ftrace - Function Tracer
  		========================
  
  Copyright 2008 Red Hat Inc.

     Author:   Steven Rostedt <srostedt@redhat.com>
    License:   The GNU Free Documentation License, Version 1.2

                 (dual licensed under the GPL v2)

  Reviewers:   Elias Oltmanns, Randy Dunlap, Andrew Morton,
  	     John Kacur, and David Teigland.

  Written for: 2.6.28-rc2

  
  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 include 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 (ftrace allows for tracer plugins, which means that the list of
  tracers can always grow).

  
  
  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

  (Note: it is more common to mount at /sys/kernel/debug, but for simplicity
   this document will use /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

  		have been compiled into the kernel. The tracers

  		listed here can be configured by echoing 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 to enable it.

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

  		format (described below).

    latency_trace: This file shows the same trace but the information

  		is organized more to display possible latencies

  		in the system (described below).

    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 is not adding more data, they will display

  		the same information every time they are read.

    trace_options: 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)

    buffer_size_kb: This sets or displays the number of kilobytes each CPU

  		buffer can hold. The tracer buffers are the same size
  		for each CPU. The displayed number is the size of the

  		CPU buffer and not total size of all buffers. The

  		trace buffers are allocated in pages (blocks of memory
  		that the kernel uses for allocation, usually 4 KB in size).

  		If the last page allocated has room for more bytes
  		than requested, the rest of the page will be used,
  		making the actual allocation bigger than requested.
  		(Note, the size may not be a multiple of the page size due
  		to buffer managment overhead.)

  
  		This can only be updated when the current_tracer

  		is set to "nop".

    tracing_cpumask: This is a mask that lets the user only trace

  		on specified CPUS. The format is a hex string
  		representing the CPUS.

    set_ftrace_filter: When dynamic ftrace is configured in (see the

  		section below "dynamic ftrace"), the code is dynamically
  		modified (code text rewrite) to disable calling of the
  		function profiler (mcount). This lets tracing be configured
  		in with practically no overhead in performance.  This also
  		has a side effect of enabling or disabling specific functions
  		to be traced. Echoing names of functions into this file
  		will limit the trace to only those functions.
  
    set_ftrace_notrace: This has an effect opposite to that of
  		set_ftrace_filter. Any function that is added here will not
  		be traced. If a function exists in both set_ftrace_filter
  		and set_ftrace_notrace,	the function will _not_ be traced.

    set_ftrace_pid: Have the function tracer only trace a single thread.

    available_filter_functions: This lists the functions that ftrace
  		has processed and can trace. These are the function
  		names that you can pass to "set_ftrace_filter" or
  		"set_ftrace_notrace". (See the section "dynamic ftrace"
  		below for more details.)

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

  Here is the list of current tracers that may be configured.

    function - function tracer that uses mcount to trace all functions.

  
    sched_switch - traces the context switches between tasks.

    irqsoff - traces the areas that disable interrupts and saves

    		the trace with the longest max latency.
  		See tracing_max_latency.  When a new max is recorded,
  		it replaces the old trace. It is best to view this

  		trace via the latency_trace file.

    preemptoff - Similar to irqsoff but traces and records the amount of
  		time for which preemption is disabled.

  
    preemptirqsoff - Similar to irqsoff and preemptoff, but traces and

  		 records the largest time for which irqs and/or preemption
  		 is disabled.

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

    nop - This is not a tracer. To remove all tracers from tracing
  		simply echo "nop" into current_tracer.

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

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

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

  Here is an example of the output format of the file "trace"

  
                               --------

  # tracer: function

  #
  #           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 tracer name that is represented by the trace.

  In this case the tracer is "function". 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 timestamp is the time at which the function was
  entered.

  The sched_switch tracer also includes tracing of task wakeups 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 are shown as

  "==>".  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 the inverse of 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

  somewhat more information to see why a latency happened. Here is 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)

  This shows that the current tracer is "irqsoff" tracing the time for which
  interrupts were disabled. It gives the trace version and the 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
  and 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 are reserved for later use. #P is the number of online CPUS (#P:2).

  The task is the process that was running when the latency occurred.

  (swapper pid: 0).

  The start and stop (the functions in which the interrupts were disabled and
  enabled respectively) 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 which the process was running on.

