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Date: Fri, 11 Jul 2008 12:16:55 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Randy Dunlap <randy.dunlap@...cle.com>,
Elias Oltmanns <eo@...ensachen.de>,
LKML <linux-kernel@...r.kernel.org>, 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>,
Jon Masters <jonathan@...masters.org>
Subject: Re: [PATCH -v2] ftrace: Documentation
On Thu, 10 Jul 2008 20:37:19 -0400 (EDT) Steven Rostedt <rostedt@...dmis.org> wrote:
>
> This is the long awaited ftrace.txt. It explains in quite detail how to
> use ftrace.
>
> Updated with comments from Elias Oltmann and Randy Dunlap.
>
> Signed-off-by: Steven Rostedt <srostedt@...hat.com>
> ---
> Documentation/ftrace.txt | 1361 +++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 1361 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 20:18:33.000000000 -0400
> @@ -0,0 +1,1361 @@
> + ftrace - Function Tracer
> + ========================
> +
> +Copyright 2008 Red Hat Inc.
> + Author: Steven Rostedt <srostedt@...hat.com>
> + License: The GNU Free Documentation License, Version 1.2
> +Reviewers: Elias Oltmanns and Randy Dunlap
> +
> +Writen for: 2.6.26-rc8 linux-2.6-tip.git tip/tracing/ftrace branch
> +
> +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
grammar bustage here
> +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.
Please enumerate "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
This should be a reference to the debugfs documentation (rofl?) rather than
a reimplementation of it.
> +
> +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
> + have been compiled into the kernel. The tracers
> + listed here can be configured by echoing in their
s/in//
> + 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.
kernel should only permit 0 or 1 (IMO). This is a kernel ABI, not a C
program.
> + trace : This file holds the output of the trace in a human readable
> + format.
"described below" (I hope)
> + 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.
We have three slightly munged versions of the same data? wtf?
Would it not be better to present all the data a single time and perform
post-processing in userspace?
It's scary, but we _can_ ship userspace code. getdelays.c has worked well.
Baby steps.
> + 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.
hrm.
> 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.
hrm. Side note: it is sad that we are learning fundamental design
decisions ages after the code was put into mainline. How did this happen?
In a better world we'd have seen this document before coding started! Or
at least prior to merging.
> + 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.
I don't understand "A, so care must be taken when B". Why must care be
taken? What might happen if I was careless? How do I take care? Type
slowly? ;)
> The trace
> + buffers are allocated in pages (blocks of memory that
> + the kernel uses for allocation, usually 4 KB in size).
> + Since each entry is smaller than a page, if the last
> + allocated page has room for more entries than were
> + requested, the rest of the page is used to allocate
> + entries.
> +
> + 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.
Why is this feature useful? (I'd have asked this prior to merging, if I'd
known it existed!)
> + set_ftrace_filter : When dynamic ftrace is configured in, the
I guess we'll learn later what "dynamic" ftrace is.
> + code is dynamically modified to disable calling
> + of the function profiler (mcount).
What does "dynamically modified" mean here? text rewriting?
> 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
s/in//
> + file will limit the trace to only these functions.
s/these/those/
> + set_ftrace_notrace: This has the opposite effect that
> + set_ftrace_filter has.
"This has an effect opposite to that of set_ftrace_filter", perhaps.
> 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.
> +
> + 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.
My head just spun off. Perhaps some more details here?
> +
> +The Tracers
> +-----------
> +
> +Here are the list of current tracers that can be configured.
s/are/is/
s/can/may/
> +
> + ftrace - function tracer that uses mcount to trace all functions.
> + It is possible to filter out which functions that are
s/that//
What does "filter out" mean here? I asusme that they are omitted? A bit
unclear.
> + to be traced when dynamic ftrace is configured in.
"a function tracer which"
> + 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.
s/off//
> + 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.
s/with/via/
> + preemptoff - Similar to irqsoff but traces and records the time
> + preemption is disabled.
s/the time/the amount of time for which/
> + preemptirqsoff - Similar to irqsoff and preemptoff, but traces and
> + records the largest time irqs and/or preemption is
> + disabled.
s/time/time for which/
This interface has a strange mix of wordsruntogether and
words_separated_by_underscores. Oh well - another consequence of
post-facto changelogging.
