-irq_exit
- ls-2269 3d..3 160us : sub_preempt_count <-irq_exit
- ls-2269 3d... 161us : __mutex_unlock_slowpath <-mutex_unlock
- ls-2269 3d... 162us+: trace_hardirqs_on <-mutex_unlock
- ls-2269 3d... 186us : <stack trace>
- => __mutex_unlock_slowpath
- => mutex_unlock
- => process_output
- => n_tty_write
- => tty_write
- => vfs_write
- => sys_write
- => system_call_fastpath
-
-This is an interesting trace. It started with kworker running and
-scheduling out and ls taking over. But as soon as ls released the
-rq lock and enabled interrupts (but not preemption) an interrupt
-triggered. When the interrupt finished, it started running softirqs.
-But while the softirq was running, another interrupt triggered.
-When an interrupt is running inside a softirq, the annotation is 'H'.
-
-
-wakeup
-------
-
-One common case that people are interested in tracing is the
-time it takes for a task that is woken to actually wake up.
-Now for non Real-Time tasks, this can be arbitrary. But tracing
-it none the less can be interesting.
-
-Without function tracing:
-
- # echo 0 > options/function-trace
- # echo wakeup > current_tracer
- # echo 1 > tracing_on
- # echo 0 > tracing_max_latency
- # chrt -f 5 sleep 1
- # echo 0 > tracing_on
- # cat trace
-# tracer: wakeup
-#
-# wakeup latency trace v1.1.5 on 3.8.0-test+
-# --------------------------------------------------------------------
-# latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
-# -----------------
-# | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
-# -----------------
-#
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| / delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- <idle>-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H
- <idle>-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
- <idle>-0 3d..3 15us : __schedule <-schedule
- <idle>-0 3d..3 15us : 0:120:R ==> [003] 312:100:R kworker/3:1H
-
-The tracer only traces the highest priority task in the system
-to avoid tracing the normal circumstances. Here we see that
-the kworker with a nice priority of -20 (not very nice), took
-just 15 microseconds from the time it woke up, to the time it
-ran.
-
-Non Real-Time tasks are not that interesting. A more interesting
-trace is to concentrate only on Real-Time tasks.
-
-wakeup_rt
----------
-
-In a Real-Time environment it is very important to know the
-wakeup time it takes for the highest priority task that is woken
-up to the time that it executes. This is also known as "schedule
-latency". I stress the point that this is about RT tasks. It is
-also important to know the scheduling latency of non-RT tasks,
-but the average schedule latency is better for non-RT tasks.
-Tools like LatencyTop are more appropriate for such
-measurements.
-
-Real-Time environments are interested in the worst case latency.
-That is the longest latency it takes for something to happen,
-and not the average. We can have a very fast scheduler that may
-only have a large latency once in a while, but that would not
-work well with Real-Time tasks. The wakeup_rt tracer was designed
-to record the worst case wakeups of RT tasks. Non-RT tasks are
-not recorded because the tracer only records one worst case and
-tracing non-RT tasks that are unpredictable will overwrite the
-worst case latency of RT tasks (just run the normal wakeup
-tracer for a while to see that effect).
-
-Since this tracer only deals with RT tasks, we will run this
-slightly differently than we did with the previous tracers.
-Instead of performing an 'ls', we will run 'sleep 1' under
-'chrt' which changes the priority of the task.
-
- # echo 0 > options/function-trace
- # echo wakeup_rt > current_tracer
- # echo 1 > tracing_on
- # echo 0 > tracing_max_latency
- # chrt -f 5 sleep 1
- # echo 0 > tracing_on
- # cat trace
-# tracer: wakeup
-#
-# tracer: wakeup_rt
-#
-# wakeup_rt latency trace v1.1.5 on 3.8.0-test+
-# --------------------------------------------------------------------
-# latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
-# -----------------
-# | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
-# -----------------
-#
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| / delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- <idle>-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep
- <idle>-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
- <idle>-0 3d..3 5us : __schedule <-schedule
- <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep
-
-
-Running this on an idle system, we see that it only took 5 microseconds
-to perform the task switch. Note, since the trace point in the schedule
-is before the actual "switch", we stop the tracing when the recorded task
-is about to schedule in. This may change if we add a new marker at the
-end of the scheduler.
-
-Notice that the recorded task is 'sleep' with the PID of 2389
-and it has an rt_prio of 5. This priority is user-space priority
-and not the internal kernel priority. The policy is 1 for
-SCHED_FIFO and 2 for SCHED_RR.
-
-Note, that the trace data shows the internal priority (99 - rtprio).
-
- <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep
-
-The 0:120:R means idle was running with a nice priority of 0 (120 - 120)
-and in the running state 'R'. The sleep task was scheduled in with
-2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
-and it too is in the running state.
-
-Doing the same with chrt -r 5 and function-trace set.
-
- echo 1 > options/function-trace
-
-# tracer: wakeup_rt
-#
-# wakeup_rt latency trace v1.1.5 on 3.8.0-test+
-# --------------------------------------------------------------------
-# latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
-# -----------------
-# | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
-# -----------------
-#
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| / delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- <idle>-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep
- <idle>-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
- <idle>-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup
- <idle>-0 3d.h3 3us : resched_curr <-check_preempt_curr
- <idle>-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup
- <idle>-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up
- <idle>-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock
- <idle>-0 3dNh2 5us : ttwu_stat <-try_to_wake_up
- <idle>-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
- <idle>-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
- <idle>-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer
- <idle>-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock
- <idle>-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt
- <idle>-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock
- <idle>-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt
- <idle>-0 3dNh1 7us : clockevents_program_event <-tick_program_event
- <idle>-0 3dNh1 8us : ktime_get <-clockevents_program_event
- <idle>-0 3dNh1 8us : lapic_next_event <-clockevents_program_event
- <idle>-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt
- <idle>-0 3dNh1 9us : sub_preempt_count <-irq_exit
- <idle>-0 3dN.2 9us : idle_cpu <-irq_exit
- <idle>-0 3dN.2 9us : rcu_irq_exit <-irq_exit
- <idle>-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
- <idle>-0 3dN.2 10us : sub_preempt_count <-irq_exit
- <idle>-0 3.N.1 11us : rcu_idle_exit <-cpu_idle
- <idle>-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
- <idle>-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle
- <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
- <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit
- <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
- <idle>-0 3dN.1 13us : cpu_load_update_nohz <-tick_nohz_idle_exit
- <idle>-0 3dN.1 13us : _raw_spin_lock <-cpu_load_update_nohz
- <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock
- <idle>-0 3dN.2 13us : __cpu_load_update <-cpu_load_update_nohz
- <idle>-0 3dN.2 14us : sched_avg_update <-__cpu_load_update
- <idle>-0 3dN.2 14us : _raw_spin_unlock <-cpu_load_update_nohz
- <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock
