tracing/syscalls: use a dedicated file header

[linux/fpc-iii.git] / Documentation / ftrace.txt

                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.

  tracing_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.

  set_graph_function:

        Set a "trigger" function where tracing should start
        with the function graph tracer (See the section
        "dynamic ftrace" for more details).

  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 call tracer to trace all kernel functions.

  "function_graph_tracer"

        Similar to the function tracer except that the
        function tracer probes the functions on their entry
        whereas the function graph tracer traces on both entry
        and exit of the functions. It then provides the ability
        to draw a graph of function calls similar to C code
        source.

  "sched_switch"

        Traces the context switches and wakeups 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.

  "hw-branch-tracer"

        Uses the BTS CPU feature on x86 CPUs to traces all
        branches executed.

  "nop"

        This is the "trace nothing" 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 (parent)
                 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 - trace all tasks that are on the runqueue, at
               every scheduling event. Will add overhead if
               there's a lot of tasks running at once.


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\n", getpid());
                write(fd, line, s);

                write(ffd, "function", 8);

                close(fd);
                close(ffd);

                execvp(argv[1], argv+1);
        }

        return 0;
}


hw-branch-tracer (x86 only)
---------------------------

This tracer uses the x86 last branch tracing hardware feature to
collect a branch trace on all cpus with relatively low overhead.

The tracer uses a fixed-size circular buffer per cpu and only
traces ring 0 branches. The trace file dumps that buffer in the
following format:

# tracer: hw-branch-tracer
#
# CPU#        TO  <-  FROM
   0  scheduler_tick+0xb5/0x1bf   <-  task_tick_idle+0x5/0x6
   2  run_posix_cpu_timers+0x2b/0x72a     <-  run_posix_cpu_timers+0x25/0x72a
   0  scheduler_tick+0x139/0x1bf          <-  scheduler_tick+0xed/0x1bf
   0  scheduler_tick+0x17c/0x1bf          <-  scheduler_tick+0x148/0x1bf
   2  run_posix_cpu_timers+0x9e/0x72a     <-  run_posix_cpu_timers+0x5e/0x72a
   0  scheduler_tick+0x1b6/0x1bf          <-  scheduler_tick+0x1aa/0x1bf


The tracer may be used to dump the trace for the oops'ing cpu on
a kernel oops into the system log. To enable this,
ftrace_dump_on_oops must be set. To set ftrace_dump_on_oops, one
can either use the sysctl function or set it via the proc system
interface.

  sysctl kernel.ftrace_dump_on_oops=1

or

  echo 1 > /proc/sys/kernel/ftrace_dump_on_oops


Here's an example of such a dump after a null pointer
dereference in a kernel module:

[57848.105921] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
[57848.106019] IP: [<ffffffffa0000006>] open+0x6/0x14 [oops]
[57848.106019] PGD 2354e9067 PUD 2375e7067 PMD 0
[57848.106019] Oops: 0002 [#1] SMP
[57848.106019] last sysfs file: /sys/devices/pci0000:00/0000:00:1e.0/0000:20:05.0/local_cpus
[57848.106019] Dumping ftrace buffer:
[57848.106019] ---------------------------------
[...]
[57848.106019]    0  chrdev_open+0xe6/0x165       <-  cdev_put+0x23/0x24
[57848.106019]    0  chrdev_open+0x117/0x165      <-  chrdev_open+0xfa/0x165
[57848.106019]    0  chrdev_open+0x120/0x165      <-  chrdev_open+0x11c/0x165
[57848.106019]    0  chrdev_open+0x134/0x165      <-  chrdev_open+0x12b/0x165
[57848.106019]    0  open+0x0/0x14 [oops]         <-  chrdev_open+0x144/0x165
[57848.106019]    0  page_fault+0x0/0x30          <-  open+0x6/0x14 [oops]
[57848.106019]    0  error_entry+0x0/0x5b         <-  page_fault+0x4/0x30
[57848.106019]    0  error_kernelspace+0x0/0x31   <-  error_entry+0x59/0x5b
[57848.106019]    0  error_sti+0x0/0x1    <-  error_kernelspace+0x2d/0x31
[57848.106019]    0  page_fault+0x9/0x30          <-  error_sti+0x0/0x1
[57848.106019]    0  do_page_fault+0x0/0x881      <-  page_fault+0x1a/0x30
[...]
[57848.106019]    0  do_page_fault+0x66b/0x881    <-  is_prefetch+0x1ee/0x1f2
[57848.106019]    0  do_page_fault+0x6e0/0x881    <-  do_page_fault+0x67a/0x881
[57848.106019]    0  oops_begin+0x0/0x96          <-  do_page_fault+0x6e0/0x881
[57848.106019]    0  trace_hw_branch_oops+0x0/0x2d        <-  oops_begin+0x9/0x96
[...]
[57848.106019]    0  ds_suspend_bts+0x2a/0xe3     <-  ds_suspend_bts+0x1a/0xe3
[57848.106019] ---------------------------------
[57848.106019] CPU 0
[57848.106019] Modules linked in: oops
[57848.106019] Pid: 5542, comm: cat Tainted: G        W  2.6.28 #23
[57848.106019] RIP: 0010:[<ffffffffa0000006>]  [<ffffffffa0000006>] open+0x6/0x14 [oops]
[57848.106019] RSP: 0018:ffff880235457d48  EFLAGS: 00010246
[...]


