path: root/Documentation/ftrace.txt

		ftrace - Function Tracer
		========================

Copyright 2008 Red Hat Inc.
Author: Steven Rostedt <srostedt@redhat.com>


Introduction
------------

Ftrace is an internal tracer designed to help out developers and
designers of systems to find what is going on inside the kernel.
It can be used for debugging or analyzing latencies and performance
issues that take place outside of user-space.

Although ftrace is the function tracer, it also includes an
infrastructure that allows for other types of tracing. Some of the
tracers that are currently in ftrace is a tracer to trace
context switches, the time it takes for a high priority task to
run after it was woken up, the time interrupts are disabled, and
more.


The File System
---------------

Ftrace uses the debugfs file system to hold the control files as well
as the files to display output.

To mount the debugfs system:

  # mkdir /debug
  # mount -t debugfs nodev /debug


That's it! (assuming that you have ftrace configured into your kernel)

After mounting the debugfs, you can see a directory called
"tracing".  This directory contains the control and output files
of ftrace. Here is a list of some of the key files:


 Note: all time values are in microseconds.

  current_tracer : This is used to set or display the current tracer
		that is configured.

  available_tracers : This holds the different types of tracers that
		has been compiled into the kernel. The tracers
		listed here can be configured by echoing in their
		name into current_tracer.

  tracing_enabled : This sets or displays whether the current_tracer
		is activated and tracing or not. Echo 0 into this
		file to disable the tracer or 1 (or non-zero) to
		enable it.

  trace : This file holds the output of the trace in a human readable
		format.

  latency_trace : This file shows the same trace but the information
		is organized more to display possible latencies
		in the system.

  trace_pipe : The output is the same as the "trace" file but this
		file is meant to be streamed with live tracing.
		Reads from this file will block until new data
		is retrieved. Unlike the "trace" and "latency_trace"
		files, this file is a consumer. This means reading
		from this file causes sequential reads to display
		more current data. Once data is read from this
		file, it is consumed, and will not be read
		again with a sequential read. The "trace" and
		"latency_trace" files are static, and if the
		tracer isn't adding more data, they will display
		the same information every time they are read.

  iter_ctrl : This file lets the user control the amount of data
		that is displayed in one of the above output
		files.

  trace_max_latency : Some of the tracers record the max latency.
		For example, the time interrupts are disabled.
		This time is saved in this file. The max trace
		will also be stored, and displayed by either
		"trace" or "latency_trace".  A new max trace will
		only be recorded if the latency is greater than
		the value in this file. (in microseconds)

  trace_entries : This sets or displays the number of trace
		entries each CPU buffer can hold. The tracer buffers
		are the same size for each CPU, so care must be
		taken when modifying the trace_entries. The number
		of actually entries will be the number given
		times the number of possible CPUS. The buffers
		are saved as individual pages, and the actual entries
		will always be rounded up to entries per page.

		This can only be updated when the current_tracer
		is set to "none".

		NOTE: It is planned on changing the allocated buffers
		      from being the number of possible CPUS to
		      the number of online CPUS.

  tracing_cpumask : This is a mask that lets the user only trace
		on specified CPUS. The format is a hex string
		representing the CPUS.

  set_ftrace_filter : When dynamic ftrace is configured in, the
		code is dynamically modified to disable calling
		of the function profiler (mcount). This lets
		tracing be configured in with practically no overhead
		in performance.  This also has a side effect of
		enabling or disabling specific functions to be
		traced.  Echoing in names of functions into this
		file will limit the trace to only those files.

  set_ftrace_notrace: This has the opposite effect that
		set_ftrace_filter has. Any function that is added
		here will not be traced. If a function exists
		in both set_ftrace_filter and set_ftrace_notrace
		the function will _not_ bet traced.

  available_filter_functions : When a function is encountered the first
		time by the dynamic tracer, it is recorded and
		later the call is converted into a nop. This file
		lists the functions that have been recorded
		by the dynamic tracer and these functions can
		be used to set the ftrace filter by the above
		"set_ftrace_filter" file.


The Tracers
-----------

Here are the list of current tracers that can be configured.

  ftrace - function tracer that uses mcount to trace all functions.
		It is possible to filter out which functions that are
		traced when dynamic ftrace is configured in.

  sched_switch - traces the context switches between tasks.

  irqsoff - traces the areas that disable interrupts and saves off
  		the trace with the longest max latency.
		See tracing_max_latency.  When a new max is recorded,
		it replaces the old trace. It is best to view this
		trace with the latency_trace file.

  preemptoff - Similar to irqsoff but traces and records the time
		preemption is disabled.

  preemptirqsoff - Similar to irqsoff and preemptoff, but traces and
		 records the largest time irqs and/or preemption is
		 disabled.

  wakeup - Traces and records the max latency that it takes for
		the highest priority task to get scheduled after
		it has been woken up.

  none - This is not a tracer. To remove all tracers from tracing
		simply echo "none" into current_tracer.


Examples of using the tracer
----------------------------

Here are typical examples of using the tracers with only controlling
them with the debugfs interface (without using any user-land utilities).

Output format:
--------------

Here's an example of the output format of the file "trace"

                             --------
# tracer: ftrace
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |
            bash-4251  [01] 10152.583854: path_put <-path_walk
            bash-4251  [01] 10152.583855: dput <-path_put
            bash-4251  [01] 10152.583855: _atomic_dec_and_lock <-dput
                             --------

A header is printed with the trace that is represented. In this case
the tracer is "ftrace". Then a header showing the format. Task name
"bash", the task PID "4251", the CPU that it was running on
"01", the timestamp in <secs>.<usecs> format, the function name that was
traced "path_put" and the parent function that called this function
"path_walk".