- <idle>-0 3dN.1 15us : calc_load_nohz_stop <-tick_nohz_idle_exit
- <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
- <idle>-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit
- <idle>-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel
- <idle>-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
- <idle>-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
- <idle>-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave
- <idle>-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16
- <idle>-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer
- <idle>-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram
- <idle>-0 3dN.2 18us : clockevents_program_event <-tick_program_event
- <idle>-0 3dN.2 18us : ktime_get <-clockevents_program_event
- <idle>-0 3dN.2 18us : lapic_next_event <-clockevents_program_event
- <idle>-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
- <idle>-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
- <idle>-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit
- <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
- <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
- <idle>-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
- <idle>-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
- <idle>-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
- <idle>-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
- <idle>-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave
- <idle>-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns
- <idle>-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns
- <idle>-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns
- <idle>-0 3dN.2 23us : clockevents_program_event <-tick_program_event
- <idle>-0 3dN.2 23us : ktime_get <-clockevents_program_event
- <idle>-0 3dN.2 23us : lapic_next_event <-clockevents_program_event
- <idle>-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
- <idle>-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
- <idle>-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit
- <idle>-0 3dN.1 24us : account_idle_time <-account_idle_ticks
- <idle>-0 3.N.1 25us : sub_preempt_count <-cpu_idle
- <idle>-0 3.N.. 25us : schedule <-cpu_idle
- <idle>-0 3.N.. 25us : __schedule <-preempt_schedule
- <idle>-0 3.N.. 26us : add_preempt_count <-__schedule
- <idle>-0 3.N.1 26us : rcu_note_context_switch <-__schedule
- <idle>-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch
- <idle>-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch
- <idle>-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule
- <idle>-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq
- <idle>-0 3dN.2 28us : put_prev_task_idle <-__schedule
- <idle>-0 3dN.2 28us : pick_next_task_stop <-pick_next_task
- <idle>-0 3dN.2 28us : pick_next_task_rt <-pick_next_task
- <idle>-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt
- <idle>-0 3d..3 29us : __schedule <-preempt_schedule
- <idle>-0 3d..3 30us : 0:120:R ==> [003] 2448: 94:R sleep
-
-This isn't that big of a trace, even with function tracing enabled,
-so I included the entire trace.
-
-The interrupt went off while when the system was idle. Somewhere
-before task_woken_rt() was called, the NEED_RESCHED flag was set,
-this is indicated by the first occurrence of the 'N' flag.
-
-Latency tracing and events
---------------------------
-As function tracing can induce a much larger latency, but without
-seeing what happens within the latency it is hard to know what
-caused it. There is a middle ground, and that is with enabling
-events.
-
- # echo 0 > options/function-trace
- # echo wakeup_rt > current_tracer
- # echo 1 > events/enable
- # echo 1 > tracing_on
- # echo 0 > tracing_max_latency
- # chrt -f 5 sleep 1
- # echo 0 > tracing_on
- # cat trace
-# tracer: wakeup_rt
-#
-# wakeup_rt latency trace v1.1.5 on 3.8.0-test+
-# --------------------------------------------------------------------
-# latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
-# -----------------
-# | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
-# -----------------
-#
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| / delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- <idle>-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep
- <idle>-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
- <idle>-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
- <idle>-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
- <idle>-0 2.N.2 2us : power_end: cpu_id=2
- <idle>-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2
- <idle>-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
- <idle>-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
- <idle>-0 2.N.2 5us : rcu_utilization: Start context switch
- <idle>-0 2.N.2 5us : rcu_utilization: End context switch
- <idle>-0 2d..3 6us : __schedule <-schedule
- <idle>-0 2d..3 6us : 0:120:R ==> [002] 5882: 94:R sleep
-
-
-Hardware Latency Detector
--------------------------
-
-The hardware latency detector is executed by enabling the "hwlat" tracer.
-
-NOTE, this tracer will affect the performance of the system as it will
-periodically make a CPU constantly busy with interrupts disabled.
-
- # echo hwlat > current_tracer
- # sleep 100
- # cat trace
-# tracer: hwlat
-#
-# _-----=> irqs-off
-# / _----=> need-resched
-# | / _---=> hardirq/softirq
-# || / _--=> preempt-depth
-# ||| / delay
-# TASK-PID CPU# |||| TIMESTAMP FUNCTION
-# | | | |||| | |
- <...>-3638 [001] d... 19452.055471: #1 inner/outer(us): 12/14 ts:1499801089.066141940
- <...>-3638 [003] d... 19454.071354: #2 inner/outer(us): 11/9 ts:1499801091.082164365
- <...>-3638 [002] dn.. 19461.126852: #3 inner/outer(us): 12/9 ts:1499801098.138150062
- <...>-3638 [001] d... 19488.340960: #4 inner/outer(us): 8/12 ts:1499801125.354139633
- <...>-3638 [003] d... 19494.388553: #5 inner/outer(us): 8/12 ts:1499801131.402150961
- <...>-3638 [003] d... 19501.283419: #6 inner/outer(us): 0/12 ts:1499801138.297435289 nmi-total:4 nmi-count:1
-
-
-The above output is somewhat the same in the header. All events will have
-interrupts disabled 'd'. Under the FUNCTION title there is:
-
- #1 - This is the count of events recorded that were greater than the
- tracing_threshold (See below).
-
- inner/outer(us): 12/14
-
- This shows two numbers as "inner latency" and "outer latency". The test
- runs in a loop checking a timestamp twice. The latency detected within
- the two timestamps is the "inner latency" and the latency detected
- after the previous timestamp and the next timestamp in the loop is
- the "outer latency".
-
- ts:1499801089.066141940
-
- The absolute timestamp that the event happened.
-
- nmi-total:4 nmi-count:1
-
- On architectures that support it, if an NMI comes in during the
- test, the time spent in NMI is reported in "nmi-total" (in
- microseconds).
-
- All architectures that have NMIs will show the "nmi-count" if an
- NMI comes in during the test.
-
-hwlat files:
-
- tracing_threshold - This gets automatically set to "10" to represent 10
- microseconds. This is the threshold of latency that
- needs to be detected before the trace will be recorded.
-
- Note, when hwlat tracer is finished (another tracer is
- written into "current_tracer"), the original value for
- tracing_threshold is placed back into this file.
-
- hwlat_detector/width - The length of time the test runs with interrupts
- disabled.
-
- hwlat_detector/window - The length of time of the window which the test
- runs. That is, the test will run for "width"
- microseconds per "window" microseconds
-
- tracing_cpumask - When the test is started. A kernel thread is created that
- runs the test. This thread will alternate between CPUs
- listed in the tracing_cpumask between each period
- (one "window"). To limit the test to specific CPUs
- set the mask in this file to only the CPUs that the test
- should run on.
-
-function
---------
-
-This tracer is the function tracer. Enabling the function tracer
-can be done from the debug file system. Make sure the
-ftrace_enabled is set; otherwise this tracer is a nop.
-See the "ftrace_enabled" section below.