function graph tracer
---------------------------

This tracer is similar to the function tracer except that it
probes a function on its entry and its exit. This is done by
using a dynamically allocated stack of return addresses in each
task_struct. On function entry the tracer overwrites the return
address of each function traced to set a custom probe. Thus the
original return address is stored on the stack of return address
in the task_struct.

Probing on both ends of a function leads to special features
such as:

- measure of a function's time execution
- having a reliable call stack to draw function calls graph

This tracer is useful in several situations:

- you want to find the reason of a strange kernel behavior and
  need to see what happens in detail on any areas (or specific
  ones).

- you are experiencing weird latencies but it's difficult to
  find its origin.

- you want to find quickly which path is taken by a specific
  function

- you just want to peek inside a working kernel and want to see
  what happens there.

# tracer: function_graph
#
# CPU  DURATION                  FUNCTION CALLS
# |     |   |                     |   |   |   |

 0)               |  sys_open() {
 0)               |    do_sys_open() {
 0)               |      getname() {
 0)               |        kmem_cache_alloc() {
 0)   1.382 us    |          __might_sleep();
 0)   2.478 us    |        }
 0)               |        strncpy_from_user() {
 0)               |          might_fault() {
 0)   1.389 us    |            __might_sleep();
 0)   2.553 us    |          }
 0)   3.807 us    |        }
 0)   7.876 us    |      }
 0)               |      alloc_fd() {
 0)   0.668 us    |        _spin_lock();
 0)   0.570 us    |        expand_files();
 0)   0.586 us    |        _spin_unlock();


There are several columns that can be dynamically
enabled/disabled. You can use every combination of options you
want, depending on your needs.

- The cpu number on which the function executed is default
  enabled.  It is sometimes better to only trace one cpu (see
  tracing_cpu_mask file) or you might sometimes see unordered
  function calls while cpu tracing switch.

        hide: echo nofuncgraph-cpu > /debug/tracing/trace_options
        show: echo funcgraph-cpu > /debug/tracing/trace_options

- The duration (function's time of execution) is displayed on
  the closing bracket line of a function or on the same line
  than the current function in case of a leaf one. It is default
  enabled.

        hide: echo nofuncgraph-duration > /debug/tracing/trace_options
        show: echo funcgraph-duration > /debug/tracing/trace_options

- The overhead field precedes the duration field in case of
  reached duration thresholds.

        hide: echo nofuncgraph-overhead > /debug/tracing/trace_options
        show: echo funcgraph-overhead > /debug/tracing/trace_options
        depends on: funcgraph-duration

  ie:

  0)               |    up_write() {
  0)   0.646 us    |      _spin_lock_irqsave();
  0)   0.684 us    |      _spin_unlock_irqrestore();
  0)   3.123 us    |    }
  0)   0.548 us    |    fput();
  0) + 58.628 us   |  }

  [...]

  0)               |      putname() {
  0)               |        kmem_cache_free() {
  0)   0.518 us    |          __phys_addr();
  0)   1.757 us    |        }
  0)   2.861 us    |      }
  0) ! 115.305 us  |    }
  0) ! 116.402 us  |  }

  + means that the function exceeded 10 usecs.
  ! means that the function exceeded 100 usecs.