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

  	    Note: If the architecture does not support a way to
  		  read the irq flags variable, an 'X' will always
  		  be printed here.

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

  	'H' - hard irq occurred inside a softirq.

  	'h' - hard irq is running
  	's' - soft irq is running
  	'.' - normal context.
  
    preempt-depth: The level of preempt_disabled
  
  The above is mostly meaningful for kernel developers.

    time: This differs from the trace file output. The trace file output

  	includes an absolute timestamp. The timestamp used by the

  	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.

  	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.

  trace_options
  -------------

  The trace_options file is used to control what gets printed in the trace

  output. To see what is available, simply cat the file:

    cat /debug/tracing/trace_options

    print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \

   noblock nostacktrace nosched-tree nouserstacktrace nosym-userobj

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

    echo noprint-parent > /debug/tracing/trace_options

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

    echo sym-offset > /debug/tracing/trace_options

  
  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.

    userstacktrace - This option changes the trace.
  		   It records a stacktrace of the current userspace thread.

    sym-userobj - when user stacktrace are enabled, look up which object the
  		address belongs to, and print a relative address
  		This is especially useful when ASLR is on, otherwise you don't
  		get a chance to resolve the address to object/file/line after the app is no
  		longer running
  
  		The lookup is performed when you read trace,trace_pipe,latency_trace. Example:
  
  		a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
  x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]

    sched-tree - TBD (any users??)
  
  
  sched_switch
  ------------

  This tracer simply records schedule switches. Here is 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 file only lists the wake ups (represented with '+')
  and context switches ('==>') with the previous task or 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 - disk sleep (uninterruptible 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 (listed below) 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 for which interrupts are disabled.
  When a new maximum latency is hit, the tracer saves the trace leading up
  to that latency point so that every time a new maximum is reached, the old
  saved trace is discarded and the new trace is saved.

  To reset the maximum, echo 0 into tracing_max_latency. Here is an

  example:
  
   # echo 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

  --------------------------------------------------------------------

   latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)

      -----------------

      | task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0)

      -----------------

   => started at: sys_setpgid
   => ended at:   sys_setpgid

  
  #                _------=> CPU#
  #               / _-----=> irqs-off
  #              | / _----=> need-resched
  #              || / _---=> hardirq/softirq
  #              ||| / _--=> preempt-depth
  #              |||| /
  #              |||||     delay
  #  cmd     pid ||||| time  |   caller
  #     \   /    |||||   \   |   /

      bash-3730  1d...    0us : _write_lock_irq (sys_setpgid)
      bash-3730  1d..1    1us+: _write_unlock_irq (sys_setpgid)
      bash-3730  1d..2   14us : trace_hardirqs_on (sys_setpgid)

  Here we see that that we had a latency of 12 microsecs (which is
  very good). The _write_lock_irq in sys_setpgid disabled interrupts.
  The difference between the 12 and the displayed timestamp 14us occurred
  because the clock was incremented between the time of recording the max
  latency and the time of recording the function that had that latency.

  Note the above example had 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)

  
  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 incur an added overhead. This overhead may

  extend the latency times. But nevertheless, this trace has provided
  some very helpful debugging information.

  
  
  preemptoff
  ----------

  When preemption is disabled, we may be able to receive interrupts but
  the task cannot be preempted and a higher priority task must wait

  for preemption to be enabled again before it can preempt a lower
  priority task.

  The preemptoff tracer traces the places that disable preemption.

  Like the irqsoff tracer, it records the maximum latency for which preemption
  was disabled. The control of preemptoff tracer is much like the irqsoff
  tracer.

  
   # echo 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)

  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 from the functions themselves that

  this is not the case.

  Notice that __do_softirq when called does not have a preempt_count.
  It may seem that we missed a preempt enabling. What really happened
  is that the preempt count is held on the thread's 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 do not need to worry about it. Having a tracer like this is good
  for letting 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.

  Consider the following code:

  
      local_irq_disable();
      call_function_with_irqs_off();
      preempt_disable();
      call_function_with_irqs_and_preemption_off();
      local_irq_enable();
      call_function_with_preemption_off();
      preempt_enable();
  
  The irqsoff tracer will record the total length of
  call_function_with_irqs_off() and
  call_function_with_irqs_and_preemption_off().
  