The sched_switch tracer also includes tracing of task wake ups and
context switches.

     ksoftirqd/1-7     [01]  1453.070013:      7:115:R   +  2916:115:S
     ksoftirqd/1-7     [01]  1453.070013:      7:115:R   +    10:115:S
     ksoftirqd/1-7     [01]  1453.070013:      7:115:R ==>    10:115:R
        events/1-10    [01]  1453.070013:     10:115:S ==>  2916:115:R
     kondemand/1-2916  [01]  1453.070013:   2916:115:S ==>     7:115:R
     ksoftirqd/1-7     [01]  1453.070013:      7:115:S ==>     0:140:R

Wake ups are represented by a "+" and the context switches show
"==>".  The format is:

 Context switches:

       Previous task              Next Task

  <pid>:<prio>:<state>  ==>  <pid>:<prio>:<state>

 Wake ups:

       Current task               Task waking up

  <pid>:<prio>:<state>    +  <pid>:<prio>:<state>

The prio is the internal kernel priority, which is inverse to the
priority that is usually displayed by user-space tools. Zero represents
the highest priority (99). Prio 100 starts the "nice" priorities with
100 being equal to nice -20 and 139 being nice 19. The prio "140" is
reserved for the idle task which is the lowest priority thread (pid 0).


Latency trace format
--------------------

For traces that display latency times, the latency_trace file gives
a bit more information to see why a latency happened. Here's a typical
trace.

# tracer: irqsoff
#
irqsoff latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: apic_timer_interrupt
 => ended at:   do_softirq

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
  <idle>-0     0d..1    0us+: trace_hardirqs_off_thunk (apic_timer_interrupt)
  <idle>-0     0d.s.   97us : __do_softirq (do_softirq)
  <idle>-0     0d.s1   98us : trace_hardirqs_on (do_softirq)


vim:ft=help


This shows that the current tracer is "irqsoff" tracing the time
interrupts are disabled. It gives the trace version and the kernel
this was executed on (2.6.26-rc8). Then it displays the max latency
in microsecs (97 us). The number of trace entries displayed
by the total number recorded (both are three: #3/3). The type of
preemption that was used (PREEMPT). VP, KP, SP, and HP are always zero
and reserved for later use. #P is the number of online CPUS (#P:2).

The task is the process that was running when the latency happened.
(swapper pid: 0).

The start and stop that caused the latencies:

  apic_timer_interrupt is where the interrupts were disabled.
  do_softirq is where they were enabled again.

The next lines after the header are the trace itself. The header
explains which is which.

  cmd: The name of the process in the trace.

  pid: The PID of that process.

  CPU#: The CPU that the process was running on.

  irqs-off: 'd' interrupts are disabled. '.' otherwise.

  need-resched: 'N' task need_resched is set, '.' otherwise.

  hardirq/softirq:
	'H' - hard irq happened inside a softirq.
	'h' - hard irq is running
	's' - soft irq is running
	'.' - normal context.

  preempt-depth: The level of preempt_disabled

The above is mostly meaningful for kernel developers.

  time: This differs from the trace output where as the trace output
	contained a absolute timestamp. This timestamp is relative
	to the start of the first entry in the the trace.

  delay: This is just to help catch your eye a bit better. And
	needs to be fixed to be only relative to the same CPU.
	The marks is determined by the difference between this
	current trace and the next trace.
	 '!' - greater than preempt_mark_thresh (default 100)
	 '+' - greater than 1 microsecond
	 ' ' - less than or equal to 1 microsecond.

  The rest is the same as the 'trace' file.


iter_ctrl
---------

The iter_ctrl file is used to control what gets printed in the trace
output. To see what is available, simply cat the file:

  cat /debug/tracing/iter_ctrl
  print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \
 noblock nostacktrace nosched-tree

To disable one of the options, echo in the option appended with "no".

  echo noprint-parent > /debug/tracing/iter_ctrl

To enable an option, leave off the "no".

  echo sym-offest > /debug/tracing/iter_ctrl

Here are the available options:

  print-parent - On function traces, display the calling function
		as well as the function being traced.

  print-parent:
   bash-4000  [01]  1477.606694: simple_strtoul <-strict_strtoul

  noprint-parent:
   bash-4000  [01]  1477.606694: simple_strtoul


  sym-offset - Display not only the function name, but also the offset
		in the function. For example, instead of seeing just
		"ktime_get" you will see "ktime_get+0xb/0x20"

  sym-offset:
   bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0

  sym-addr - this will also display the function address as well as
		the function name.

  sym-addr:
   bash-4000  [01]  1477.606694: simple_strtoul <c0339346>

  verbose - This deals with the latency_trace file.

    bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
    (+0.000ms): simple_strtoul (strict_strtoul)

  raw - This will display raw numbers. This option is best for use with
	user applications that can translate the raw numbers better than
	having it done in the kernel.

  hex - similar to raw, but the numbers will be in a hexadecimal format.

  bin - This will print out the formats in raw binary.

  block - TBD (needs update)

  stacktrace - This is one of the options that changes the trace itself.
		When a trace is recorded, so is the stack of functions.
		This allows for back traces of trace sites.

  sched-tree - TBD (any users??)


sched_switch
------------

This tracer simply records schedule switches. Here's an example
on how to implement it.

 # echo sched_switch > /debug/tracing/current_tracer
 # echo 1 > /debug/tracing/tracing_enabled
 # sleep 1
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/trace

# tracer: sched_switch
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |
            bash-3997  [01]   240.132281:   3997:120:R   +  4055:120:R
            bash-3997  [01]   240.132284:   3997:120:R ==>  4055:120:R
           sleep-4055  [01]   240.132371:   4055:120:S ==>  3997:120:R
            bash-3997  [01]   240.132454:   3997:120:R   +  4055:120:S
            bash-3997  [01]   240.132457:   3997:120:R ==>  4055:120:R
           sleep-4055  [01]   240.132460:   4055:120:D ==>  3997:120:R
            bash-3997  [01]   240.132463:   3997:120:R   +  4055:120:D
            bash-3997  [01]   240.132465:   3997:120:R ==>  4055:120:R
          <idle>-0     [00]   240.132589:      0:140:R   +     4:115:S
          <idle>-0     [00]   240.132591:      0:140:R ==>     4:115:R
     ksoftirqd/0-4     [00]   240.132595:      4:115:S ==>     0:140:R
          <idle>-0     [00]   240.132598:      0:140:R   +     4:115:S
          <idle>-0     [00]   240.132599:      0:140:R ==>     4:115:R
     ksoftirqd/0-4     [00]   240.132603:      4:115:S ==>     0:140:R
           sleep-4055  [01]   240.133058:   4055:120:S ==>  3997:120:R
 [...]