> + 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.
Does the system then run at full performance levels?
> +
> +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).
s/with only controlling them/when controlling them only/
> +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
> + --------
pids are no longer unique system-wide, and any part of the kernel ABI which
exports them to userspace is, basically, broken. Oh well.
> +A header is printed with the trace that is represented.
s/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".
Please spell out what the timestamp represents. The time at which that
function was entered?
> +The sched_switch tracer also includes tracing of task wake ups and
"wakeups" would be a more typical spelling.
> +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:
s/show/are shown as/
> +
> + 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
s/inverse to/the inverse of/
> +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).
Would it not be better to convert these back into their userland
representation or userland presentation?
> +
> +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
s/a bit/somewhat/ (IMO)
> +trace.
General nit: apostrophes are suitable for conversation, but not for formal
docmentation. Please consider s/'s/ is/g.
> +# 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)
The kernel prints all that stuff out of a debugfs file?
What have we done? :(
> +
> +vim:ft=help
What's this?
> +
> +
> +This shows that the current tracer is "irqsoff" tracing the time
s/time/time for which/
> +interrupts are disabled. It gives the trace version and the kernel
s/are/were/
s/kernel/version of the kernel upon which/
> +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
s/by/and/???
> +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).
s/reserved/are reserved/
> +
> +The task is the process that was running when the latency happened.
s/happened/occurred/
> +(swapper pid: 0).
> +
> +The start and stop that caused the latencies:
"start and stop" what? events? function calls?
> +
> + 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.
s/that/which/
> +
> + 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 file output. The trace file output
> + included an absolute timestamp. The timestamp used by the
s/included/includes/?
> + latency_trace file is relative to the start of 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.
eh?
> + The marks are 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 prepended with "no".
> +
> + echo noprint-parent > /debug/tracing/iter_ctrl
> +
> +To enable an option, leave off the "no".
> +
> + echo sym-offset > /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.
ooh, does this mean that we get to delete all the other interfaces?
I mean, honestly, all this pretty-printing and post-processing could and
should have been done in userspace. As much as poss. We suck.
> + hex - Similar to raw, but the numbers will be in a hexadecimal format.
Does really this need to exist? Again, it is the sort of thing which would
have needed justification during the pre-merge review. But afaik it was
jammed into the tree without knowledge or comment.
There are lessons here.
> + bin - This will print out the formats in raw binary.
I don't understand this.
> + 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
> +of how to use 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 '+')
s/sched_switch/sched_switch file/?
> +and context switches ('==>') with the previous task or current
s/current/current task/?
> +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)
"uninterruptible sleep", please. no need to invent new (and hence
unfamilar) terms!
> + 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.
What are "the following tracers" here?
> +
> +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 to the time
"te time for which interrupts are disabled." will suffice.
> +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.
"the tracer saves the stack trace leading up to that latency point so that"
> 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)
istr writing stuff which does this in 1999 ;)
> +
> +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.
spin_lock disables interrutps?
> +The difference between the 6 and the displayed timestamp 7us is
> +because
"occurred because"
> the clock must have incremented between the time of recording
s/must have/was/
> +the max latency and recording the function that had that latency.
s/recording/the time of recording/
> +
> +Note the above had ftrace_enabled not set. If we set the ftrace_enabled,
s/above/above example/
> +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 by enabling
> +function tracing, we endure an added overhead. This overhead may
s/endure/incur/
> +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, it records the maximum latency that preemption
s/that/for which/
> +was disabled. The control of preemptoff is much like the irqsoff.
s/the//
> + # 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
s/by/from/
> +this is not the case.
> +
> +Notice that the __do_softirq when called doesn't have a preempt_count.
s/the//
> +It may seem that we missed a preempt enabled. What really happened
s/enabled/enabling/?
> +is that the preempt count is held on the threads stack and we
s/threads/thread's/
> +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.
s/to let/for letting/
> +
> +
> +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:
s/The/Consider the/?
