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Date: Thu, 10 Jul 2008 12:46:01 -0400 (EDT)
From: Steven Rostedt <rostedt@...dmis.org>
To: LKML <linux-kernel@...r.kernel.org>
cc: Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Clark Williams <clark.williams@...il.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Randy Dunlap <randy.dunlap@...cle.com>,
Jon Masters <jonathan@...masters.org>
Subject: [PATCH] ftrace: Documentation
This is the long awaited ftrace.txt. It explains in quite detail how to
use ftrace and the various tracers.
Signed-off-by: Steven Rostedt <srostedt@...hat.com>
---
Documentation/ftrace.txt | 1353 +++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 1353 insertions(+)
Index: linux-tip.git/Documentation/ftrace.txt
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-tip.git/Documentation/ftrace.txt 2008-07-10 12:23:06.000000000 -0400
@@ -0,0 +1,1353 @@
+ ftrace - Function Tracer
+ ========================
+
+Copyright 2008 Red Hat Inc.
+Author: Steven Rostedt <srostedt@...hat.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 the function tracer, it also includes an
+infrastructure that allows for other types of tracing. Some of the
+tracers that are currently in ftrace is a tracer to trace
+context switches, the time it takes for a high priority task to
+run after it was woken up, the time interrupts are disabled, and
+more.
+
+
+The File System
+---------------
+
+Ftrace uses the debugfs file system to hold the control files as well
+as the files to display output.
+
+To mount the debugfs system:
+
+ # mkdir /debug
+ # mount -t debugfs nodev /debug
+
+
+That's it! (assuming that you have ftrace configured into your kernel)
+
+After mounting the debugfs, you can see a directory called
+"tracing". This directory contains 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
+ has been compiled into the kernel. The tracers
+ listed here can be configured by echoing in their
+ name into current_tracer.
+
+ tracing_enabled : This sets or displays whether the current_tracer
+ is activated and tracing or not. Echo 0 into this
+ file to disable the tracer or 1 (or non-zero) to
+ enable it.
+
+ trace : This file holds the output of the trace in a human readable
+ format.
+
+ latency_trace : This file shows the same trace but the information
+ is organized more to display possible latencies
+ in the system.
+
+ 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" and "latency_trace"
+ files, 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" and
+ "latency_trace" files are static, and if the
+ tracer isn't adding more data, they will display
+ the same information every time they are read.
+
+ iter_ctrl : This file lets the user control the amount of data
+ that is displayed in one of the above output
+ files.
+
+ trace_max_latency : Some of the tracers record the max latency.
+ For example, the time interrupts are disabled.
+ This time is saved in this file. The max trace
+ will also be stored, and displayed by either
+ "trace" or "latency_trace". A new max trace will
+ only be recorded if the latency is greater than
+ the value in this file. (in microseconds)
+
+ trace_entries : This sets or displays the number of trace
+ entries each CPU buffer can hold. The tracer buffers
+ are the same size for each CPU, so care must be
+ taken when modifying the trace_entries. The number
+ of actually entries will be the number given
+ times the number of possible CPUS. The buffers
+ are saved as individual pages, and the actual entries
+ will always be rounded up to entries per page.
+
+ This can only be updated when the current_tracer
+ is set to "none".
+
+ NOTE: It is planned on changing the allocated buffers
+ from being the number of possible CPUS to
+ the number of online CPUS.
+
+ 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, the
+ code is dynamically modified 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 in names of functions into this
+ file will limit the trace to only those files.
+
+ set_ftrace_notrace: This has the opposite effect that
+ set_ftrace_filter has. 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_ bet traced.
+
+ available_filter_functions : When a function is encountered the first
+ time by the dynamic tracer, it is recorded and
+ later the call is converted into a nop. This file
+ lists the functions that have been recorded
+ by the dynamic tracer and these functions can
+ be used to set the ftrace filter by the above
+ "set_ftrace_filter" file.
+
+
+The Tracers
+-----------
+
+Here are the list of current tracers that can be configured.
+
+ ftrace - function tracer that uses mcount to trace all functions.
+ It is possible to filter out which functions that are
+ traced when dynamic ftrace is configured in.
+
+ sched_switch - traces the context switches between tasks.
+
+ irqsoff - traces the areas that disable interrupts and saves off
+ 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_trace file.
+
+ preemptoff - Similar to irqsoff but traces and records the time
+ preemption is disabled.
+
+ preemptirqsoff - Similar to irqsoff and preemptoff, but traces and
+ records the largest time 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.
+
+ none - This is not a tracer. To remove all tracers from tracing
+ simply echo "none" into current_tracer.
+
+
+Examples of using the tracer
+----------------------------
+
+Here are typical examples of using the tracers with only controlling
+them with the debugfs interface (without using any user-land utilities).
+
+Output format:
+--------------
+
+Here's an example of the output format of the file "trace"
+
+ --------
+# tracer: ftrace
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+ bash-4251 [01] 10152.583854: path_put <-path_walk
+ bash-4251 [01] 10152.583855: dput <-path_put
+ bash-4251 [01] 10152.583855: _atomic_dec_and_lock <-dput
+ --------
+
+A header is printed with the trace that is represented. In this case
+the tracer is "ftrace". Then a header showing the format. Task name
+"bash", the task PID "4251", the CPU that it was running on
+"01", the timestamp in <secs>.<usecs> format, the function name that was
+traced "path_put" and the parent function that called this function
+"path_walk".