As we have discussed previously about this format, the header shows
the name of the trace and points to the options. The "FUNCTION"
is a misnomer since here it represents the wake ups and context
switches.

The sched_switch only lists the wake ups (represented with '+')
and context switches ('==>') with the previous task or current
first followed by the next task or task waking up. The format for both
of these is PID:KERNEL-PRIO:TASK-STATE. Remember that the KERNEL-PRIO
is the inverse of the actual priority with zero (0) being the highest
priority and the nice values starting at 100 (nice -20). Below is
a quick chart to map the kernel priority to user land priorities.

  Kernel priority: 0 to 99    ==> user RT priority 99 to 0
  Kernel priority: 100 to 139 ==> user nice -20 to 19
  Kernel priority: 140        ==> idle task priority

The task states are:

 R - running : wants to run, may not actually be running
 S - sleep   : process is waiting to be woken up (handles signals)
 D - deep sleep : process must be woken up (ignores signals)
 T - stopped : process suspended
 t - traced  : process is being traced (with something like gdb)
 Z - zombie  : process waiting to be cleaned up
 X - unknown


ftrace_enabled
--------------

The following tracers give different output depending on whether
or not the sysctl ftrace_enabled is set. To set ftrace_enabled,
one can either use the sysctl function or set it via the proc
file system interface.

  sysctl kernel.ftrace_enabled=1

 or

  echo 1 > /proc/sys/kernel/ftrace_enabled

To disable ftrace_enabled simply replace the '1' with '0' in
the above commands.

When ftrace_enabled is set the tracers will also record the functions
that are within the trace. The descriptions of the tracers
will also show an example with ftrace enabled.


irqsoff
-------

When interrupts are disabled, the CPU can not react to any other
external event (besides NMIs and SMIs). This prevents the timer
interrupt from triggering or the mouse interrupt from letting the
kernel know of a new mouse event. The result is a latency with the
reaction time.

The irqsoff tracer tracks the time interrupts are disabled and when
they are re-enabled. When a new maximum latency is hit, it saves off
the trace so that it may be retrieved at a later time. Every time a
new maximum in reached, the old saved trace is discarded and the new
trace is saved.

To reset the maximum, echo 0 into tracing_max_latency. Here's an
example:

 # echo irqsoff > /debug/tracing/current_tracer
 # echo 0 > /debug/tracing/tracing_max_latency
 # echo 1 > /debug/tracing/tracing_enabled
 # ls -ltr
 [...]
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/latency_trace
# tracer: irqsoff
#
irqsoff latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 6 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: bash-4269 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: copy_page_range
 => ended at:   copy_page_range

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
    bash-4269  1...1    0us+: _spin_lock (copy_page_range)
    bash-4269  1...1    7us : _spin_unlock (copy_page_range)
    bash-4269  1...2    7us : trace_preempt_on (copy_page_range)


vim:ft=help

Here we see that that we had a latency of 6 microsecs (which is
very good). The spin_lock in copy_page_range disabled interrupts.
The difference between the 6 and the displayed timestamp 7us is
because the clock must have incremented between the time of recording
the max latency and recording the function that had that latency.

Note the above had ftrace_enabled not set. If we set the ftrace_enabled
we get a much larger output:

# tracer: irqsoff
#
irqsoff latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: __alloc_pages_internal
 => ended at:   __alloc_pages_internal

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
      ls-4339  0...1    0us+: get_page_from_freelist (__alloc_pages_internal)
      ls-4339  0d..1    3us : rmqueue_bulk (get_page_from_freelist)
      ls-4339  0d..1    3us : _spin_lock (rmqueue_bulk)
      ls-4339  0d..1    4us : add_preempt_count (_spin_lock)
      ls-4339  0d..2    4us : __rmqueue (rmqueue_bulk)
      ls-4339  0d..2    5us : __rmqueue_smallest (__rmqueue)
      ls-4339  0d..2    5us : __mod_zone_page_state (__rmqueue_smallest)
      ls-4339  0d..2    6us : __rmqueue (rmqueue_bulk)
      ls-4339  0d..2    6us : __rmqueue_smallest (__rmqueue)
      ls-4339  0d..2    7us : __mod_zone_page_state (__rmqueue_smallest)
      ls-4339  0d..2    7us : __rmqueue (rmqueue_bulk)
      ls-4339  0d..2    8us : __rmqueue_smallest (__rmqueue)
[...]
      ls-4339  0d..2   46us : __rmqueue_smallest (__rmqueue)
      ls-4339  0d..2   47us : __mod_zone_page_state (__rmqueue_smallest)
      ls-4339  0d..2   47us : __rmqueue (rmqueue_bulk)
      ls-4339  0d..2   48us : __rmqueue_smallest (__rmqueue)
      ls-4339  0d..2   48us : __mod_zone_page_state (__rmqueue_smallest)
      ls-4339  0d..2   49us : _spin_unlock (rmqueue_bulk)
      ls-4339  0d..2   49us : sub_preempt_count (_spin_unlock)
      ls-4339  0d..1   50us : get_page_from_freelist (__alloc_pages_internal)
      ls-4339  0d..2   51us : trace_hardirqs_on (__alloc_pages_internal)


vim:ft=help


Here we traced a 50 microsecond latency. But we also see all the
functions that were called during that time. Note that enabling
function tracing we endure an added overhead. This overhead may
extend the latency times. But never the less, this trace has provided
some very helpful debugging.


preemptoff
----------

When preemption is disabled we may be able to receive interrupts but
the task can not be preempted and a higher priority task must wait
for preemption to be enabled again before it can preempt a lower
priority task.