> +
> + 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 preemptirqsoff > /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
s/with/via/
> +and the trace started. We see that a schedule took place to run
s/started/is started/? (unclear)
> +sshd. When the interrupts were enabled, we took an interrupt.
> +On return from the interrupt handler, the softirq ran. We took another
> +interrupt while running the softirq as we see with the capital 'H'.
s/with/from/
> +
> +
> +wakeup
> +------
> +
> +In Real-Time environment it is very important to know the wakeup
s/In/In a/
> +time it takes for the highest priority task that wakes up to the
s/wakes up/is woken up/
> +time it executes. This is also known as "schedule latency".
s/time/time that/
> +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 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
> +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 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. On x86_32 with 4K stacks
> +configured, the interrupt and softirq runs with their own stack.
> +Some information is held on the top of the task's stack (need_resched
> +and preempt_count are both stored there). The setting of the NEED_RESCHED
> +bit is done directly to the task's stack, but the reading of the
> +NEED_RESCHED is done by looking at the current stack, which in this case
> +is the stack for the hard interrupt. This hides the fact that NEED_RESCHED
> +has been set. We don't see the 'N' until we switch back to the task's
> +assigned 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.
What does this mean?
> It is also easier to
> +stop the tracing at the point that you hit the part that you are
s/at the point that/when/? (unclear)
> +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
s/you/your/?
(better would be "the controlling application")
> +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
s/then//
> +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.
So some config option enabled -pg?
> +When dynamic ftrace is initialized, it calls kstop_machine to make
> +the machine 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 changed 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.
s/recordable/measurable/?
> +One special side-effect to the recording of the functions being
> +traced, is that we can now selectively choose which functions we
s/,//
> +want to trace and which ones we want the mcount calls to remain as
s/want/wish/g
> +nops.
> +
> +Two files are used, one for enabling and one for disabling the tracing
> +of recorded functions. They are:
"tracing of recorded" doesn't make a lot of sense. "recording of traced"?
> + 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 /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:
s/what/which/
> + # 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 are currently available
> +
> + <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
> +
> +Thats all the wild cards that are allowed.
"These are the only wildcards which are supported"?
> + <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 are not any, then
> +it simply goes back to sleep. But if there are some, it will call
> +kstop_machine to convert the calls to nops.
> +
> +There may be a case that you do not want this added latency.
s/that/in which/
> +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.
Oh. Is the term "ftraced" the name of a kernel thread? I'd been thinking
it referred to "something which is being ftraced".
> +
> + # echo 0 > /debug/tracing/ftraced_enabled
> +
> +This will disable the calling of the kstop_machine to update the
"of kstop_machine"
> +mcount calls to nops. Remember that there's a large overhead
> +to calling mcount. Without this kernel thread, that overhead will
> +exist.
> +
> +If there are recorded calls to mcount, any write to the ftraced_enabled
> +file will cause the kstop_machine to run. 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,
"The trace_pipe file has the same contents as the trace file"? (unclear)
> 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.
"the trace_pipe file"
> +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
s/some/an/
> +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.
How do I know the number of possible CPUs? Within an order of magnitude?
Is it in dmesg, perhaps?
> + # cat /debug/tracing/trace_entries
> +65620
> +
> +Note, to modify this, you must have tracing completely disabled. To do that,
> +echo "none" into the current_tracer.
What happens if I forgot?
> + # 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
s/by/in/
> +to fulfill the request. If more entries may fit on the last page
> +it will add them.
s/it will add them/then they will be added/
> + # echo 1 > /debug/tracing/trace_entries
> + # cat /debug/tracing/trace_entries
> +85
> +
> +This shows us that 85 entries can fit on a single page.
s/on/in/
> +The number of pages that will be allocated is a percentage of available
s/that/which/
s/is a/is limited to a/? (unclear)
> +memory. Allocating too much will produce an error.
s/much/many/
> + # echo 1000000000000 > /debug/tracing/trace_entries
> +-bash: echo: write error: Cannot allocate memory
> + # cat /debug/tracing/trace_entries
> +85
> +
>
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