+
+The sched_switch tracer also includes tracing of task wake ups and
+context switches.
+
+ ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 2916:115:S
+ ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 10:115:S
+ ksoftirqd/1-7 [01] 1453.070013: 7:115:R ==> 10:115:R
+ events/1-10 [01] 1453.070013: 10:115:S ==> 2916:115:R
+ kondemand/1-2916 [01] 1453.070013: 2916:115:S ==> 7:115:R
+ ksoftirqd/1-7 [01] 1453.070013: 7:115:S ==> 0:140:R
+
+Wake ups are represented by a "+" and the context switches show
+"==>". The format is:
+
+ Context switches:
+
+ Previous task Next Task
+
+ <pid>:<prio>:<state> ==> <pid>:<prio>:<state>
+
+ Wake ups:
+
+ Current task Task waking up
+
+ <pid>:<prio>:<state> + <pid>:<prio>:<state>
+
+The prio is the internal kernel priority, which is inverse to the
+priority that is usually displayed by user-space tools. Zero represents
+the highest priority (99). Prio 100 starts the "nice" priorities with
+100 being equal to nice -20 and 139 being nice 19. The prio "140" is
+reserved for the idle task which is the lowest priority thread (pid 0).
+
+
+Latency trace format
+--------------------
+
+For traces that display latency times, the latency_trace file gives
+a bit more information to see why a latency happened. Here's a typical
+trace.
+
+# tracer: irqsoff
+#
+irqsoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: apic_timer_interrupt
+ => ended at: do_softirq
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ <idle>-0 0d..1 0us+: trace_hardirqs_off_thunk (apic_timer_interrupt)
+ <idle>-0 0d.s. 97us : __do_softirq (do_softirq)
+ <idle>-0 0d.s1 98us : trace_hardirqs_on (do_softirq)
+
+
+vim:ft=help
+
+
+This shows that the current tracer is "irqsoff" tracing the time
+interrupts are disabled. It gives the trace version and the kernel
+this was executed on (2.6.26-rc8). Then it displays the max latency
+in microsecs (97 us). The number of trace entries displayed
+by the total number recorded (both are three: #3/3). The type of
+preemption that was used (PREEMPT). VP, KP, SP, and HP are always zero
+and reserved for later use. #P is the number of online CPUS (#P:2).
+
+The task is the process that was running when the latency happened.
+(swapper pid: 0).
+
+The start and stop that caused the latencies:
+
+ apic_timer_interrupt is where the interrupts were disabled.
+ do_softirq 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 that the process was running on.
+
+ irqs-off: 'd' interrupts are disabled. '.' otherwise.
+
+ need-resched: 'N' task need_resched is set, '.' otherwise.
+
+ hardirq/softirq:
+ 'H' - hard irq happened 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: This differs from the trace output where as the trace output
+ contained a absolute timestamp. This timestamp is relative
+ to the start of the first entry in the the trace.
+
+ 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 is determined by the difference between this
+ current trace and the next trace.
+ '!' - greater than preempt_mark_thresh (default 100)
+ '+' - greater than 1 microsecond
+ ' ' - less than or equal to 1 microsecond.
+
+ The rest is the same as the 'trace' file.
+
+
+iter_ctrl
+---------
+
+The iter_ctrl file is used to control what gets printed in the trace
+output. To see what is available, simply cat the file:
+
+ cat /debug/tracing/iter_ctrl
+ print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \
+ noblock nostacktrace nosched-tree
+
+To disable one of the options, echo in the option appended with "no".
+
+ echo noprint-parent > /debug/tracing/iter_ctrl
+
+To enable an option, leave off the "no".
+
+ echo sym-offest > /debug/tracing/iter_ctrl
+
+Here are the available options:
+
+ print-parent - On function traces, display the calling function
+ as well as the function being traced.
+
+ print-parent:
+ bash-4000 [01] 1477.606694: simple_strtoul <-strict_strtoul
+
+ 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 latency_trace file.
+
+ bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
+ (+0.000ms): simple_strtoul (strict_strtoul)
+
+ 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 - TBD (needs update)
+
+ stacktrace - This is one of the options that changes the trace itself.
+ When a trace is recorded, so is the stack of functions.
+ This allows for back traces of trace sites.
+
+ sched-tree - TBD (any users??)
+
+
+sched_switch
+------------
+
+This tracer simply records schedule switches. Here's an example
+on how to implement it.
+
+ # echo sched_switch > /debug/tracing/current_tracer
+ # echo 1 > /debug/tracing/tracing_enabled
+ # sleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/trace
+
+# tracer: sched_switch
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+ bash-3997 [01] 240.132281: 3997:120:R + 4055:120:R
+ bash-3997 [01] 240.132284: 3997:120:R ==> 4055:120:R
+ sleep-4055 [01] 240.132371: 4055:120:S ==> 3997:120:R
+ bash-3997 [01] 240.132454: 3997:120:R + 4055:120:S
+ bash-3997 [01] 240.132457: 3997:120:R ==> 4055:120:R
+ sleep-4055 [01] 240.132460: 4055:120:D ==> 3997:120:R
+ bash-3997 [01] 240.132463: 3997:120:R + 4055:120:D
+ bash-3997 [01] 240.132465: 3997:120:R ==> 4055:120:R
+ <idle>-0 [00] 240.132589: 0:140:R + 4:115:S
+ <idle>-0 [00] 240.132591: 0:140:R ==> 4:115:R
+ ksoftirqd/0-4 [00] 240.132595: 4:115:S ==> 0:140:R
+ <idle>-0 [00] 240.132598: 0:140:R + 4:115:S
+ <idle>-0 [00] 240.132599: 0:140:R ==> 4:115:R
+ ksoftirqd/0-4 [00] 240.132603: 4:115:S ==> 0:140:R
+ sleep-4055 [01] 240.133058: 4055:120:S ==> 3997:120:R
+ [...]