The preemptoff tracer traces the places that disables preemption.
Like the irqsoff, it records the maximum latency that preemption
was disabled. The control of preemptoff is much like the irqsoff.

 # echo preemptoff > /debug/tracing/current_tracer
 # echo 0 > /debug/tracing/tracing_max_latency
 # echo 1 > /debug/tracing/tracing_enabled
 # ls -ltr
 [...]
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/latency_trace
# tracer: preemptoff
#
preemptoff latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: do_IRQ
 => ended at:   __do_softirq

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
    sshd-4261  0d.h.    0us+: irq_enter (do_IRQ)
    sshd-4261  0d.s.   29us : _local_bh_enable (__do_softirq)
    sshd-4261  0d.s1   30us : trace_preempt_on (__do_softirq)


vim:ft=help

This has some more changes. Preemption was disabled when an interrupt
came in (notice the 'h'), and was enabled while doing a softirq.
(notice the 's'). But we also see that interrupts have been disabled
when entering the preempt off section and leaving it (the 'd').
We do not know if interrupts were enabled in the mean time.

# tracer: preemptoff
#
preemptoff latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: remove_wait_queue
 => ended at:   __do_softirq

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
    sshd-4261  0d..1    0us : _spin_lock_irqsave (remove_wait_queue)
    sshd-4261  0d..1    1us : _spin_unlock_irqrestore (remove_wait_queue)
    sshd-4261  0d..1    2us : do_IRQ (common_interrupt)
    sshd-4261  0d..1    2us : irq_enter (do_IRQ)
    sshd-4261  0d..1    2us : idle_cpu (irq_enter)
    sshd-4261  0d..1    3us : add_preempt_count (irq_enter)
    sshd-4261  0d.h1    3us : idle_cpu (irq_enter)
    sshd-4261  0d.h.    4us : handle_fasteoi_irq (do_IRQ)
[...]
    sshd-4261  0d.h.   12us : add_preempt_count (_spin_lock)
    sshd-4261  0d.h1   12us : ack_ioapic_quirk_irq (handle_fasteoi_irq)
    sshd-4261  0d.h1   13us : move_native_irq (ack_ioapic_quirk_irq)
    sshd-4261  0d.h1   13us : _spin_unlock (handle_fasteoi_irq)
    sshd-4261  0d.h1   14us : sub_preempt_count (_spin_unlock)
    sshd-4261  0d.h1   14us : irq_exit (do_IRQ)
    sshd-4261  0d.h1   15us : sub_preempt_count (irq_exit)
    sshd-4261  0d..2   15us : do_softirq (irq_exit)
    sshd-4261  0d...   15us : __do_softirq (do_softirq)
    sshd-4261  0d...   16us : __local_bh_disable (__do_softirq)
    sshd-4261  0d...   16us+: add_preempt_count (__local_bh_disable)
    sshd-4261  0d.s4   20us : add_preempt_count (__local_bh_disable)
    sshd-4261  0d.s4   21us : sub_preempt_count (local_bh_enable)
    sshd-4261  0d.s5   21us : sub_preempt_count (local_bh_enable)
[...]
    sshd-4261  0d.s6   41us : add_preempt_count (__local_bh_disable)
    sshd-4261  0d.s6   42us : sub_preempt_count (local_bh_enable)
    sshd-4261  0d.s7   42us : sub_preempt_count (local_bh_enable)
    sshd-4261  0d.s5   43us : add_preempt_count (__local_bh_disable)
    sshd-4261  0d.s5   43us : sub_preempt_count (local_bh_enable_ip)
    sshd-4261  0d.s6   44us : sub_preempt_count (local_bh_enable_ip)
    sshd-4261  0d.s5   44us : add_preempt_count (__local_bh_disable)
    sshd-4261  0d.s5   45us : sub_preempt_count (local_bh_enable)
[...]
    sshd-4261  0d.s.   63us : _local_bh_enable (__do_softirq)
    sshd-4261  0d.s1   64us : trace_preempt_on (__do_softirq)


The above is an example of the preemptoff trace with ftrace_enabled
set. Here we see that interrupts were disabled the entire time.
The irq_enter code lets us know that we entered an interrupt 'h'.
Before that, the functions being traced still show that it is not
in an interrupt, but we can see by the functions themselves that
this is not the case.

Notice that the __do_softirq when called doesn't have a preempt_count.
It may seem that we missed a preempt enabled. What really happened
is that the preempt count is held on the threads stack and we
switched to the softirq stack (4K stacks in effect). The code
does not copy the preempt count, but because interrupts are disabled
we don't need to worry about it. Having a tracer like this is good
to let people know what really happens inside the kernel.


preemptirqsoff
--------------

Knowing the locations that have interrupts disabled or preemption
disabled for the longest times is helpful. But sometimes we would
like to know when either preemption and/or interrupts are disabled.

The following code:

    local_irq_disable();
    call_function_with_irqs_off();
    preempt_disable();
    call_function_with_irqs_and_preemption_off();
    local_irq_enable();
    call_function_with_preemption_off();
    preempt_enable();

The irqsoff tracer will record the total length of
call_function_with_irqs_off() and
call_function_with_irqs_and_preemption_off().

The preemptoff tracer will record the total length of
call_function_with_irqs_and_preemption_off() and
call_function_with_preemption_off().

But neither will trace the time that interrupts and/or preemption
is disabled. This total time is the time that we can not schedule.
To record this time, use the preemptirqsoff tracer.

Again, using this trace is much like the irqsoff and preemptoff tracers.