-
- # sysctl kernel.ftrace_enabled=1
- # echo function > current_tracer
- # echo 1 > tracing_on
- # usleep 1
- # echo 0 > tracing_on
- # cat trace
-# tracer: function
-#
-# entries-in-buffer/entries-written: 24799/24799 #P:4
-#
-# _-----=> irqs-off
-# / _----=> need-resched
-# | / _---=> hardirq/softirq
-# || / _--=> preempt-depth
-# ||| / delay
-# TASK-PID CPU# |||| TIMESTAMP FUNCTION
-# | | | |||| | |
- bash-1994 [002] .... 3082.063030: mutex_unlock <-rb_simple_write
- bash-1994 [002] .... 3082.063031: __mutex_unlock_slowpath <-mutex_unlock
- bash-1994 [002] .... 3082.063031: __fsnotify_parent <-fsnotify_modify
- bash-1994 [002] .... 3082.063032: fsnotify <-fsnotify_modify
- bash-1994 [002] .... 3082.063032: __srcu_read_lock <-fsnotify
- bash-1994 [002] .... 3082.063032: add_preempt_count <-__srcu_read_lock
- bash-1994 [002] ...1 3082.063032: sub_preempt_count <-__srcu_read_lock
- bash-1994 [002] .... 3082.063033: __srcu_read_unlock <-fsnotify
-[...]
-
-
-Note: function tracer uses ring buffers to store the above
-entries. The newest data may overwrite the oldest data.
-Sometimes using echo to stop the trace is not sufficient because
-the tracing could have overwritten the data that you wanted to
-record. For this reason, it is sometimes better to disable
-tracing directly from a program. This allows you to stop the
-tracing at the point that you hit the part that you are
-interested in. To disable the tracing directly from a C program,
-something like following code snippet can be used:
-
-int trace_fd;
-[...]
-int main(int argc, char *argv[]) {
- [...]
- trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
- [...]
- if (condition_hit()) {
- write(trace_fd, "0", 1);
- }
- [...]
-}
-
-
-Single thread tracing
----------------------
-
-By writing into set_ftrace_pid you can trace a
-single thread. For example:
-
-# cat set_ftrace_pid
-no pid
-# echo 3111 > set_ftrace_pid
-# cat set_ftrace_pid
-3111
-# echo function > current_tracer
-# cat trace | head
- # tracer: function
- #
- # TASK-PID CPU# TIMESTAMP FUNCTION
- # | | | | |
- yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return
- yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
- yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
- yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
- yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll
- yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll
-# echo > set_ftrace_pid
-# cat trace |head
- # tracer: function
- #
- # TASK-PID CPU# TIMESTAMP FUNCTION
- # | | | | |
- ##### CPU 3 buffer started ####
- yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait
- yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry
- yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry
- yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit
- yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit
-
-If you want to trace a function when executing, you could use
-something like this simple program:
-
-#include <stdio.h>
-#include <stdlib.h>
-#include <sys/types.h>
-#include <sys/stat.h>
-#include <fcntl.h>
-#include <unistd.h>
-#include <string.h>
-
-#define _STR(x) #x
-#define STR(x) _STR(x)
-#define MAX_PATH 256
-
-const char *find_tracefs(void)
-{
- static char tracefs[MAX_PATH+1];
- static int tracefs_found;
- char type[100];
- FILE *fp;
-
- if (tracefs_found)
- return tracefs;
-
- if ((fp = fopen("/proc/mounts","r")) == NULL) {
- perror("/proc/mounts");
- return NULL;
- }
-
- while (fscanf(fp, "%*s %"
- STR(MAX_PATH)
- "s %99s %*s %*d %*d\n",
- tracefs, type) == 2) {
- if (strcmp(type, "tracefs") == 0)
- break;
- }
- fclose(fp);
-
- if (strcmp(type, "tracefs") != 0) {
- fprintf(stderr, "tracefs not mounted");
- return NULL;
- }
-
- strcat(tracefs, "/tracing/");
- tracefs_found = 1;
-
- return tracefs;
-}
-
-const char *tracing_file(const char *file_name)
-{
- static char trace_file[MAX_PATH+1];
- snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name);
- return trace_file;
-}
-
-int main (int argc, char **argv)
-{
- if (argc < 1)
- exit(-1);
-
- if (fork() > 0) {
- int fd, ffd;
- char line[64];
- int s;
-
- ffd = open(tracing_file("current_tracer"), O_WRONLY);
- if (ffd < 0)
- exit(-1);
- write(ffd, "nop", 3);
-
- fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
- s = sprintf(line, "%d\n", getpid());
- write(fd, line, s);
-
- write(ffd, "function", 8);
-
- close(fd);
- close(ffd);
-
- execvp(argv[1], argv+1);
- }
-
- return 0;
-}
-
-Or this simple script!
-
-------
-#!/bin/bash
-
-tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts`
-echo nop > $tracefs/tracing/current_tracer
-echo 0 > $tracefs/tracing/tracing_on
-echo $$ > $tracefs/tracing/set_ftrace_pid
-echo function > $tracefs/tracing/current_tracer
-echo 1 > $tracefs/tracing/tracing_on
-exec "$@"
-------
-
-
-function graph tracer
----------------------------
-
-This tracer is similar to the function tracer except that it
-probes a function on its entry and its exit. This is done by
-using a dynamically allocated stack of return addresses in each
-task_struct. On function entry the tracer overwrites the return
-address of each function traced to set a custom probe. Thus the
-original return address is stored on the stack of return address
-in the task_struct.
-
-Probing on both ends of a function leads to special features
-such as:
-
-- measure of a function's time execution
-- having a reliable call stack to draw function calls graph
-
-This tracer is useful in several situations:
-
-- you want to find the reason of a strange kernel behavior and
- need to see what happens in detail on any areas (or specific
- ones).
-
-- you are experiencing weird latencies but it's difficult to
- find its origin.
-
-- you want to find quickly which path is taken by a specific
- function
-
-- you just want to peek inside a working kernel and want to see
- what happens there.
-
-# tracer: function_graph
-#
-# CPU DURATION FUNCTION CALLS
-# | | | | | | |
-
- 0) | sys_open() {
- 0) | do_sys_open() {
- 0) | getname() {
- 0) | kmem_cache_alloc() {
- 0) 1.382 us | __might_sleep();
- 0) 2.478 us | }
- 0) | strncpy_from_user() {
- 0) | might_fault() {
- 0) 1.389 us | __might_sleep();
- 0) 2.553 us | }
- 0) 3.807 us | }
- 0) 7.876 us | }
- 0) | alloc_fd() {
- 0) 0.668 us | _spin_lock();
- 0) 0.570 us | expand_files();
- 0) 0.586 us | _spin_unlock();
-
-
-There are several columns that can be dynamically
-enabled/disabled. You can use every combination of options you
-want, depending on your needs.