+
+
+As we have discussed previously about this format, the header shows
+the name of the trace and points to the options. The "FUNCTION"
+is a misnomer since here it represents the wake ups and context
+switches.
+
+The sched_switch only lists the wake ups (represented with '+')
+and context switches ('==>') with the previous task or current
+first followed by the next task or task waking up. The format for both
+of these is PID:KERNEL-PRIO:TASK-STATE. Remember that the KERNEL-PRIO
+is the inverse of the actual priority with zero (0) being the highest
+priority and the nice values starting at 100 (nice -20). Below is
+a quick chart to map the kernel priority to user land priorities.
+
+ Kernel priority: 0 to 99 ==> user RT priority 99 to 0
+ Kernel priority: 100 to 139 ==> user nice -20 to 19
+ Kernel priority: 140 ==> idle task priority
+
+The task states are:
+
+ R - running : wants to run, may not actually be running
+ S - sleep : process is waiting to be woken up (handles signals)
+ D - deep sleep : process must be woken up (ignores signals)
+ T - stopped : process suspended
+ t - traced : process is being traced (with something like gdb)
+ Z - zombie : process waiting to be cleaned up
+ X - unknown
+
+
+ftrace_enabled
+--------------
+
+The following tracers give different output depending on whether
+or not the sysctl ftrace_enabled is set. To set ftrace_enabled,
+one can either use the sysctl function or set it via the proc
+file system interface.
+
+ sysctl kernel.ftrace_enabled=1
+
+ or
+
+ echo 1 > /proc/sys/kernel/ftrace_enabled
+
+To disable ftrace_enabled simply replace the '1' with '0' in
+the above commands.
+
+When ftrace_enabled is set the tracers will also record the functions
+that are within the trace. The descriptions of the tracers
+will also show an example with ftrace enabled.
+
+
+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 interrupts are disabled and when
+they are re-enabled. When a new maximum latency is hit, it saves off
+the trace so that it may be retrieved at a later time. Every time a
+new maximum in reached, the old saved trace is discarded and the new
+trace is saved.
+
+To reset the maximum, echo 0 into tracing_max_latency. Here's an
+example:
+
+ # echo irqsoff > /debug/tracing/current_tracer
+ # echo 0 > /debug/tracing/tracing_max_latency
+ # echo 1 > /debug/tracing/tracing_enabled
+ # ls -ltr
+ [...]
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/latency_trace
+# tracer: irqsoff
+#
+irqsoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 6 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: bash-4269 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: copy_page_range
+ => ended at: copy_page_range
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ bash-4269 1...1 0us+: _spin_lock (copy_page_range)
+ bash-4269 1...1 7us : _spin_unlock (copy_page_range)
+ bash-4269 1...2 7us : trace_preempt_on (copy_page_range)
+
+
+vim:ft=help
+
+Here we see that that we had a latency of 6 microsecs (which is
+very good). The spin_lock in copy_page_range disabled interrupts.
+The difference between the 6 and the displayed timestamp 7us is
+because the clock must have incremented between the time of recording
+the max latency and recording the function that had that latency.
+
+Note the above had ftrace_enabled not set. If we set the ftrace_enabled
+we get a much larger output:
+
+# tracer: irqsoff
+#
+irqsoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: __alloc_pages_internal
+ => ended at: __alloc_pages_internal
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ ls-4339 0...1 0us+: get_page_from_freelist (__alloc_pages_internal)
+ ls-4339 0d..1 3us : rmqueue_bulk (get_page_from_freelist)
+ ls-4339 0d..1 3us : _spin_lock (rmqueue_bulk)
+ ls-4339 0d..1 4us : add_preempt_count (_spin_lock)
+ ls-4339 0d..2 4us : __rmqueue (rmqueue_bulk)
+ ls-4339 0d..2 5us : __rmqueue_smallest (__rmqueue)
+ ls-4339 0d..2 5us : __mod_zone_page_state (__rmqueue_smallest)
+ ls-4339 0d..2 6us : __rmqueue (rmqueue_bulk)
+ ls-4339 0d..2 6us : __rmqueue_smallest (__rmqueue)
+ ls-4339 0d..2 7us : __mod_zone_page_state (__rmqueue_smallest)
+ ls-4339 0d..2 7us : __rmqueue (rmqueue_bulk)
+ ls-4339 0d..2 8us : __rmqueue_smallest (__rmqueue)
+[...]