 # echo preemptoff > /debug/tracing/current_tracer
 # echo 0 > /debug/tracing/tracing_max_latency
 # echo 1 > /debug/tracing/tracing_enabled
 # ls -ltr
 [...]
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/latency_trace
# tracer: preemptirqsoff
#
preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: apic_timer_interrupt
 => ended at:   __do_softirq

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
      ls-4860  0d...    0us!: trace_hardirqs_off_thunk (apic_timer_interrupt)
      ls-4860  0d.s.  294us : _local_bh_enable (__do_softirq)
      ls-4860  0d.s1  294us : trace_preempt_on (__do_softirq)


vim:ft=help


The trace_hardirqs_off_thunk is called from assembly on x86 when
interrupts are disabled in the assembly code. Without the function
tracing, we don't know if interrupts were enabled within the preemption
points. We do see that it started with preemption enabled.

Here is a trace with ftrace_enabled set:


# tracer: preemptirqsoff
#
preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
    -----------------
 => started at: write_chan
 => ended at:   __do_softirq

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
      ls-4473  0.N..    0us : preempt_schedule (write_chan)
      ls-4473  0dN.1    1us : _spin_lock (schedule)
      ls-4473  0dN.1    2us : add_preempt_count (_spin_lock)
      ls-4473  0d..2    2us : put_prev_task_fair (schedule)
[...]
      ls-4473  0d..2   13us : set_normalized_timespec (ktime_get_ts)
      ls-4473  0d..2   13us : __switch_to (schedule)
    sshd-4261  0d..2   14us : finish_task_switch (schedule)
    sshd-4261  0d..2   14us : _spin_unlock_irq (finish_task_switch)
    sshd-4261  0d..1   15us : add_preempt_count (_spin_lock_irqsave)
    sshd-4261  0d..2   16us : _spin_unlock_irqrestore (hrtick_set)
    sshd-4261  0d..2   16us : do_IRQ (common_interrupt)
    sshd-4261  0d..2   17us : irq_enter (do_IRQ)
    sshd-4261  0d..2   17us : idle_cpu (irq_enter)
    sshd-4261  0d..2   18us : add_preempt_count (irq_enter)
    sshd-4261  0d.h2   18us : idle_cpu (irq_enter)
    sshd-4261  0d.h.   18us : handle_fasteoi_irq (do_IRQ)
    sshd-4261  0d.h.   19us : _spin_lock (handle_fasteoi_irq)
    sshd-4261  0d.h.   19us : add_preempt_count (_spin_lock)
    sshd-4261  0d.h1   20us : _spin_unlock (handle_fasteoi_irq)
    sshd-4261  0d.h1   20us : sub_preempt_count (_spin_unlock)
[...]
    sshd-4261  0d.h1   28us : _spin_unlock (handle_fasteoi_irq)
    sshd-4261  0d.h1   29us : sub_preempt_count (_spin_unlock)
    sshd-4261  0d.h2   29us : irq_exit (do_IRQ)
    sshd-4261  0d.h2   29us : sub_preempt_count (irq_exit)
    sshd-4261  0d..3   30us : do_softirq (irq_exit)
    sshd-4261  0d...   30us : __do_softirq (do_softirq)
    sshd-4261  0d...   31us : __local_bh_disable (__do_softirq)
    sshd-4261  0d...   31us+: add_preempt_count (__local_bh_disable)
    sshd-4261  0d.s4   34us : add_preempt_count (__local_bh_disable)
[...]
    sshd-4261  0d.s3   43us : sub_preempt_count (local_bh_enable_ip)
    sshd-4261  0d.s4   44us : sub_preempt_count (local_bh_enable_ip)
    sshd-4261  0d.s3   44us : smp_apic_timer_interrupt (apic_timer_interrupt)
    sshd-4261  0d.s3   45us : irq_enter (smp_apic_timer_interrupt)
    sshd-4261  0d.s3   45us : idle_cpu (irq_enter)
    sshd-4261  0d.s3   46us : add_preempt_count (irq_enter)
    sshd-4261  0d.H3   46us : idle_cpu (irq_enter)
    sshd-4261  0d.H3   47us : hrtimer_interrupt (smp_apic_timer_interrupt)
    sshd-4261  0d.H3   47us : ktime_get (hrtimer_interrupt)
[...]
    sshd-4261  0d.H3   81us : tick_program_event (hrtimer_interrupt)
    sshd-4261  0d.H3   82us : ktime_get (tick_program_event)
    sshd-4261  0d.H3   82us : ktime_get_ts (ktime_get)
    sshd-4261  0d.H3   83us : getnstimeofday (ktime_get_ts)
    sshd-4261  0d.H3   83us : set_normalized_timespec (ktime_get_ts)
    sshd-4261  0d.H3   84us : clockevents_program_event (tick_program_event)
    sshd-4261  0d.H3   84us : lapic_next_event (clockevents_program_event)
    sshd-4261  0d.H3   85us : irq_exit (smp_apic_timer_interrupt)
    sshd-4261  0d.H3   85us : sub_preempt_count (irq_exit)
    sshd-4261  0d.s4   86us : sub_preempt_count (irq_exit)
    sshd-4261  0d.s3   86us : add_preempt_count (__local_bh_disable)
[...]
    sshd-4261  0d.s1   98us : sub_preempt_count (net_rx_action)
    sshd-4261  0d.s.   99us : add_preempt_count (_spin_lock_irq)
    sshd-4261  0d.s1   99us+: _spin_unlock_irq (run_timer_softirq)
    sshd-4261  0d.s.  104us : _local_bh_enable (__do_softirq)
    sshd-4261  0d.s.  104us : sub_preempt_count (_local_bh_enable)
    sshd-4261  0d.s.  105us : _local_bh_enable (__do_softirq)
    sshd-4261  0d.s1  105us : trace_preempt_on (__do_softirq)


This is a very interesting trace. It started with the preemption of
the ls task. We see that the task had the "need_resched" bit set
with the 'N' in the trace.  Interrupts are disabled in the spin_lock
and the trace started. We see that a schedule took place to run
sshd.  When the interrupts were enabled we took an interrupt.
On return of the interrupt the softirq ran. We took another interrupt
while running the softirq as we see with the capital 'H'.


wakeup
------

In Real-Time environment it is very important to know the wakeup
time it takes for the highest priority task that wakes up to the
time it executes. This is also known as "schedule latency".
I stress the point that this is about RT tasks. It is also important
to know the scheduling latency of non-RT tasks, but the average
schedule latency is better for non-RT tasks. Tools like
LatencyTop is more appropriate for such measurements.