  The preemptoff tracer will record the total length of
  call_function_with_irqs_and_preemption_off() and
  call_function_with_preemption_off().
  
  But neither will trace the time that interrupts and/or preemption
  is disabled. This total time is the time that we can not schedule.
  To record this time, use the preemptirqsoff tracer.
  
  Again, using this trace is much like the irqsoff and preemptoff tracers.

   # echo 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)

  
  The trace_hardirqs_off_thunk is called from assembly on x86 when
  interrupts are disabled in the assembly code. Without the function

  tracing, we do not know if interrupts were enabled within the preemption

  points. We do see that it started with preemption enabled.
  
  Here is a trace with 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

  via the 'N' in the trace.  Interrupts were disabled before the spin_lock
  at the beginning of the trace. We see that a schedule took place to run

  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 from the capital 'H'.

  
  
  wakeup
  ------

  In a Real-Time environment it is very important to know the wakeup
  time it takes for the highest priority task that is woken up to the
  time that it executes. This is also known as "schedule latency".

  I stress the point that this is about RT tasks. It is also important
  to know the scheduling latency of non-RT tasks, but the average
  schedule latency is better for non-RT tasks. Tools like

  LatencyTop are more appropriate for such measurements.

  Real-Time environments are interested in the worst case latency.

  That is the longest latency it takes for something to happen, and
  not the average. We can have a very fast scheduler that may only
  have a large latency once in a while, but that would not work well
  with Real-Time tasks.  The wakeup 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)

  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 did not 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 run 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 do not see the 'N' until we switch back to the task's

  assigned stack.

  function
  --------

  This tracer is the function tracer. Enabling the function tracer
  can be done from the debug file system. Make sure the ftrace_enabled is
  set; otherwise this tracer is a nop.

  
   # sysctl kernel.ftrace_enabled=1

   # echo function > /debug/tracing/current_tracer

   # echo 1 > /debug/tracing/tracing_enabled
   # usleep 1
   # echo 0 > /debug/tracing/tracing_enabled
   # cat /debug/tracing/trace

  # tracer: function

  #
  #           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: function tracer uses ring buffers to store the above entries.
  The newest data may overwrite the oldest data. Sometimes using echo to
  stop the trace is not sufficient because the tracing could have overwritten
  the data that you wanted to record. For this reason, it is sometimes better to

  disable tracing directly from a program. This allows you to stop the
  tracing at the point that you hit the part that you are interested in.
  To disable the tracing directly from a C program, something like following
  code snippet can be used:

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

  		write(trace_fd, "0", 1);

  	}
  	[...]
  }

  Note: Here we hard coded the path name. The debugfs mount is not
  guaranteed to be at /debug (and is more commonly at /sys/kernel/debug).
  For simple one time traces, the above is sufficent. For anything else,
  a search through /proc/mounts may be needed to find where the debugfs
  file-system is mounted.

  
  Single thread tracing
  ---------------------
  
  By writing into /debug/tracing/set_ftrace_pid you can trace a
  single thread. For example:
  
  # cat /debug/tracing/set_ftrace_pid
  no pid
  # echo 3111 > /debug/tracing/set_ftrace_pid
  # cat /debug/tracing/set_ftrace_pid
  3111
  # echo function > /debug/tracing/current_tracer
  # cat /debug/tracing/trace | head
   # tracer: function
   #
   #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
   #              | |       |          |         |
       yum-updatesd-3111  [003]  1637.254676: finish_task_switch <-thread_return
       yum-updatesd-3111  [003]  1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
       yum-updatesd-3111  [003]  1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
       yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
       yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
       yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll
  # echo -1 > /debug/tracing/set_ftrace_pid
  # cat /debug/tracing/trace |head
   # tracer: function
   #
   #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
   #              | |       |          |         |
   ##### CPU 3 buffer started ####
       yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
       yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
       yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
       yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
       yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit
  
  If you want to trace a function when executing, you could use
  something like this simple program:
  