+ ls-4339 0d..2 46us : __rmqueue_smallest (__rmqueue)
+ ls-4339 0d..2 47us : __mod_zone_page_state (__rmqueue_smallest)
+ ls-4339 0d..2 47us : __rmqueue (rmqueue_bulk)
+ ls-4339 0d..2 48us : __rmqueue_smallest (__rmqueue)
+ ls-4339 0d..2 48us : __mod_zone_page_state (__rmqueue_smallest)
+ ls-4339 0d..2 49us : _spin_unlock (rmqueue_bulk)
+ ls-4339 0d..2 49us : sub_preempt_count (_spin_unlock)
+ ls-4339 0d..1 50us : get_page_from_freelist (__alloc_pages_internal)
+ ls-4339 0d..2 51us : trace_hardirqs_on (__alloc_pages_internal)
+
+
+vim:ft=help
+
+
+Here we traced a 50 microsecond latency. But we also see all the
+functions that were called during that time. Note that enabling
+function tracing we endure an added overhead. This overhead may
+extend the latency times. But never the less, this trace has provided
+some very helpful debugging.
+
+
+preemptoff
+----------
+
+When preemption is disabled we may be able to receive interrupts but
+the task can not 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 disables preemption.
+Like the irqsoff, it records the maximum latency that preemption
+was disabled. The control of preemptoff is much like the irqsoff.
+
+ # echo preemptoff > /debug/tracing/current_tracer
+ # echo 0 > /debug/tracing/tracing_max_latency
+ # echo 1 > /debug/tracing/tracing_enabled
+ # ls -ltr
+ [...]
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/latency_trace
+# tracer: preemptoff
+#
+preemptoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: do_IRQ
+ => ended at: __do_softirq
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ sshd-4261 0d.h. 0us+: irq_enter (do_IRQ)
+ sshd-4261 0d.s. 29us : _local_bh_enable (__do_softirq)
+ sshd-4261 0d.s1 30us : trace_preempt_on (__do_softirq)
+
+
+vim:ft=help
+
+This has some more changes. Preemption was disabled when an interrupt
+came in (notice the 'h'), and was enabled while doing a softirq.
+(notice the 's'). 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.
+
+# tracer: preemptoff
+#
+preemptoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: remove_wait_queue
+ => ended at: __do_softirq
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ sshd-4261 0d..1 0us : _spin_lock_irqsave (remove_wait_queue)
+ sshd-4261 0d..1 1us : _spin_unlock_irqrestore (remove_wait_queue)
+ sshd-4261 0d..1 2us : do_IRQ (common_interrupt)
+ sshd-4261 0d..1 2us : irq_enter (do_IRQ)
+ sshd-4261 0d..1 2us : idle_cpu (irq_enter)
+ sshd-4261 0d..1 3us : add_preempt_count (irq_enter)
+ sshd-4261 0d.h1 3us : idle_cpu (irq_enter)
+ sshd-4261 0d.h. 4us : handle_fasteoi_irq (do_IRQ)
+[...]
+ sshd-4261 0d.h. 12us : add_preempt_count (_spin_lock)
+ sshd-4261 0d.h1 12us : ack_ioapic_quirk_irq (handle_fasteoi_irq)
+ sshd-4261 0d.h1 13us : move_native_irq (ack_ioapic_quirk_irq)
+ sshd-4261 0d.h1 13us : _spin_unlock (handle_fasteoi_irq)
+ sshd-4261 0d.h1 14us : sub_preempt_count (_spin_unlock)
+ sshd-4261 0d.h1 14us : irq_exit (do_IRQ)
+ sshd-4261 0d.h1 15us : sub_preempt_count (irq_exit)
+ sshd-4261 0d..2 15us : do_softirq (irq_exit)
+ sshd-4261 0d... 15us : __do_softirq (do_softirq)
+ sshd-4261 0d... 16us : __local_bh_disable (__do_softirq)
+ sshd-4261 0d... 16us+: add_preempt_count (__local_bh_disable)
+ sshd-4261 0d.s4 20us : add_preempt_count (__local_bh_disable)
+ sshd-4261 0d.s4 21us : sub_preempt_count (local_bh_enable)
+ sshd-4261 0d.s5 21us : sub_preempt_count (local_bh_enable)
+[...]
+ sshd-4261 0d.s6 41us : add_preempt_count (__local_bh_disable)
+ sshd-4261 0d.s6 42us : sub_preempt_count (local_bh_enable)
+ sshd-4261 0d.s7 42us : sub_preempt_count (local_bh_enable)
+ sshd-4261 0d.s5 43us : add_preempt_count (__local_bh_disable)
+ sshd-4261 0d.s5 43us : sub_preempt_count (local_bh_enable_ip)
+ sshd-4261 0d.s6 44us : sub_preempt_count (local_bh_enable_ip)
+ sshd-4261 0d.s5 44us : add_preempt_count (__local_bh_disable)
+ sshd-4261 0d.s5 45us : sub_preempt_count (local_bh_enable)
+[...]
+ sshd-4261 0d.s. 63us : _local_bh_enable (__do_softirq)
+ sshd-4261 0d.s1 64us : trace_preempt_on (__do_softirq)
+
+
+The above is an example of the preemptoff trace with ftrace_enabled
+set. Here we see that interrupts were 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 by the functions themselves that
+this is not the case.
+
+Notice that the __do_softirq when called doesn't have a preempt_count.
+It may seem that we missed a preempt enabled. What really happened
+is that the preempt count is held on the threads stack and we
+switched to the softirq stack (4K stacks in effect). The code
+does not copy the preempt count, but because interrupts are disabled
+we don't need to worry about it. Having a tracer like this is good
+to let people know what really happens inside the kernel.