-
-- The cpu number on which the function executed is default
- enabled. It is sometimes better to only trace one cpu (see
- tracing_cpu_mask file) or you might sometimes see unordered
- function calls while cpu tracing switch.
-
- hide: echo nofuncgraph-cpu > trace_options
- show: echo funcgraph-cpu > trace_options
-
-- The duration (function's time of execution) is displayed on
- the closing bracket line of a function or on the same line
- than the current function in case of a leaf one. It is default
- enabled.
-
- hide: echo nofuncgraph-duration > trace_options
- show: echo funcgraph-duration > trace_options
-
-- The overhead field precedes the duration field in case of
- reached duration thresholds.
-
- hide: echo nofuncgraph-overhead > trace_options
- show: echo funcgraph-overhead > trace_options
- depends on: funcgraph-duration
-
- ie:
-
- 3) # 1837.709 us | } /* __switch_to */
- 3) | finish_task_switch() {
- 3) 0.313 us | _raw_spin_unlock_irq();
- 3) 3.177 us | }
- 3) # 1889.063 us | } /* __schedule */
- 3) ! 140.417 us | } /* __schedule */
- 3) # 2034.948 us | } /* schedule */
- 3) * 33998.59 us | } /* schedule_preempt_disabled */
-
- [...]
-
- 1) 0.260 us | msecs_to_jiffies();
- 1) 0.313 us | __rcu_read_unlock();
- 1) + 61.770 us | }
- 1) + 64.479 us | }
- 1) 0.313 us | rcu_bh_qs();
- 1) 0.313 us | __local_bh_enable();
- 1) ! 217.240 us | }
- 1) 0.365 us | idle_cpu();
- 1) | rcu_irq_exit() {
- 1) 0.417 us | rcu_eqs_enter_common.isra.47();
- 1) 3.125 us | }
- 1) ! 227.812 us | }
- 1) ! 457.395 us | }
- 1) @ 119760.2 us | }
-
- [...]
-
- 2) | handle_IPI() {
- 1) 6.979 us | }
- 2) 0.417 us | scheduler_ipi();
- 1) 9.791 us | }
- 1) + 12.917 us | }
- 2) 3.490 us | }
- 1) + 15.729 us | }
- 1) + 18.542 us | }
- 2) $ 3594274 us | }
-
- + means that the function exceeded 10 usecs.
- ! means that the function exceeded 100 usecs.
- # means that the function exceeded 1000 usecs.
- * means that the function exceeded 10 msecs.
- @ means that the function exceeded 100 msecs.
- $ means that the function exceeded 1 sec.
-
-
-- The task/pid field displays the thread cmdline and pid which
- executed the function. It is default disabled.
-
- hide: echo nofuncgraph-proc > trace_options
- show: echo funcgraph-proc > trace_options
-
- ie:
-
- # tracer: function_graph
- #
- # CPU TASK/PID DURATION FUNCTION CALLS
- # | | | | | | | | |
- 0) sh-4802 | | d_free() {
- 0) sh-4802 | | call_rcu() {
- 0) sh-4802 | | __call_rcu() {
- 0) sh-4802 | 0.616 us | rcu_process_gp_end();
- 0) sh-4802 | 0.586 us | check_for_new_grace_period();
- 0) sh-4802 | 2.899 us | }
- 0) sh-4802 | 4.040 us | }
- 0) sh-4802 | 5.151 us | }
- 0) sh-4802 | + 49.370 us | }
-
-
-- The absolute time field is an absolute timestamp given by the
- system clock since it started. A snapshot of this time is
- given on each entry/exit of functions
-
- hide: echo nofuncgraph-abstime > trace_options
- show: echo funcgraph-abstime > trace_options
-
- ie:
-
- #
- # TIME CPU DURATION FUNCTION CALLS
- # | | | | | | | |
- 360.774522 | 1) 0.541 us | }
- 360.774522 | 1) 4.663 us | }
- 360.774523 | 1) 0.541 us | __wake_up_bit();
- 360.774524 | 1) 6.796 us | }
- 360.774524 | 1) 7.952 us | }
- 360.774525 | 1) 9.063 us | }
- 360.774525 | 1) 0.615 us | journal_mark_dirty();
- 360.774527 | 1) 0.578 us | __brelse();
- 360.774528 | 1) | reiserfs_prepare_for_journal() {
- 360.774528 | 1) | unlock_buffer() {
- 360.774529 | 1) | wake_up_bit() {
- 360.774529 | 1) | bit_waitqueue() {
- 360.774530 | 1) 0.594 us | __phys_addr();
-
-
-The function name is always displayed after the closing bracket
-for a function if the start of that function is not in the
-trace buffer.
-
-Display of the function name after the closing bracket may be
-enabled for functions whose start is in the trace buffer,
-allowing easier searching with grep for function durations.
-It is default disabled.
-
- hide: echo nofuncgraph-tail > trace_options
- show: echo funcgraph-tail > trace_options
-
- Example with nofuncgraph-tail (default):
- 0) | putname() {
- 0) | kmem_cache_free() {
- 0) 0.518 us | __phys_addr();
- 0) 1.757 us | }
- 0) 2.861 us | }
-
- Example with funcgraph-tail:
- 0) | putname() {
- 0) | kmem_cache_free() {
- 0) 0.518 us | __phys_addr();
- 0) 1.757 us | } /* kmem_cache_free() */
- 0) 2.861 us | } /* putname() */
-
-You can put some comments on specific functions by using
-trace_printk() For example, if you want to put a comment inside
-the __might_sleep() function, you just have to include
-<linux/ftrace.h> and call trace_printk() inside __might_sleep()
-
-trace_printk("I'm a comment!\n")
-
-will produce:
-
- 1) | __might_sleep() {
- 1) | /* I'm a comment! */
- 1) 1.449 us | }
-
-
-You might find other useful features for this tracer in the
-following "dynamic ftrace" section such as tracing only specific
-functions or tasks.
-
-dynamic ftrace
---------------
-
-If CONFIG_DYNAMIC_FTRACE is set, the system will run with
-virtually no overhead when function tracing is disabled. The way
-this works is the mcount function call (placed at the start of
-every kernel function, produced by the -pg switch in gcc),
-starts of pointing to a simple return. (Enabling FTRACE will
-include the -pg switch in the compiling of the kernel.)
-
-At compile time every C file object is run through the
-recordmcount program (located in the scripts directory). This
-program will parse the ELF headers in the C object to find all
-the locations in the .text section that call mcount. Starting
-with gcc verson 4.6, the -mfentry has been added for x86, which
-calls "__fentry__" instead of "mcount". Which is called before
-the creation of the stack frame.
-
-Note, not all sections are traced. They may be prevented by either
-a notrace, or blocked another way and all inline functions are not
-traced. Check the "available_filter_functions" file to see what functions
-can be traced.
-
-A section called "__mcount_loc" is created that holds
-references to all the mcount/fentry call sites in the .text section.