Real-Time environments is interested in the worst case latency.
That is the longest latency it takes for something to happen, and
not the average. We can have a very fast scheduler that may only
have a large latency once in a while, but that would not work well
with Real-Time tasks.  The wakeup tracer was designed to record
the worst case wakeups of RT tasks. Non-RT tasks are not recorded
because the tracer only records one worst case and tracing non-RT
tasks that are unpredictable will overwrite the worst case latency
of RT tasks.

Since this tracer only deals with RT tasks, we will run this slightly
different than we did with the previous tracers. Instead of performing
an 'ls' we will run 'sleep 1' under 'chrt' which changes the
priority of the task.

 # echo wakeup > /debug/tracing/current_tracer
 # echo 0 > /debug/tracing/tracing_max_latency
 # echo 1 > /debug/tracing/tracing_enabled
 # chrt -f 5 sleep 1
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/latency_trace
# tracer: wakeup
#
wakeup latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: sleep-4901 (uid:0 nice:0 policy:1 rt_prio:5)
    -----------------

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
  <idle>-0     1d.h4    0us+: try_to_wake_up (wake_up_process)
  <idle>-0     1d..4    4us : schedule (cpu_idle)


vim:ft=help


Running this on an idle system we see that it only took 4 microseconds
to perform the task switch.  Note, since the trace marker in the
schedule is before the actual "switch" we stop the tracing when
the recorded task is about to schedule in. This may change if
we add a new marker at the end of the scheduler.

Notice that the recorded task is 'sleep' with the PID of 4901 and it
has an rt_prio of 5. This priority is user-space priority and not
the internal kernel priority. The policy is 1 for SCHED_FIFO and 2
for SCHED_RR.

Doing the same with chrt -r 5 and ftrace_enabled set.

# tracer: wakeup
#
wakeup latency trace v1.1.5 on 2.6.26-rc8
--------------------------------------------------------------------
 latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
    -----------------
    | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5)
    -----------------

#                _------=> CPU#
#               / _-----=> irqs-off
#              | / _----=> need-resched
#              || / _---=> hardirq/softirq
#              ||| / _--=> preempt-depth
#              |||| /
#              |||||     delay
#  cmd     pid ||||| time  |   caller
#     \   /    |||||   \   |   /
ksoftirq-7     1d.H3    0us : try_to_wake_up (wake_up_process)
ksoftirq-7     1d.H4    1us : sub_preempt_count (marker_probe_cb)
ksoftirq-7     1d.H3    2us : check_preempt_wakeup (try_to_wake_up)
ksoftirq-7     1d.H3    3us : update_curr (check_preempt_wakeup)
ksoftirq-7     1d.H3    4us : calc_delta_mine (update_curr)
ksoftirq-7     1d.H3    5us : __resched_task (check_preempt_wakeup)
ksoftirq-7     1d.H3    6us : task_wake_up_rt (try_to_wake_up)
ksoftirq-7     1d.H3    7us : _spin_unlock_irqrestore (try_to_wake_up)
[...]
ksoftirq-7     1d.H2   17us : irq_exit (smp_apic_timer_interrupt)
ksoftirq-7     1d.H2   18us : sub_preempt_count (irq_exit)
ksoftirq-7     1d.s3   19us : sub_preempt_count (irq_exit)
ksoftirq-7     1..s2   20us : rcu_process_callbacks (__do_softirq)
[...]
ksoftirq-7     1..s2   26us : __rcu_process_callbacks (rcu_process_callbacks)
ksoftirq-7     1d.s2   27us : _local_bh_enable (__do_softirq)
ksoftirq-7     1d.s2   28us : sub_preempt_count (_local_bh_enable)
ksoftirq-7     1.N.3   29us : sub_preempt_count (ksoftirqd)
ksoftirq-7     1.N.2   30us : _cond_resched (ksoftirqd)
ksoftirq-7     1.N.2   31us : __cond_resched (_cond_resched)
ksoftirq-7     1.N.2   32us : add_preempt_count (__cond_resched)
ksoftirq-7     1.N.2   33us : schedule (__cond_resched)
ksoftirq-7     1.N.2   33us : add_preempt_count (schedule)
ksoftirq-7     1.N.3   34us : hrtick_clear (schedule)
ksoftirq-7     1dN.3   35us : _spin_lock (schedule)
ksoftirq-7     1dN.3   36us : add_preempt_count (_spin_lock)
ksoftirq-7     1d..4   37us : put_prev_task_fair (schedule)
ksoftirq-7     1d..4   38us : update_curr (put_prev_task_fair)
[...]
ksoftirq-7     1d..5   47us : _spin_trylock (tracing_record_cmdline)
ksoftirq-7     1d..5   48us : add_preempt_count (_spin_trylock)
ksoftirq-7     1d..6   49us : _spin_unlock (tracing_record_cmdline)
ksoftirq-7     1d..6   49us : sub_preempt_count (_spin_unlock)
ksoftirq-7     1d..4   50us : schedule (__cond_resched)

The interrupt went off while running ksoftirqd. This task runs at
SCHED_OTHER. Why didn't we see the 'N' set early? This may be
a harmless bug with x86_32 and 4K stacks. The need_reched() function
that tests if we need to reschedule looks on the actual stack.
Where as the setting of the NEED_RESCHED bit happens on the
task's stack. But because we are in a hard interrupt, the test
is with the interrupts stack which has that to be false. We don't
see the 'N' until we switch back to the task's stack.

ftrace
------

ftrace is not only the name of the tracing infrastructure, but it
is also a name of one of the tracers. The tracer is the function
tracer. Enabling the function tracer can be done from the
debug file system. Make sure the ftrace_enabled is set otherwise
this tracer is a nop.