+
+
+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.
+
+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 preemptoff > /debug/tracing/current_tracer
+ # echo 0 > /debug/tracing/tracing_max_latency
+ # echo 1 > /debug/tracing/tracing_enabled
+ # ls -ltr
+ [...]
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/latency_trace
+# tracer: preemptirqsoff
+#
+preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: apic_timer_interrupt
+ => ended at: __do_softirq
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ ls-4860 0d... 0us!: trace_hardirqs_off_thunk (apic_timer_interrupt)
+ ls-4860 0d.s. 294us : _local_bh_enable (__do_softirq)
+ ls-4860 0d.s1 294us : trace_preempt_on (__do_softirq)
+
+
+vim:ft=help
+
+
+The trace_hardirqs_off_thunk is called from assembly on x86 when
+interrupts are disabled in the assembly code. Without the function
+tracing, we don't know if interrupts were enabled within the preemption
+points. We do see that it started with preemption enabled.
+
+Here is a trace with ftrace_enabled set:
+
+
+# tracer: preemptirqsoff
+#
+preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: write_chan
+ => ended at: __do_softirq
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ ls-4473 0.N.. 0us : preempt_schedule (write_chan)
+ ls-4473 0dN.1 1us : _spin_lock (schedule)
+ ls-4473 0dN.1 2us : add_preempt_count (_spin_lock)
+ ls-4473 0d..2 2us : put_prev_task_fair (schedule)
+[...]
+ ls-4473 0d..2 13us : set_normalized_timespec (ktime_get_ts)
+ ls-4473 0d..2 13us : __switch_to (schedule)
+ sshd-4261 0d..2 14us : finish_task_switch (schedule)
+ sshd-4261 0d..2 14us : _spin_unlock_irq (finish_task_switch)
+ sshd-4261 0d..1 15us : add_preempt_count (_spin_lock_irqsave)
+ sshd-4261 0d..2 16us : _spin_unlock_irqrestore (hrtick_set)
+ sshd-4261 0d..2 16us : do_IRQ (common_interrupt)
+ sshd-4261 0d..2 17us : irq_enter (do_IRQ)
+ sshd-4261 0d..2 17us : idle_cpu (irq_enter)
+ sshd-4261 0d..2 18us : add_preempt_count (irq_enter)
+ sshd-4261 0d.h2 18us : idle_cpu (irq_enter)
+ sshd-4261 0d.h. 18us : handle_fasteoi_irq (do_IRQ)
+ sshd-4261 0d.h. 19us : _spin_lock (handle_fasteoi_irq)
+ sshd-4261 0d.h. 19us : add_preempt_count (_spin_lock)
+ sshd-4261 0d.h1 20us : _spin_unlock (handle_fasteoi_irq)
+ sshd-4261 0d.h1 20us : sub_preempt_count (_spin_unlock)
+[...]
+ sshd-4261 0d.h1 28us : _spin_unlock (handle_fasteoi_irq)
+ sshd-4261 0d.h1 29us : sub_preempt_count (_spin_unlock)
+ sshd-4261 0d.h2 29us : irq_exit (do_IRQ)
+ sshd-4261 0d.h2 29us : sub_preempt_count (irq_exit)
+ sshd-4261 0d..3 30us : do_softirq (irq_exit)
+ sshd-4261 0d... 30us : __do_softirq (do_softirq)
+ sshd-4261 0d... 31us : __local_bh_disable (__do_softirq)
+ sshd-4261 0d... 31us+: add_preempt_count (__local_bh_disable)
+ sshd-4261 0d.s4 34us : add_preempt_count (__local_bh_disable)
+[...]
+ sshd-4261 0d.s3 43us : sub_preempt_count (local_bh_enable_ip)
+ sshd-4261 0d.s4 44us : sub_preempt_count (local_bh_enable_ip)
+ sshd-4261 0d.s3 44us : smp_apic_timer_interrupt (apic_timer_interrupt)
+ sshd-4261 0d.s3 45us : irq_enter (smp_apic_timer_interrupt)
+ sshd-4261 0d.s3 45us : idle_cpu (irq_enter)
+ sshd-4261 0d.s3 46us : add_preempt_count (irq_enter)
+ sshd-4261 0d.H3 46us : idle_cpu (irq_enter)
+ sshd-4261 0d.H3 47us : hrtimer_interrupt (smp_apic_timer_interrupt)
+ sshd-4261 0d.H3 47us : ktime_get (hrtimer_interrupt)
+[...]
+ sshd-4261 0d.H3 81us : tick_program_event (hrtimer_interrupt)
+ sshd-4261 0d.H3 82us : ktime_get (tick_program_event)
+ sshd-4261 0d.H3 82us : ktime_get_ts (ktime_get)
+ sshd-4261 0d.H3 83us : getnstimeofday (ktime_get_ts)
+ sshd-4261 0d.H3 83us : set_normalized_timespec (ktime_get_ts)
+ sshd-4261 0d.H3 84us : clockevents_program_event (tick_program_event)
+ sshd-4261 0d.H3 84us : lapic_next_event (clockevents_program_event)
+ sshd-4261 0d.H3 85us : irq_exit (smp_apic_timer_interrupt)
+ sshd-4261 0d.H3 85us : sub_preempt_count (irq_exit)
+ sshd-4261 0d.s4 86us : sub_preempt_count (irq_exit)
+ sshd-4261 0d.s3 86us : add_preempt_count (__local_bh_disable)
+[...]