-The recordmcount program re-links this section back into the
-original object. The final linking stage of the kernel will add all these
-references into a single table.
-
-On boot up, before SMP is initialized, the dynamic ftrace code
-scans this table and updates all the locations into nops. It
-also records the locations, which are added to the
-available_filter_functions list. Modules are processed as they
-are loaded and before they are executed. When a module is
-unloaded, it also removes its functions from the ftrace function
-list. This is automatic in the module unload code, and the
-module author does not need to worry about it.
-
-When tracing is enabled, the process of modifying the function
-tracepoints is dependent on architecture. The old method is to use
-kstop_machine to prevent races with the CPUs executing code being
-modified (which can cause the CPU to do undesirable things, especially
-if the modified code crosses cache (or page) boundaries), and the nops are
-patched back to calls. But this time, they do not call mcount
-(which is just a function stub). They now call into the ftrace
-infrastructure.
-
-The new method of modifying the function tracepoints is to place
-a breakpoint at the location to be modified, sync all CPUs, modify
-the rest of the instruction not covered by the breakpoint. Sync
-all CPUs again, and then remove the breakpoint with the finished
-version to the ftrace call site.
-
-Some archs do not even need to monkey around with the synchronization,
-and can just slap the new code on top of the old without any
-problems with other CPUs executing it at the same time.
-
-One special side-effect to the recording of the functions being
-traced is that we can now selectively choose which functions we
-wish to trace and which ones we want the mcount calls to remain
-as nops.
-
-Two files are used, one for enabling and one for disabling the
-tracing of specified functions. They are:
-
- set_ftrace_filter
-
-and
-
- set_ftrace_notrace
-
-A list of available functions that you can add to these files is
-listed in:
-
- available_filter_functions
-
- # cat available_filter_functions
-put_prev_task_idle
-kmem_cache_create
-pick_next_task_rt
-get_online_cpus
-pick_next_task_fair
-mutex_lock
-[...]
-
-If I am only interested in sys_nanosleep and hrtimer_interrupt:
-
- # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
- # echo function > current_tracer
- # echo 1 > tracing_on
- # usleep 1
- # echo 0 > tracing_on
- # cat trace
-# tracer: function
-#
-# entries-in-buffer/entries-written: 5/5 #P:4
-#
-# _-----=> irqs-off
-# / _----=> need-resched
-# | / _---=> hardirq/softirq
-# || / _--=> preempt-depth
-# ||| / delay
-# TASK-PID CPU# |||| TIMESTAMP FUNCTION
-# | | | |||| | |
- usleep-2665 [001] .... 4186.475355: sys_nanosleep <-system_call_fastpath
- <idle>-0 [001] d.h1 4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
- usleep-2665 [001] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
- <idle>-0 [003] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
- <idle>-0 [002] d.h1 4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
-
-To see which functions are being traced, you can cat the file:
-
- # cat set_ftrace_filter
-hrtimer_interrupt
-sys_nanosleep
-
-
-Perhaps this is not enough. The filters also allow glob(7) matching.
-
- <match>* - will match functions that begin with <match>
- *<match> - will match functions that end with <match>
- *<match>* - will match functions that have <match> in it
- <match1>*<match2> - will match functions that begin with
- <match1> and end with <match2>
-
-Note: It is better to use quotes to enclose the wild cards,
- otherwise the shell may expand the parameters into names
- of files in the local directory.
-
- # echo 'hrtimer_*' > set_ftrace_filter
-
-Produces:
-
-# tracer: function
-#
-# entries-in-buffer/entries-written: 897/897 #P:4
-#
-# _-----=> irqs-off
-# / _----=> need-resched
-# | / _---=> hardirq/softirq
-# || / _--=> preempt-depth
-# ||| / delay
-# TASK-PID CPU# |||| TIMESTAMP FUNCTION
-# | | | |||| | |
- <idle>-0 [003] dN.1 4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
- <idle>-0 [003] dN.1 4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
- <idle>-0 [003] dN.2 4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
- <idle>-0 [003] dN.1 4228.547805: hrtimer_forward <-tick_nohz_idle_exit
- <idle>-0 [003] dN.1 4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
- <idle>-0 [003] d..1 4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
- <idle>-0 [003] d..1 4228.547859: hrtimer_start <-__tick_nohz_idle_enter
- <idle>-0 [003] d..2 4228.547860: hrtimer_force_reprogram <-__rem
-
-Notice that we lost the sys_nanosleep.
-
- # cat set_ftrace_filter
-hrtimer_run_queues
-hrtimer_run_pending
-hrtimer_init
-hrtimer_cancel
-hrtimer_try_to_cancel
-hrtimer_forward
-hrtimer_start
-hrtimer_reprogram
-hrtimer_force_reprogram
-hrtimer_get_next_event
-hrtimer_interrupt
-hrtimer_nanosleep
-hrtimer_wakeup
-hrtimer_get_remaining
-hrtimer_get_res
-hrtimer_init_sleeper
-
-
-This is because the '>' and '>>' act just like they do in bash.
-To rewrite the filters, use '>'
-To append to the filters, use '>>'
-
-To clear out a filter so that all functions will be recorded
-again:
-
- # echo > set_ftrace_filter
- # cat set_ftrace_filter
- #
-
-Again, now we want to append.
-
- # echo sys_nanosleep > set_ftrace_filter
- # cat set_ftrace_filter
-sys_nanosleep
- # echo 'hrtimer_*' >> set_ftrace_filter
- # cat set_ftrace_filter
-hrtimer_run_queues
-hrtimer_run_pending
-hrtimer_init
-hrtimer_cancel
-hrtimer_try_to_cancel
-hrtimer_forward
-hrtimer_start
-hrtimer_reprogram
-hrtimer_force_reprogram
-hrtimer_get_next_event
-hrtimer_interrupt
-sys_nanosleep
-hrtimer_nanosleep
-hrtimer_wakeup
-hrtimer_get_remaining
-hrtimer_get_res
-hrtimer_init_sleeper
-
-
-The set_ftrace_notrace prevents those functions from being
-traced.
-
- # echo '*preempt*' '*lock*' > set_ftrace_notrace
-
-Produces:
-
-# tracer: function
-#
-# entries-in-buffer/entries-written: 39608/39608 #P:4
-#
-# _-----=> irqs-off
-# / _----=> need-resched
-# | / _---=> hardirq/softirq
-# || / _--=> preempt-depth
-# ||| / delay
-# TASK-PID CPU# |||| TIMESTAMP FUNCTION
-# | | | |||| | |
- bash-1994 [000] .... 4342.324896: file_ra_state_init <-do_dentry_open
- bash-1994 [000] .... 4342.324897: open_check_o_direct <-do_last
- bash-1994 [000] .... 4342.324897: ima_file_check <-do_last
- bash-1994 [000] .... 4342.324898: process_measurement <-ima_file_check
- bash-1994 [000] .... 4342.324898: ima_get_action <-process_measurement
- bash-1994 [000] .... 4342.324898: ima_match_policy <-ima_get_action
- bash-1994 [000] .... 4342.324899: do_truncate <-do_last
- bash-1994 [000] .... 4342.324899: should_remove_suid <-do_truncate
- bash-1994 [000] .... 4342.324899: notify_change <-do_truncate
- bash-1994 [000] .... 4342.324900: current_fs_time <-notify_change
- bash-1994 [000] .... 4342.324900: current_kernel_time <-current_fs_time
- bash-1994 [000] .... 4342.324900: timespec_trunc <-current_fs_time
-
-We can see that there's no more lock or preempt tracing.