 # sysctl kernel.ftrace_enabled=1
 # echo ftrace > /debug/tracing/current_tracer
 # echo 1 > /debug/tracing/tracing_enabled
 # usleep 1
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/trace
# tracer: ftrace
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |
            bash-4003  [00]   123.638713: finish_task_switch <-schedule
            bash-4003  [00]   123.638714: _spin_unlock_irq <-finish_task_switch
            bash-4003  [00]   123.638714: sub_preempt_count <-_spin_unlock_irq
            bash-4003  [00]   123.638715: hrtick_set <-schedule
            bash-4003  [00]   123.638715: _spin_lock_irqsave <-hrtick_set
            bash-4003  [00]   123.638716: add_preempt_count <-_spin_lock_irqsave
            bash-4003  [00]   123.638716: _spin_unlock_irqrestore <-hrtick_set
            bash-4003  [00]   123.638717: sub_preempt_count <-_spin_unlock_irqrestore
            bash-4003  [00]   123.638717: hrtick_clear <-hrtick_set
            bash-4003  [00]   123.638718: sub_preempt_count <-schedule
            bash-4003  [00]   123.638718: sub_preempt_count <-preempt_schedule
            bash-4003  [00]   123.638719: wait_for_completion <-__stop_machine_run
            bash-4003  [00]   123.638719: wait_for_common <-wait_for_completion
            bash-4003  [00]   123.638720: _spin_lock_irq <-wait_for_common
            bash-4003  [00]   123.638720: add_preempt_count <-_spin_lock_irq
[...]


Note: It is sometimes better to enable or disable tracing directly from
a program, because the buffer may be overflowed by the echo commands
before you get to the point you want to trace. It is also easier to
stop the tracing at the point that you hit the part that you are
interested in. Since the ftrace buffer is a ring buffer with the
oldest data being overwritten, usually it is sufficient to start the
tracer with an echo command but have you code stop it. Something
like the following is usually appropriate for this.

int trace_fd;
[...]
int main(int argc, char *argv[]) {
	[...]
	trace_fd = open("/debug/tracing/tracing_enabled", O_WRONLY);
	[...]
	if (condition_hit()) {
	write(trace_fd, "0", 1);
	}
	[...]
}


dynamic ftrace
--------------

If CONFIG_DYNAMIC_FTRACE is set, then the system will run with
virtually no overhead when function tracing is disabled. The way
this works is the mcount function call (placed at the start of
every kernel function, produced by the -pg switch in gcc), starts
of pointing to a simple return.

When dynamic ftrace is initialized, it calls kstop_machine to make it
act like a uniprocessor so that it can freely modify code without
worrying about other processors executing that same code.  At
initialization, the mcount calls are change to call a "record_ip"
function.  After this, the first time a kernel function is called,
it has the calling address saved in a hash table.

Later on the ftraced kernel thread is awoken and will again call
kstop_machine if new functions have been recorded. The ftraced thread
will change all calls to mcount to "nop".  Just calling mcount
and having mcount return has shown a 10% overhead. By converting
it to a nop, there is no recordable overhead to the system.

One special side-effect to the recording of the functions being
traced, is that we can now selectively choose which functions we
want to trace and which ones we want the mcount calls to remain as
nops.

Two files that contain to the enabling and disabling of recorded
functions are:

  set_ftrace_filter

and

  set_ftrace_notrace

A list of available functions that you can add to this files is listed
in:

   available_filter_functions

 # cat /debug/tracing/available_filter_functions
put_prev_task_idle
kmem_cache_create
pick_next_task_rt
get_online_cpus
pick_next_task_fair
mutex_lock
[...]

If I'm only interested in sys_nanosleep and hrtimer_interrupt:

 # echo sys_nanosleep hrtimer_interrupt \
		> /debug/tracing/set_ftrace_filter
 # echo ftrace > /debug/tracing/current_tracer
 # echo 1 > /debug/tracing/tracing_enabled
 # usleep 1
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/trace
# tracer: ftrace
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |
          usleep-4134  [00]  1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt
          usleep-4134  [00]  1317.070111: sys_nanosleep <-syscall_call
          <idle>-0     [00]  1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt

To see what functions are being traced, you can cat the file:

 # cat /debug/tracing/set_ftrace_filter
hrtimer_interrupt
sys_nanosleep


Perhaps this isn't enough. The filters also allow simple wild cards.
Only the following is currently available

  <match>*  - will match functions that begins with <match>
  *<match>  - will match functions that end with <match>
  *<match>* - will match functions that have <match> in it

Thats all the wild cards that are allowed.

  <match>*<match> will not work.

 # echo hrtimer_* > /debug/tracing/set_ftrace_filter

Produces:

# tracer: ftrace
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |
            bash-4003  [00]  1480.611794: hrtimer_init <-copy_process
            bash-4003  [00]  1480.611941: hrtimer_start <-hrtick_set
            bash-4003  [00]  1480.611956: hrtimer_cancel <-hrtick_clear
            bash-4003  [00]  1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel
          <idle>-0     [00]  1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt
          <idle>-0     [00]  1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt
          <idle>-0     [00]  1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt
          <idle>-0     [00]  1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt
          <idle>-0     [00]  1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt


Notice that we lost the sys_nanosleep.