+ sshd-4261 0d.s1 98us : sub_preempt_count (net_rx_action)
+ sshd-4261 0d.s. 99us : add_preempt_count (_spin_lock_irq)
+ sshd-4261 0d.s1 99us+: _spin_unlock_irq (run_timer_softirq)
+ sshd-4261 0d.s. 104us : _local_bh_enable (__do_softirq)
+ sshd-4261 0d.s. 104us : sub_preempt_count (_local_bh_enable)
+ sshd-4261 0d.s. 105us : _local_bh_enable (__do_softirq)
+ sshd-4261 0d.s1 105us : trace_preempt_on (__do_softirq)
+
+
+This is a very interesting trace. It started with the preemption of
+the ls task. We see that the task had the "need_resched" bit set
+with the 'N' in the trace. Interrupts are disabled in the spin_lock
+and the trace started. We see that a schedule took place to run
+sshd. When the interrupts were enabled we took an interrupt.
+On return of the interrupt the softirq ran. We took another interrupt
+while running the softirq as we see with the capital 'H'.
+
+
+wakeup
+------
+
+In Real-Time environment it is very important to know the wakeup
+time it takes for the highest priority task that wakes up to the
+time 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 is more appropriate for such measurements.
+
+Real-Time environments is 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 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.
+
+Since this tracer only deals with RT tasks, we will run this slightly
+different 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 wakeup > /debug/tracing/current_tracer
+ # echo 0 > /debug/tracing/tracing_max_latency
+ # echo 1 > /debug/tracing/tracing_enabled
+ # chrt -f 5 sleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/latency_trace
+# tracer: wakeup
+#
+wakeup latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: sleep-4901 (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 1d.h4 0us+: try_to_wake_up (wake_up_process)
+ <idle>-0 1d..4 4us : schedule (cpu_idle)
+
+
+vim:ft=help
+
+
+Running this on an idle system we see that it only took 4 microseconds
+to perform the task switch. Note, since the trace marker 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 4901 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.
+
+Doing the same with chrt -r 5 and ftrace_enabled set.
+
+# tracer: wakeup
+#
+wakeup latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5)
+ -----------------
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ksoftirq-7 1d.H3 0us : try_to_wake_up (wake_up_process)
+ksoftirq-7 1d.H4 1us : sub_preempt_count (marker_probe_cb)
+ksoftirq-7 1d.H3 2us : check_preempt_wakeup (try_to_wake_up)
+ksoftirq-7 1d.H3 3us : update_curr (check_preempt_wakeup)
+ksoftirq-7 1d.H3 4us : calc_delta_mine (update_curr)
+ksoftirq-7 1d.H3 5us : __resched_task (check_preempt_wakeup)
+ksoftirq-7 1d.H3 6us : task_wake_up_rt (try_to_wake_up)
+ksoftirq-7 1d.H3 7us : _spin_unlock_irqrestore (try_to_wake_up)
+[...]
+ksoftirq-7 1d.H2 17us : irq_exit (smp_apic_timer_interrupt)
+ksoftirq-7 1d.H2 18us : sub_preempt_count (irq_exit)
+ksoftirq-7 1d.s3 19us : sub_preempt_count (irq_exit)
+ksoftirq-7 1..s2 20us : rcu_process_callbacks (__do_softirq)
+[...]
+ksoftirq-7 1..s2 26us : __rcu_process_callbacks (rcu_process_callbacks)
+ksoftirq-7 1d.s2 27us : _local_bh_enable (__do_softirq)
+ksoftirq-7 1d.s2 28us : sub_preempt_count (_local_bh_enable)
+ksoftirq-7 1.N.3 29us : sub_preempt_count (ksoftirqd)
+ksoftirq-7 1.N.2 30us : _cond_resched (ksoftirqd)
+ksoftirq-7 1.N.2 31us : __cond_resched (_cond_resched)
+ksoftirq-7 1.N.2 32us : add_preempt_count (__cond_resched)
+ksoftirq-7 1.N.2 33us : schedule (__cond_resched)
+ksoftirq-7 1.N.2 33us : add_preempt_count (schedule)
+ksoftirq-7 1.N.3 34us : hrtick_clear (schedule)
+ksoftirq-7 1dN.3 35us : _spin_lock (schedule)
+ksoftirq-7 1dN.3 36us : add_preempt_count (_spin_lock)
+ksoftirq-7 1d..4 37us : put_prev_task_fair (schedule)
+ksoftirq-7 1d..4 38us : update_curr (put_prev_task_fair)
+[...]
+ksoftirq-7 1d..5 47us : _spin_trylock (tracing_record_cmdline)
+ksoftirq-7 1d..5 48us : add_preempt_count (_spin_trylock)
+ksoftirq-7 1d..6 49us : _spin_unlock (tracing_record_cmdline)
+ksoftirq-7 1d..6 49us : sub_preempt_count (_spin_unlock)
+ksoftirq-7 1d..4 50us : schedule (__cond_resched)
+
+The interrupt went off while running ksoftirqd. This task runs at
+SCHED_OTHER. Why didn't we see the 'N' set early? This may be
+a harmless bug with x86_32 and 4K stacks. The need_reched() function
+that tests if we need to reschedule looks on the actual stack.