-
-
-Dynamic ftrace with the function graph tracer
----------------------------------------------
-
-Although what has been explained above concerns both the
-function tracer and the function-graph-tracer, there are some
-special features only available in the function-graph tracer.
-
-If you want to trace only one function and all of its children,
-you just have to echo its name into set_graph_function:
-
- echo __do_fault > set_graph_function
-
-will produce the following "expanded" trace of the __do_fault()
-function:
-
- 0) | __do_fault() {
- 0) | filemap_fault() {
- 0) | find_lock_page() {
- 0) 0.804 us | find_get_page();
- 0) | __might_sleep() {
- 0) 1.329 us | }
- 0) 3.904 us | }
- 0) 4.979 us | }
- 0) 0.653 us | _spin_lock();
- 0) 0.578 us | page_add_file_rmap();
- 0) 0.525 us | native_set_pte_at();
- 0) 0.585 us | _spin_unlock();
- 0) | unlock_page() {
- 0) 0.541 us | page_waitqueue();
- 0) 0.639 us | __wake_up_bit();
- 0) 2.786 us | }
- 0) + 14.237 us | }
- 0) | __do_fault() {
- 0) | filemap_fault() {
- 0) | find_lock_page() {
- 0) 0.698 us | find_get_page();
- 0) | __might_sleep() {
- 0) 1.412 us | }
- 0) 3.950 us | }
- 0) 5.098 us | }
- 0) 0.631 us | _spin_lock();
- 0) 0.571 us | page_add_file_rmap();
- 0) 0.526 us | native_set_pte_at();
- 0) 0.586 us | _spin_unlock();
- 0) | unlock_page() {
- 0) 0.533 us | page_waitqueue();
- 0) 0.638 us | __wake_up_bit();
- 0) 2.793 us | }
- 0) + 14.012 us | }
-
-You can also expand several functions at once:
-
- echo sys_open > set_graph_function
- echo sys_close >> set_graph_function
-
-Now if you want to go back to trace all functions you can clear
-this special filter via:
-
- echo > set_graph_function
-
-
-ftrace_enabled
---------------
-
-Note, the proc sysctl ftrace_enable is a big on/off switch for the
-function tracer. By default it is enabled (when function tracing is
-enabled in the kernel). If it is disabled, all function tracing is
-disabled. This includes not only the function tracers for ftrace, but
-also for any other uses (perf, kprobes, stack tracing, profiling, etc).
-
-Please disable this with care.
-
-This can be disable (and enabled) with:
-
- sysctl kernel.ftrace_enabled=0
- sysctl kernel.ftrace_enabled=1
-
- or
-
- echo 0 > /proc/sys/kernel/ftrace_enabled
- echo 1 > /proc/sys/kernel/ftrace_enabled
-
-
-Filter commands
----------------
-
-A few commands are supported by the set_ftrace_filter interface.
-Trace commands have the following format:
-
-<function>:<command>:<parameter>
-
-The following commands are supported:
-
-- mod
- This command enables function filtering per module. The
- parameter defines the module. For example, if only the write*
- functions in the ext3 module are desired, run:
-
- echo 'write*:mod:ext3' > set_ftrace_filter
-
- This command interacts with the filter in the same way as
- filtering based on function names. Thus, adding more functions
- in a different module is accomplished by appending (>>) to the
- filter file. Remove specific module functions by prepending
- '!':
-
- echo '!writeback*:mod:ext3' >> set_ftrace_filter
-
- Mod command supports module globbing. Disable tracing for all
- functions except a specific module:
-
- echo '!*:mod:!ext3' >> set_ftrace_filter
-
- Disable tracing for all modules, but still trace kernel:
-
- echo '!*:mod:*' >> set_ftrace_filter
-
- Enable filter only for kernel:
-
- echo '*write*:mod:!*' >> set_ftrace_filter
-
- Enable filter for module globbing:
-
- echo '*write*:mod:*snd*' >> set_ftrace_filter
-
-- traceon/traceoff
- These commands turn tracing on and off when the specified
- functions are hit. The parameter determines how many times the
- tracing system is turned on and off. If unspecified, there is
- no limit. For example, to disable tracing when a schedule bug
- is hit the first 5 times, run:
-
- echo '__schedule_bug:traceoff:5' > set_ftrace_filter
-
- To always disable tracing when __schedule_bug is hit:
-
- echo '__schedule_bug:traceoff' > set_ftrace_filter
-
- These commands are cumulative whether or not they are appended
- to set_ftrace_filter. To remove a command, prepend it by '!'
- and drop the parameter:
-
- echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
-
- The above removes the traceoff command for __schedule_bug
- that have a counter. To remove commands without counters:
-
- echo '!__schedule_bug:traceoff' > set_ftrace_filter
-
-- snapshot
- Will cause a snapshot to be triggered when the function is hit.
-
- echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
-
- To only snapshot once:
-
- echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
-
- To remove the above commands:
-
- echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
- echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
-
-- enable_event/disable_event
- These commands can enable or disable a trace event. Note, because
- function tracing callbacks are very sensitive, when these commands
- are registered, the trace point is activated, but disabled in
- a "soft" mode. That is, the tracepoint will be called, but
- just will not be traced. The event tracepoint stays in this mode
- as long as there's a command that triggers it.
-
- echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
- set_ftrace_filter
-
- The format is:
-
- <function>:enable_event:<system>:<event>[:count]
- <function>:disable_event:<system>:<event>[:count]
-
- To remove the events commands:
-
-
- echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
- set_ftrace_filter
- echo '!schedule:disable_event:sched:sched_switch' > \
- set_ftrace_filter
-
-- dump
- When the function is hit, it will dump the contents of the ftrace
- ring buffer to the console. This is useful if you need to debug
- something, and want to dump the trace when a certain function
- is hit. Perhaps its a function that is called before a tripple
- fault happens and does not allow you to get a regular dump.
-
-- cpudump
- When the function is hit, it will dump the contents of the ftrace
- ring buffer for the current CPU to the console. Unlike the "dump"
- command, it only prints out the contents of the ring buffer for the
- CPU that executed the function that triggered the dump.
-
-trace_pipe
-----------
-
-The trace_pipe outputs the same content as the trace file, but
-the effect on the tracing is different. Every read from
-trace_pipe is consumed. This means that subsequent reads will be
-different. The trace is live.