- The task/pid field displays the thread cmdline and pid which
  executed the function. It is default disabled.

        hide: echo nofuncgraph-proc > /debug/tracing/trace_options
        show: echo funcgraph-proc > /debug/tracing/trace_options

  ie:

  # tracer: function_graph
  #
  # CPU  TASK/PID        DURATION                  FUNCTION CALLS
  # |    |    |           |   |                     |   |   |   |
  0)    sh-4802     |               |                  d_free() {
  0)    sh-4802     |               |                    call_rcu() {
  0)    sh-4802     |               |                      __call_rcu() {
  0)    sh-4802     |   0.616 us    |                        rcu_process_gp_end();
  0)    sh-4802     |   0.586 us    |                        check_for_new_grace_period();
  0)    sh-4802     |   2.899 us    |                      }
  0)    sh-4802     |   4.040 us    |                    }
  0)    sh-4802     |   5.151 us    |                  }
  0)    sh-4802     | + 49.370 us   |                }


- The absolute time field is an absolute timestamp given by the
  system clock since it started. A snapshot of this time is
  given on each entry/exit of functions

        hide: echo nofuncgraph-abstime > /debug/tracing/trace_options
        show: echo funcgraph-abstime > /debug/tracing/trace_options

  ie:

  #
  #      TIME       CPU  DURATION                  FUNCTION CALLS
  #       |         |     |   |                     |   |   |   |
  360.774522 |   1)   0.541 us    |                                          }
  360.774522 |   1)   4.663 us    |                                        }
  360.774523 |   1)   0.541 us    |                                        __wake_up_bit();
  360.774524 |   1)   6.796 us    |                                      }
  360.774524 |   1)   7.952 us    |                                    }
  360.774525 |   1)   9.063 us    |                                  }
  360.774525 |   1)   0.615 us    |                                  journal_mark_dirty();
  360.774527 |   1)   0.578 us    |                                  __brelse();
  360.774528 |   1)               |                                  reiserfs_prepare_for_journal() {
  360.774528 |   1)               |                                    unlock_buffer() {
  360.774529 |   1)               |                                      wake_up_bit() {
  360.774529 |   1)               |                                        bit_waitqueue() {
  360.774530 |   1)   0.594 us    |                                          __phys_addr();


You can put some comments on specific functions by using
trace_printk() For example, if you want to put a comment inside
the __might_sleep() function, you just have to include
<linux/ftrace.h> and call trace_printk() inside __might_sleep()

trace_printk("I'm a comment!\n")

will produce:

 1)               |             __might_sleep() {
 1)               |                /* I'm a comment! */
 1)   1.449 us    |             }


You might find other useful features for this tracer in the
following "dynamic ftrace" section such as tracing only specific
functions or tasks.

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.


Dynamic ftrace with the function graph tracer
---------------------------------------------

Although what has been explained above concerns both the
function tracer and the function-graph-tracer, there are some
special features only available in the function-graph tracer.

If you want to trace only one function and all of its children,
you just have to echo its name into set_graph_function:

 echo __do_fault > set_graph_function

will produce the following "expanded" trace of the __do_fault()
function:

 0)               |  __do_fault() {
 0)               |    filemap_fault() {
 0)               |      find_lock_page() {
 0)   0.804 us    |        find_get_page();
 0)               |        __might_sleep() {
 0)   1.329 us    |        }
 0)   3.904 us    |      }
 0)   4.979 us    |    }
 0)   0.653 us    |    _spin_lock();
 0)   0.578 us    |    page_add_file_rmap();
 0)   0.525 us    |    native_set_pte_at();
 0)   0.585 us    |    _spin_unlock();
 0)               |    unlock_page() {
 0)   0.541 us    |      page_waitqueue();
 0)   0.639 us    |      __wake_up_bit();
 0)   2.786 us    |    }
 0) + 14.237 us   |  }
 0)               |  __do_fault() {
 0)               |    filemap_fault() {
 0)               |      find_lock_page() {
 0)   0.698 us    |        find_get_page();
 0)               |        __might_sleep() {
 0)   1.412 us    |        }
 0)   3.950 us    |      }
 0)   5.098 us    |    }
 0)   0.631 us    |    _spin_lock();
 0)   0.571 us    |    page_add_file_rmap();
 0)   0.526 us    |    native_set_pte_at();
 0)   0.586 us    |    _spin_unlock();
 0)               |    unlock_page() {
 0)   0.533 us    |      page_waitqueue();
 0)   0.638 us    |      __wake_up_bit();
 0)   2.793 us    |    }
 0) + 14.012 us   |  }

You can also expand several functions at once:

 echo sys_open > set_graph_function
 echo sys_close >> set_graph_function

Now if you want to go back to trace all functions you can clear
this special filter via:

 echo > set_graph_function


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

-----------

More details can be found in the source code, in the
kernel/tracing/*.c files.