+Where as the setting of the NEED_RESCHED bit happens on the
+task's stack. But because we are in a hard interrupt, the test
+is with the interrupts stack which has that to be false. We don't
+see the 'N' until we switch back to the task's stack.
+
+ftrace
+------
+
+ftrace is not only the name of the tracing infrastructure, but it
+is also a name of one of the tracers. The 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.
+
+ # sysctl kernel.ftrace_enabled=1
+ # echo ftrace > /debug/tracing/current_tracer
+ # echo 1 > /debug/tracing/tracing_enabled
+ # usleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/trace
+# tracer: ftrace
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+ bash-4003 [00] 123.638713: finish_task_switch <-schedule
+ bash-4003 [00] 123.638714: _spin_unlock_irq <-finish_task_switch
+ bash-4003 [00] 123.638714: sub_preempt_count <-_spin_unlock_irq
+ bash-4003 [00] 123.638715: hrtick_set <-schedule
+ bash-4003 [00] 123.638715: _spin_lock_irqsave <-hrtick_set
+ bash-4003 [00] 123.638716: add_preempt_count <-_spin_lock_irqsave
+ bash-4003 [00] 123.638716: _spin_unlock_irqrestore <-hrtick_set
+ bash-4003 [00] 123.638717: sub_preempt_count <-_spin_unlock_irqrestore
+ bash-4003 [00] 123.638717: hrtick_clear <-hrtick_set
+ bash-4003 [00] 123.638718: sub_preempt_count <-schedule
+ bash-4003 [00] 123.638718: sub_preempt_count <-preempt_schedule
+ bash-4003 [00] 123.638719: wait_for_completion <-__stop_machine_run
+ bash-4003 [00] 123.638719: wait_for_common <-wait_for_completion
+ bash-4003 [00] 123.638720: _spin_lock_irq <-wait_for_common
+ bash-4003 [00] 123.638720: add_preempt_count <-_spin_lock_irq
+[...]
+
+
+Note: It is sometimes better to enable or disable tracing directly from
+a program, because the buffer may be overflowed by the echo commands
+before you get to the point you want to trace. It is also easier to
+stop the tracing at the point that you hit the part that you are
+interested in. Since the ftrace buffer is a ring buffer with the
+oldest data being overwritten, usually it is sufficient to start the
+tracer with an echo command but have you code stop it. Something
+like the following is usually appropriate for this.
+
+int trace_fd;
+[...]
+int main(int argc, char *argv[]) {
+ [...]
+ trace_fd = open("/debug/tracing/tracing_enabled", O_WRONLY);
+ [...]
+ if (condition_hit()) {
+ write(trace_fd, "0", 1);
+ }
+ [...]
+}
+
+
+dynamic ftrace
+--------------
+
+If CONFIG_DYNAMIC_FTRACE is set, then 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.
+
+When dynamic ftrace is initialized, it calls kstop_machine to make it
+act like a uniprocessor so that it can freely modify code without
+worrying about other processors executing that same code. At
+initialization, the mcount calls are change to call a "record_ip"
+function. After this, the first time a kernel function is called,
+it has the calling address saved in a hash table.
+
+Later on the ftraced kernel thread is awoken and will again call
+kstop_machine if new functions have been recorded. The ftraced thread
+will change all calls to mcount to "nop". Just calling mcount
+and having mcount return has shown a 10% overhead. By converting
+it to a nop, there is no recordable overhead to the system.
+
+One special side-effect to the recording of the functions being
+traced, is that we can now selectively choose which functions we
+want to trace and which ones we want the mcount calls to remain as
+nops.
+
+Two files that contain to the enabling and disabling of recorded
+functions are:
+
+ set_ftrace_filter
+
+and
+
+ set_ftrace_notrace
+
+A list of available functions that you can add to this files is listed
+in:
+
+ available_filter_functions
+
+ # cat /debug/tracing/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'm only interested in sys_nanosleep and hrtimer_interrupt:
+
+ # echo sys_nanosleep hrtimer_interrupt \
+ > /debug/tracing/set_ftrace_filter
+ # echo ftrace > /debug/tracing/current_tracer
+ # echo 1 > /debug/tracing/tracing_enabled
+ # usleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/trace
+# tracer: ftrace
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+ usleep-4134 [00] 1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt
+ usleep-4134 [00] 1317.070111: sys_nanosleep <-syscall_call
+ <idle>-0 [00] 1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt
+
+To see what functions are being traced, you can cat the file:
+
+ # cat /debug/tracing/set_ftrace_filter
+hrtimer_interrupt
+sys_nanosleep
+
+
+Perhaps this isn't enough. The filters also allow simple wild cards.
+Only the following is currently available
+
+ <match>* - will match functions that begins with <match>
+ *<match> - will match functions that end with <match>
+ *<match>* - will match functions that have <match> in it
+
+Thats all the wild cards that are allowed.
+
+ <match>*<match> will not work.
+
+ # echo hrtimer_* > /debug/tracing/set_ftrace_filter
+
+Produces:
+
+# tracer: ftrace
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+ bash-4003 [00] 1480.611794: hrtimer_init <-copy_process
+ bash-4003 [00] 1480.611941: hrtimer_start <-hrtick_set
+ bash-4003 [00] 1480.611956: hrtimer_cancel <-hrtick_clear
+ bash-4003 [00] 1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel
+ <idle>-0 [00] 1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt
+ <idle>-0 [00] 1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt
+ <idle>-0 [00] 1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt
+ <idle>-0 [00] 1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt
+ <idle>-0 [00] 1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt
+
+
+Notice that we lost the sys_nanosleep.