-
- # echo function > current_tracer
- # cat trace_pipe > /tmp/trace.out &
-[1] 4153
- # echo 1 > tracing_on
- # usleep 1
- # echo 0 > tracing_on
- # cat trace
-# tracer: function
-#
-# entries-in-buffer/entries-written: 0/0 #P:4
-#
-# _-----=> irqs-off
-# / _----=> need-resched
-# | / _---=> hardirq/softirq
-# || / _--=> preempt-depth
-# ||| / delay
-# TASK-PID CPU# |||| TIMESTAMP FUNCTION
-# | | | |||| | |
-
- #
- # cat /tmp/trace.out
- bash-1994 [000] .... 5281.568961: mutex_unlock <-rb_simple_write
- bash-1994 [000] .... 5281.568963: __mutex_unlock_slowpath <-mutex_unlock
- bash-1994 [000] .... 5281.568963: __fsnotify_parent <-fsnotify_modify
- bash-1994 [000] .... 5281.568964: fsnotify <-fsnotify_modify
- bash-1994 [000] .... 5281.568964: __srcu_read_lock <-fsnotify
- bash-1994 [000] .... 5281.568964: add_preempt_count <-__srcu_read_lock
- bash-1994 [000] ...1 5281.568965: sub_preempt_count <-__srcu_read_lock
- bash-1994 [000] .... 5281.568965: __srcu_read_unlock <-fsnotify
- bash-1994 [000] .... 5281.568967: sys_dup2 <-system_call_fastpath
-
-
-Note, reading the trace_pipe file will block until more input is
-added.
-
-trace entries
--------------
-
-Having too much or not enough data can be troublesome in
-diagnosing an issue in the kernel. The file buffer_size_kb is
-used to modify the size of the internal trace buffers. The
-number listed is the number of entries that can be recorded per
-CPU. To know the full size, multiply the number of possible CPUs
-with the number of entries.
-
- # cat buffer_size_kb
-1408 (units kilobytes)
-
-Or simply read buffer_total_size_kb
-
- # cat buffer_total_size_kb
-5632
-
-To modify the buffer, simple echo in a number (in 1024 byte segments).
-
- # echo 10000 > buffer_size_kb
- # cat buffer_size_kb
-10000 (units kilobytes)
-
-It will try to allocate as much as possible. If you allocate too
-much, it can cause Out-Of-Memory to trigger.
-
- # echo 1000000000000 > buffer_size_kb
--bash: echo: write error: Cannot allocate memory
- # cat buffer_size_kb
-85
-
-The per_cpu buffers can be changed individually as well:
-
- # echo 10000 > per_cpu/cpu0/buffer_size_kb
- # echo 100 > per_cpu/cpu1/buffer_size_kb
-
-When the per_cpu buffers are not the same, the buffer_size_kb
-at the top level will just show an X
-
- # cat buffer_size_kb
-X
-
-This is where the buffer_total_size_kb is useful:
-
- # cat buffer_total_size_kb
-12916
-
-Writing to the top level buffer_size_kb will reset all the buffers
-to be the same again.
-
-Snapshot
---------
-CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
-available to all non latency tracers. (Latency tracers which
-record max latency, such as "irqsoff" or "wakeup", can't use
-this feature, since those are already using the snapshot
-mechanism internally.)
-
-Snapshot preserves a current trace buffer at a particular point
-in time without stopping tracing. Ftrace swaps the current
-buffer with a spare buffer, and tracing continues in the new
-current (=previous spare) buffer.
-
-The following tracefs files in "tracing" are related to this
-feature:
-
- snapshot:
-
- This is used to take a snapshot and to read the output
- of the snapshot. Echo 1 into this file to allocate a
- spare buffer and to take a snapshot (swap), then read
- the snapshot from this file in the same format as
- "trace" (described above in the section "The File
- System"). Both reads snapshot and tracing are executable
- in parallel. When the spare buffer is allocated, echoing
- 0 frees it, and echoing else (positive) values clear the
- snapshot contents.
- More details are shown in the table below.
-
- status\input | 0 | 1 | else |
- --------------+------------+------------+------------+
- not allocated |(do nothing)| alloc+swap |(do nothing)|
- --------------+------------+------------+------------+
- allocated | free | swap | clear |
- --------------+------------+------------+------------+
-
-Here is an example of using the snapshot feature.
-
- # echo 1 > events/sched/enable
- # echo 1 > snapshot
- # cat snapshot
-# tracer: nop
-#
-# entries-in-buffer/entries-written: 71/71 #P:8
-#
-# _-----=> irqs-off
-# / _----=> need-resched
-# | / _---=> hardirq/softirq
-# || / _--=> preempt-depth
-# ||| / delay
-# TASK-PID CPU# |||| TIMESTAMP FUNCTION
-# | | | |||| | |
- <idle>-0 [005] d... 2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120
- sleep-2242 [005] d... 2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120
-[...]
- <idle>-0 [002] d... 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120
-
- # cat trace
-# tracer: nop
-#
-# entries-in-buffer/entries-written: 77/77 #P:8
-#
-# _-----=> irqs-off
-# / _----=> need-resched
-# | / _---=> hardirq/softirq
-# || / _--=> preempt-depth
-# ||| / delay
-# TASK-PID CPU# |||| TIMESTAMP FUNCTION
-# | | | |||| | |
- <idle>-0 [007] d... 2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120
- snapshot-test-2-2229 [002] d... 2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120
-[...]
-
-
-If you try to use this snapshot feature when current tracer is
-one of the latency tracers, you will get the following results.
-
- # echo wakeup > current_tracer
- # echo 1 > snapshot
-bash: echo: write error: Device or resource busy
- # cat snapshot
-cat: snapshot: Device or resource busy
-
-
-Instances
----------
-In the tracefs tracing directory is a directory called "instances".
-This directory can have new directories created inside of it using
-mkdir, and removing directories with rmdir. The directory created
-with mkdir in this directory will already contain files and other
-directories after it is created.
-
- # mkdir instances/foo
- # ls instances/foo
-buffer_size_kb buffer_total_size_kb events free_buffer per_cpu
-set_event snapshot trace trace_clock trace_marker trace_options
-trace_pipe tracing_on
-
-As you can see, the new directory looks similar to the tracing directory
-itself. In fact, it is very similar, except that the buffer and
-events are agnostic from the main director, or from any other
-instances that are created.
-
-The files in the new directory work just like the files with the
-same name in the tracing directory except the buffer that is used
-is a separate and new buffer. The files affect that buffer but do not
-affect the main buffer with the exception of trace_options. Currently,
-the trace_options affect all instances and the top level buffer
-the same, but this may change in future releases. That is, options
-may become specific to the instance they reside in.
-
-Notice that none of the function tracer files are there, nor is
-current_tracer and available_tracers. This is because the buffers
-can currently only have events enabled for them.
