Re: [PATCH -v2] ftrace: Documentation

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 10 Jul 2008 20:37:19 -0400 (EDT) Steven Rostedt <rostedt@goodmis.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@redhat.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@redhat.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
> +
>