+
+ # cat /debug/tracing/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 > /debug/tracing/set_ftrace_filter
+ # cat /debug/tracing/set_ftrace_filter
+ #
+
+Again, now we want to append.
+
+ # echo sys_nanosleep > /debug/tracing/set_ftrace_filter
+ # cat /debug/tracing/set_ftrace_filter
+sys_nanosleep
+ # echo hrtimer_* >> /debug/tracing/set_ftrace_filter
+ # cat /debug/tracing/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*' > /debug/tracing/set_ftrace_notrace
+
+Produces:
+
+# tracer: ftrace
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+ bash-4043 [01] 115.281644: finish_task_switch <-schedule
+ bash-4043 [01] 115.281645: hrtick_set <-schedule
+ bash-4043 [01] 115.281645: hrtick_clear <-hrtick_set
+ bash-4043 [01] 115.281646: wait_for_completion <-__stop_machine_run
+ bash-4043 [01] 115.281647: wait_for_common <-wait_for_completion
+ bash-4043 [01] 115.281647: kthread_stop <-stop_machine_run
+ bash-4043 [01] 115.281648: init_waitqueue_head <-kthread_stop
+ bash-4043 [01] 115.281648: wake_up_process <-kthread_stop
+ bash-4043 [01] 115.281649: try_to_wake_up <-wake_up_process
+
+We can see that there's no more lock or preempt tracing.
+
+ftraced
+-------
+
+As mentioned above, when dynamic ftrace is configured in, a kernel
+thread wakes up once a second and checks to see if there are mcount
+calls that need to be converted into nops. If there is not, then
+it simply goes back to sleep. But if there is, it will call
+kstop_machine to convert the calls to nops.
+
+There may be a case that you do not want this added latency.
+Perhaps you are doing some audio recording and this activity might
+cause skips in the playback. There is an interface to disable
+and enable the ftraced kernel thread.
+
+ # echo 0 > /debug/tracing/ftraced_enabled
+
+This will disable the calling of the kstop_machine to update the
+mcount calls to nops. Remember that there's a large overhead
+to calling mcount. Without this kernel thread, that overhead will
+exist.
+
+Any write to the ftraced_enabled file will cause the kstop_machine
+to run if there are recorded calls to mcount. This means that a
+user can manually perform the updates when they want to by simply
+echoing a '0' into the ftraced_enabled file.
+
+The updates are also done at the beginning of enabling a tracer
+that uses ftrace function recording.
+
+
+trace_pipe
+----------
+
+The trace_pipe outputs the same as trace, 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 ftrace > /debug/tracing/current_tracer
+ # cat /debug/tracing/trace_pipe > /tmp/trace.out &
+[1] 4153
+ # echo 1 > /debug/tracing/tracing_enabled
+ # usleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/trace
+# tracer: ftrace
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+
+ #
+ # cat /tmp/trace.out
+ bash-4043 [00] 41.267106: finish_task_switch <-schedule
+ bash-4043 [00] 41.267106: hrtick_set <-schedule
+ bash-4043 [00] 41.267107: hrtick_clear <-hrtick_set
+ bash-4043 [00] 41.267108: wait_for_completion <-__stop_machine_run
+ bash-4043 [00] 41.267108: wait_for_common <-wait_for_completion
+ bash-4043 [00] 41.267109: kthread_stop <-stop_machine_run
+ bash-4043 [00] 41.267109: init_waitqueue_head <-kthread_stop
+ bash-4043 [00] 41.267110: wake_up_process <-kthread_stop
+ bash-4043 [00] 41.267110: try_to_wake_up <-wake_up_process
+ bash-4043 [00] 41.267111: select_task_rq_rt <-try_to_wake_up
+
+
+Note, reading the trace_pipe will block until more input is added.
+By changing the tracer, trace_pipe will issue an EOF. We needed
+to set the ftrace tracer _before_ cating the trace_pipe file.
+
+
+trace entries
+-------------
+
+Having too much or not enough data can be troublesome in diagnosing
+some issue in the kernel. The file trace_entries is used to modify
+the size of the internal trace buffers. The numbers 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 /debug/tracing/trace_entries
+65620
+
+Note, to modify this you must have tracing fulling disabled. To do that,
+echo "none" into the current_tracer.
+
+ # echo none > /debug/tracing/current_tracer
+ # echo 100000 > /debug/tracing/trace_entries
+ # cat /debug/tracing/trace_entries
+100045
+
+
+Notice that we echoed in 100,000 but the size is 100,045. The entries
+are held by individual pages. It allocates the number of pages it takes
+to fulfill the request. If more entries may fit on the last page
+it will add them.
+
+ # echo 1 > /debug/tracing/trace_entries
+ # cat /debug/tracing/trace_entries
+85
+
+This shows us that 85 entries can fit on a single page.
+
+The number of pages that will be allocated is a percentage of available
+memory. Allocating too much will produces an error.
+
+ # echo 1000000000000 > /debug/tracing/trace_entries
+-bash: echo: write error: Cannot allocate memory
+ # cat /debug/tracing/trace_entries
+85
+
--
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