 # cat /debug/tracing/set_ftrace_filter
hrtimer_run_queues
hrtimer_run_pending
hrtimer_init
hrtimer_cancel
hrtimer_try_to_cancel
hrtimer_forward
hrtimer_start
hrtimer_reprogram
hrtimer_force_reprogram
hrtimer_get_next_event
hrtimer_interrupt
hrtimer_nanosleep
hrtimer_wakeup
hrtimer_get_remaining
hrtimer_get_res
hrtimer_init_sleeper


This is because the '>' and '>>' act just like they do in bash.
To rewrite the filters, use '>'
To append to the filters, use '>>'

To clear out a filter so that all functions will be recorded again.

 # echo > /debug/tracing/set_ftrace_filter
 # cat /debug/tracing/set_ftrace_filter
 #

Again, now we want to append.

 # echo sys_nanosleep > /debug/tracing/set_ftrace_filter
 # cat /debug/tracing/set_ftrace_filter
sys_nanosleep
 # echo hrtimer_* >> /debug/tracing/set_ftrace_filter
 # cat /debug/tracing/set_ftrace_filter
hrtimer_run_queues
hrtimer_run_pending
hrtimer_init
hrtimer_cancel
hrtimer_try_to_cancel
hrtimer_forward
hrtimer_start
hrtimer_reprogram
hrtimer_force_reprogram
hrtimer_get_next_event
hrtimer_interrupt
sys_nanosleep
hrtimer_nanosleep
hrtimer_wakeup
hrtimer_get_remaining
hrtimer_get_res
hrtimer_init_sleeper


The set_ftrace_notrace prevents those functions from being traced.

 # echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace

Produces:

# tracer: ftrace
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |
            bash-4043  [01]   115.281644: finish_task_switch <-schedule
            bash-4043  [01]   115.281645: hrtick_set <-schedule
            bash-4043  [01]   115.281645: hrtick_clear <-hrtick_set
            bash-4043  [01]   115.281646: wait_for_completion <-__stop_machine_run
            bash-4043  [01]   115.281647: wait_for_common <-wait_for_completion
            bash-4043  [01]   115.281647: kthread_stop <-stop_machine_run
            bash-4043  [01]   115.281648: init_waitqueue_head <-kthread_stop
            bash-4043  [01]   115.281648: wake_up_process <-kthread_stop
            bash-4043  [01]   115.281649: try_to_wake_up <-wake_up_process

We can see that there's no more lock or preempt tracing.

ftraced
-------

As mentioned above, when dynamic ftrace is configured in, a kernel
thread wakes up once a second and checks to see if there are mcount
calls that need to be converted into nops. If there is not, then
it simply goes back to sleep. But if there is, it will call
kstop_machine to convert the calls to nops.

There may be a case that you do not want this added latency.
Perhaps you are doing some audio recording and this activity might
cause skips in the playback. There is an interface to disable
and enable the ftraced kernel thread.

 # echo 0 > /debug/tracing/ftraced_enabled

This will disable the calling of the kstop_machine to update the
mcount calls to nops. Remember that there's a large overhead
to calling mcount. Without this kernel thread, that overhead will
exist.

Any write to the ftraced_enabled file will cause the kstop_machine
to run if there are recorded calls to mcount. This means that a
user can manually perform the updates when they want to by simply
echoing a '0' into the ftraced_enabled file.

The updates are also done at the beginning of enabling a tracer
that uses ftrace function recording.


trace_pipe
----------

The trace_pipe outputs the same as trace, but the effect on the
tracing is different. Every read from trace_pipe is consumed.
This means that subsequent reads will be different. The trace
is live.

 # echo ftrace > /debug/tracing/current_tracer
 # cat /debug/tracing/trace_pipe > /tmp/trace.out &
[1] 4153
 # echo 1 > /debug/tracing/tracing_enabled
 # usleep 1
 # echo 0 > /debug/tracing/tracing_enabled
 # cat /debug/tracing/trace
# tracer: ftrace
#
#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
#              | |      |          |         |

 #
 # cat /tmp/trace.out
            bash-4043  [00] 41.267106: finish_task_switch <-schedule
            bash-4043  [00] 41.267106: hrtick_set <-schedule
            bash-4043  [00] 41.267107: hrtick_clear <-hrtick_set
            bash-4043  [00] 41.267108: wait_for_completion <-__stop_machine_run
            bash-4043  [00] 41.267108: wait_for_common <-wait_for_completion
            bash-4043  [00] 41.267109: kthread_stop <-stop_machine_run
            bash-4043  [00] 41.267109: init_waitqueue_head <-kthread_stop
            bash-4043  [00] 41.267110: wake_up_process <-kthread_stop
            bash-4043  [00] 41.267110: try_to_wake_up <-wake_up_process
            bash-4043  [00] 41.267111: select_task_rq_rt <-try_to_wake_up


Note, reading the trace_pipe will block until more input is added.
By changing the tracer, trace_pipe will issue an EOF. We needed
to set the ftrace tracer _before_ cating the trace_pipe file.


trace entries
-------------

Having too much or not enough data can be troublesome in diagnosing
some issue in the kernel. The file trace_entries is used to modify
the size of the internal trace buffers. The numbers listed
is the number of entries that can be recorded per CPU. To know
the full size, multiply the number of possible CPUS with the
number of entries.

 # cat /debug/tracing/trace_entries
65620

Note, to modify this you must have tracing fulling disabled. To do that,
echo "none" into the current_tracer.

 # echo none > /debug/tracing/current_tracer
 # echo 100000 > /debug/tracing/trace_entries
 # cat /debug/tracing/trace_entries
100045


Notice that we echoed in 100,000 but the size is 100,045. The entries
are held by individual pages. It allocates the number of pages it takes
to fulfill the request. If more entries may fit on the last page
it will add them.

 # echo 1 > /debug/tracing/trace_entries
 # cat /debug/tracing/trace_entries
85

This shows us that 85 entries can fit on a single page.

The number of pages that will be allocated is a percentage of available
memory. Allocating too much will produces an error.

 # echo 1000000000000 > /debug/tracing/trace_entries
-bash: echo: write error: Cannot allocate memory
 # cat /debug/tracing/trace_entries
85