  #include <stdio.h>
  #include <stdlib.h>
  #include <sys/types.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  #include <unistd.h>
  
  int main (int argc, char **argv)
  {
          if (argc < 1)
                  exit(-1);
  
          if (fork() > 0) {
                  int fd, ffd;
                  char line[64];
                  int s;
  
                  ffd = open("/debug/tracing/current_tracer", O_WRONLY);
                  if (ffd < 0)
                          exit(-1);
                  write(ffd, "nop", 3);
  
                  fd = open("/debug/tracing/set_ftrace_pid", O_WRONLY);
                  s = sprintf(line, "%d
  ", getpid());
                  write(fd, line, s);
  
                  write(ffd, "function", 8);
  
                  close(fd);
                  close(ffd);
  
                  execvp(argv[1], argv+1);
          }
  
          return 0;
  }

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

  If CONFIG_DYNAMIC_FTRACE is set, the system will run with

  virtually no overhead when function tracing is disabled. The way
  this works is the mcount function call (placed at the start of
  every kernel function, produced by the -pg switch in gcc), starts

  of pointing to a simple return. (Enabling FTRACE will include the
  -pg switch in the compiling of the kernel.)

  At compile time every C file object is run through the
  recordmcount.pl script (located in the scripts directory). This
  script will process the C object using objdump to find all the
  locations in the .text section that call mcount. (Note, only
  the .text section is processed, since processing other sections
  like .init.text may cause races due to those sections being freed).
  
  A new section called "__mcount_loc" is created that holds references
  to all the mcount call sites in the .text section. This section is
  compiled back into the original object. The final linker will add
  all these references into a single table.
  
  On boot up, before SMP is initialized, the dynamic ftrace code
  scans this table and updates all the locations into nops. It also
  records the locations, which are added to the available_filter_functions
  list.  Modules are processed as they are loaded and before they are
  executed.  When a module is unloaded, it also removes its functions from
  the ftrace function list. This is automatic in the module unload
  code, and the module author does not need to worry about it.
  
  When tracing is enabled, kstop_machine is called to prevent races
  with the CPUS executing code being modified (which can cause the
  CPU to do undesireable things), and the nops are patched back
  to calls. But this time, they do not call mcount (which is just
  a function stub). They now call into the ftrace infrastructure.

  
  One special side-effect to the recording of the functions being

  traced is that we can now selectively choose which functions we
  wish to trace and which ones we want the mcount calls to remain as

  nops.

  Two files are used, one for enabling and one for disabling the tracing

  of specified functions. They are:

  
    set_ftrace_filter
  
  and
  
    set_ftrace_notrace

  A list of available functions that you can add to these files is listed

  in:
  
     available_filter_functions
  
   # cat /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 am 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 which functions are being traced, you can cat the file:

  
   # cat /debug/tracing/set_ftrace_filter
  hrtimer_interrupt
  sys_nanosleep

  Perhaps this is not 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

  These are the only wild cards which are supported.

  
    <match>*<match> will not work.

  Note: It is better to use quotes to enclose the wild cards, otherwise
    the shell may expand the parameters into names of files in the local
    directory.
  
   # echo 'hrtimer_*' > /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.

  trace_pipe
  ----------

  The trace_pipe outputs the same content as the trace file, but the effect
  on the tracing is different. Every read from trace_pipe is consumed.

  This means that subsequent reads will be different. The trace
  is live.

   # echo function > /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: function

  #
  #           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 file will block until more input is added.

  By changing the tracer, trace_pipe will issue an EOF. We needed

  to set the function tracer _before_ we "cat" the trace_pipe file.

  
  
  trace entries
  -------------
  
  Having too much or not enough data can be troublesome in diagnosing

  an issue in the kernel. The file buffer_size_kb is used to modify

  the size of the internal trace buffers. The number listed

  is the number of entries that can be recorded per CPU. To know
  the full size, multiply the number of possible CPUS with the
  number of entries.

   # cat /debug/tracing/buffer_size_kb

  1408 (units kilobytes)

  Note, to modify this, you must have tracing completely disabled. To do that,

  echo "nop" into the current_tracer. If the current_tracer is not set
  to "nop", an EINVAL error will be returned.

   # echo nop > /debug/tracing/current_tracer

   # echo 10000 > /debug/tracing/buffer_size_kb

   # cat /debug/tracing/buffer_size_kb

  10000 (units kilobytes)

  The number of pages which will be allocated is limited to a percentage
  of available memory. Allocating too much will produce an error.

   # echo 1000000000000 > /debug/tracing/buffer_size_kb

  -bash: echo: write error: Cannot allocate memory

   # cat /debug/tracing/buffer_size_kb

  85