-
- # mkdir instances/foo
- # mkdir instances/bar
- # mkdir instances/zoot
- # echo 100000 > buffer_size_kb
- # echo 1000 > instances/foo/buffer_size_kb
- # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
- # echo function > current_trace
- # echo 1 > instances/foo/events/sched/sched_wakeup/enable
- # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
- # echo 1 > instances/foo/events/sched/sched_switch/enable
- # echo 1 > instances/bar/events/irq/enable
- # echo 1 > instances/zoot/events/syscalls/enable
- # cat trace_pipe
-CPU:2 [LOST 11745 EVENTS]
- bash-2044 [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
- bash-2044 [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
- bash-2044 [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
- bash-2044 [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
- bash-2044 [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
- bash-2044 [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
- bash-2044 [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
- bash-2044 [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
- bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics
- bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics
- bash-2044 [002] .... 10594.481035: arch_dup_task_struct <-copy_process
-[...]
-
- # cat instances/foo/trace_pipe
- bash-1998 [000] d..4 136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
- bash-1998 [000] dN.4 136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
- <idle>-0 [003] d.h3 136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
- <idle>-0 [003] d..3 136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120
- rcu_preempt-9 [003] d..3 136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120
- bash-1998 [000] d..4 136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
- bash-1998 [000] dN.4 136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
- bash-1998 [000] d..3 136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120
- kworker/0:1-59 [000] d..4 136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
- kworker/0:1-59 [000] d..3 136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120
-[...]
-
- # cat instances/bar/trace_pipe
- migration/1-14 [001] d.h3 138.732674: softirq_raise: vec=3 [action=NET_RX]
- <idle>-0 [001] dNh3 138.732725: softirq_raise: vec=3 [action=NET_RX]
- bash-1998 [000] d.h1 138.733101: softirq_raise: vec=1 [action=TIMER]
- bash-1998 [000] d.h1 138.733102: softirq_raise: vec=9 [action=RCU]
- bash-1998 [000] ..s2 138.733105: softirq_entry: vec=1 [action=TIMER]
- bash-1998 [000] ..s2 138.733106: softirq_exit: vec=1 [action=TIMER]
- bash-1998 [000] ..s2 138.733106: softirq_entry: vec=9 [action=RCU]
- bash-1998 [000] ..s2 138.733109: softirq_exit: vec=9 [action=RCU]
- sshd-1995 [001] d.h1 138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
- sshd-1995 [001] d.h1 138.733280: irq_handler_exit: irq=21 ret=unhandled
- sshd-1995 [001] d.h1 138.733281: irq_handler_entry: irq=21 name=eth0
- sshd-1995 [001] d.h1 138.733283: irq_handler_exit: irq=21 ret=handled
-[...]
-
- # cat instances/zoot/trace
-# tracer: nop
-#
-# entries-in-buffer/entries-written: 18996/18996 #P:4
-#
-# _-----=> irqs-off
-# / _----=> need-resched
-# | / _---=> hardirq/softirq
-# || / _--=> preempt-depth
-# ||| / delay
-# TASK-PID CPU# |||| TIMESTAMP FUNCTION
-# | | | |||| | |
- bash-1998 [000] d... 140.733501: sys_write -> 0x2
- bash-1998 [000] d... 140.733504: sys_dup2(oldfd: a, newfd: 1)
- bash-1998 [000] d... 140.733506: sys_dup2 -> 0x1
- bash-1998 [000] d... 140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
- bash-1998 [000] d... 140.733509: sys_fcntl -> 0x1
- bash-1998 [000] d... 140.733510: sys_close(fd: a)
- bash-1998 [000] d... 140.733510: sys_close -> 0x0
- bash-1998 [000] d... 140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
- bash-1998 [000] d... 140.733515: sys_rt_sigprocmask -> 0x0
- bash-1998 [000] d... 140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
- bash-1998 [000] d... 140.733516: sys_rt_sigaction -> 0x0
-
-You can see that the trace of the top most trace buffer shows only
-the function tracing. The foo instance displays wakeups and task
-switches.
-
-To remove the instances, simply delete their directories:
-
- # rmdir instances/foo
- # rmdir instances/bar
- # rmdir instances/zoot
-
-Note, if a process has a trace file open in one of the instance
-directories, the rmdir will fail with EBUSY.
-
-
-Stack trace
------------
-Since the kernel has a fixed sized stack, it is important not to
-waste it in functions. A kernel developer must be conscience of
-what they allocate on the stack. If they add too much, the system
-can be in danger of a stack overflow, and corruption will occur,
-usually leading to a system panic.
-
-There are some tools that check this, usually with interrupts
-periodically checking usage. But if you can perform a check
-at every function call that will become very useful. As ftrace provides
-a function tracer, it makes it convenient to check the stack size
-at every function call. This is enabled via the stack tracer.
-
-CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
-To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
-
- # echo 1 > /proc/sys/kernel/stack_tracer_enabled
-
-You can also enable it from the kernel command line to trace
-the stack size of the kernel during boot up, by adding "stacktrace"
-to the kernel command line parameter.
-
-After running it for a few minutes, the output looks like:
-
- # cat stack_max_size
-2928
-
- # cat stack_trace
- Depth Size Location (18 entries)
- ----- ---- --------
- 0) 2928 224 update_sd_lb_stats+0xbc/0x4ac
- 1) 2704 160 find_busiest_group+0x31/0x1f1
- 2) 2544 256 load_balance+0xd9/0x662
- 3) 2288 80 idle_balance+0xbb/0x130
- 4) 2208 128 __schedule+0x26e/0x5b9
- 5) 2080 16 schedule+0x64/0x66
- 6) 2064 128 schedule_timeout+0x34/0xe0
- 7) 1936 112 wait_for_common+0x97/0xf1
- 8) 1824 16 wait_for_completion+0x1d/0x1f
- 9) 1808 128 flush_work+0xfe/0x119
- 10) 1680 16 tty_flush_to_ldisc+0x1e/0x20
- 11) 1664 48 input_available_p+0x1d/0x5c
- 12) 1616 48 n_tty_poll+0x6d/0x134
- 13) 1568 64 tty_poll+0x64/0x7f
- 14) 1504 880 do_select+0x31e/0x511
- 15) 624 400 core_sys_select+0x177/0x216
- 16) 224 96 sys_select+0x91/0xb9
- 17) 128 128 system_call_fastpath+0x16/0x1b
-
-Note, if -mfentry is being used by gcc, functions get traced before
-they set up the stack frame. This means that leaf level functions
-are not tested by the stack tracer when -mfentry is used.
-
-Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
-
----------
-
-More details can be found in the source code, in the
-kernel/trace/*.c files.
diff --git a/Documentation/trace/index.rst b/Documentation/trace/index.rst
index 61b5551..947c6db 100644
--- a/Documentation/trace/index.rst
+++ b/Documentation/trace/index.rst
@@ -7,4 +7,5 @@ Linux Tracing Technologies
ftrace-design
tracepoint-analysis
+ ftrace
ftrace-uses
--
2.7.4
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