FreeBSD/Linux Kernel Cross Reference
sys/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.
    
      iter_ctrl: This file lets the user control the amount of data
                    that is displayed in one of the above output
                    files.
    
      trace_max_latency: Some of the tracers record the max latency.
                    For example, the time interrupts are disabled.
                    This time is saved in this file. The max trace
                    will also be stored, and displayed by either
                    "trace" or "latency_trace".  A new max trace will
                    only be recorded if the latency is greater than
                    the value in this file. (in microseconds)
    
      trace_entries: This sets or displays the number of bytes 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.
   
     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.
   
   
   iter_ctrl
   ---------
   
   The iter_ctrl file is used to control what gets printed in the trace
   output. To see what is available, simply cat the file:
   
     cat /debug/tracing/iter_ctrl
     print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \
    noblock nostacktrace nosched-tree
   
   To disable one of the options, echo in the option prepended with "no".
   
     echo noprint-parent > /debug/tracing/iter_ctrl
   
   To enable an option, leave off the "no".
   
     echo sym-offset > /debug/tracing/iter_ctrl
   
   Here are the available options:
   
     print-parent - On function traces, display the calling function
                   as well as the function being traced.
   
     print-parent:
      bash-4000  [01]  1477.606694: simple_strtoul <-strict_strtoul
   
     noprint-parent:
      bash-4000  [01]  1477.606694: simple_strtoul
   
   
     sym-offset - Display not only the function name, but also the offset
                   in the function. For example, instead of seeing just
                   "ktime_get", you will see "ktime_get+0xb/0x20".
   
     sym-offset:
      bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0
   
     sym-addr - this will also display the function address as well as
                   the function name.
   
     sym-addr:
      bash-4000  [01]  1477.606694: simple_strtoul <c0339346>
   
     verbose - This deals with the latency_trace file.
   
       bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
       (+0.000ms): simple_strtoul (strict_strtoul)
   
     raw - This will display raw numbers. This option is best for use with
           user applications that can translate the raw numbers better than
           having it done in the kernel.
   
     hex - Similar to raw, but the numbers will be in a hexadecimal format.
   
     bin - This will print out the formats in raw binary.
   
     block - TBD (needs update)
   
     stacktrace - This is one of the options that changes the trace itself.
                   When a trace is recorded, so is the stack of functions.
                   This allows for back traces of trace sites.
   
     sched-tree - TBD (any users??)
   
   
   sched_switch
   ------------
   
   This tracer simply records schedule switches. Here 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.
  
  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.
  
   # 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 trace_entries 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/trace_entries
  65620
  
  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 100000 > /debug/tracing/trace_entries
   # cat /debug/tracing/trace_entries
  100045
  
  
  Notice that we echoed in 100,000 but the size is 100,045. The entries
  are held in individual pages. It allocates the number of pages it takes
  to fulfill the request. If more entries may fit on the last page
  then they will be added.
  
   # echo 1 > /debug/tracing/trace_entries
   # cat /debug/tracing/trace_entries
  85
  
  This shows us that 85 entries can fit in a single page.
  
  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/trace_entries
  -bash: echo: write error: Cannot allocate memory
   # cat /debug/tracing/trace_entries
  85
  

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