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                   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).
  
  
  Implementation Details
  ----------------------
  
  See ftrace-design.txt for details for arch porters and such.
  
  
  The File System
  ---------------
  
  Ftrace uses the debugfs file system to hold the control files as
  well as the files to display output.
  
  When debugfs is configured into the kernel (which selecting any ftrace
  option will do) the directory /sys/kernel/debug will be created. To mount
  this directory, you can add to your /etc/fstab file:
  
   debugfs       /sys/kernel/debug          debugfs defaults        0       0
  
  Or you can mount it at run time with:
  
   mount -t debugfs nodev /sys/kernel/debug
  
  For quicker access to that directory you may want to make a soft link to
  it:
  
   ln -s /sys/kernel/debug /debug
  
  Any selected ftrace option will also create a directory called tracing
  within the debugfs. The rest of the document will assume that you are in
  the ftrace directory (cd /sys/kernel/debug/tracing) and will only concentrate
  on the files within that directory and not distract from the content with
  the extended "/sys/kernel/debug/tracing" path name.
  
  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_on:
  
          This sets or displays whether writing to the trace
          ring buffer is enabled. 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).
  
    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" file, 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" file is 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 "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 management 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.
 
         This interface also allows for commands to be used. See the
         "Filter commands" section for more details.
 
   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"
 
         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.
 
   "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 with the latency-format option enabled.
 
   "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.
         Traces all tasks as an average developer would expect.
 
   "wakeup_rt"
 
         Traces and records the max latency that it takes for just
         RT tasks (as the current "wakeup" does). This is useful
         for those interested in wake up timings of RT tasks.
 
   "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.
 
 Latency trace format
 --------------------
 
 When the latency-format option is enabled, the 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: When the latency-format option is enabled, the trace file
         output includes a timestamp relative to the start of the
         trace. This differs from the output when latency-format
         is disabled, which includes an absolute timestamp.
 
   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 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 > trace_options
 
 To enable an option, leave off the "no".
 
   echo sym-offset > 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 trace file when the
             latency-format option is enabled.
 
     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. 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.
 
   latency-format - This option changes the trace. When
                    it is enabled, the trace displays
                    additional information about the
                    latencies, as described in "Latency
                    trace format".
 
   overwrite - This controls what happens when the trace buffer is
               full. If "1" (default), the oldest events are
               discarded and overwritten. If "0", then the newest
               events are discarded.
 
 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 > current_tracer
  # echo latency-format > trace_options
  # echo 0 > tracing_max_latency
  # echo 1 > tracing_on
  # ls -ltr
  [...]
  # echo 0 > tracing_on
  # cat 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 > current_tracer
  # echo latency-format > trace_options
  # echo 0 > tracing_max_latency
  # echo 1 > tracing_on
  # ls -ltr
  [...]
  # echo 0 > tracing_on
  # cat 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 > current_tracer
  # echo latency-format > trace_options
  # echo 0 > tracing_max_latency
  # echo 1 > tracing_on
  # ls -ltr
  [...]
  # echo 0 > tracing_on
  # cat 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 > current_tracer
  # echo latency-format > trace_options
  # echo 0 > tracing_max_latency
  # echo 1 > tracing_on
  # chrt -f 5 sleep 1
  # echo 0 > tracing_on
  # cat 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 > current_tracer
  # echo 1 > tracing_on
  # usleep 1
  # echo 0 > tracing_on
  # cat 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(tracing_file("tracing_on"), O_WRONLY);
         [...]
         if (condition_hit()) {
                 write(trace_fd, "0", 1);
         }
         [...]
 }
 
 
 Single thread tracing
 ---------------------
 
 By writing into set_ftrace_pid you can trace a
 single thread. For example:
 
 # cat set_ftrace_pid
 no pid
 # echo 3111 > set_ftrace_pid
 # cat set_ftrace_pid
 3111
 # echo function > current_tracer
 # cat 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 > set_ftrace_pid
 # cat 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>
 #include <string.h>
 
 #define _STR(x) #x
 #define STR(x) _STR(x)
 #define MAX_PATH 256
 
 const char *find_debugfs(void)
 {
        static char debugfs[MAX_PATH+1];
        static int debugfs_found;
        char type[100];
        FILE *fp;
 
        if (debugfs_found)
                return debugfs;
 
        if ((fp = fopen("/proc/mounts","r")) == NULL) {
                perror("/proc/mounts");
                return NULL;
        }
 
        while (fscanf(fp, "%*s %"
                      STR(MAX_PATH)
                      "s %99s %*s %*d %*d\n",
                      debugfs, type) == 2) {
                if (strcmp(type, "debugfs") == 0)
                        break;
        }
        fclose(fp);
 
        if (strcmp(type, "debugfs") != 0) {
                fprintf(stderr, "debugfs not mounted");
                return NULL;
        }
 
        strcat(debugfs, "/tracing/");
        debugfs_found = 1;
 
        return debugfs;
 }
 
 const char *tracing_file(const char *file_name)
 {
        static char trace_file[MAX_PATH+1];
        snprintf(trace_file, MAX_PATH, "%s/%s", find_debugfs(), file_name);
        return trace_file;
 }
 
 int main (int argc, char **argv)
 {
         if (argc < 1)
                 exit(-1);
 
         if (fork() > 0) {
                 int fd, ffd;
                 char line[64];
                 int s;
 
                 ffd = open(tracing_file("current_tracer"), O_WRONLY);
                 if (ffd < 0)
                         exit(-1);
                 write(ffd, "nop", 3);
 
                 fd = open(tracing_file("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=n
 
 or
 
   echo n > /proc/sys/kernel/ftrace_dump_on_oops
 
 If n = 1, ftrace will dump buffers of all CPUs, if n = 2 ftrace will
 only dump the buffer of the CPU that triggered the 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 > trace_options
         show: echo funcgraph-cpu > 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 > trace_options
         show: echo funcgraph-duration > trace_options
 
 - The overhead field precedes the duration field in case of
   reached duration thresholds.
 
         hide: echo nofuncgraph-overhead > trace_options
         show: echo funcgraph-overhead > 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 > trace_options
         show: echo funcgraph-proc > 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 > trace_options
         show: echo funcgraph-abstime > 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 undesirable 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 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 \
                 > set_ftrace_filter
  # echo function > current_tracer
  # echo 1 > tracing_on
  # usleep 1
  # echo 0 > tracing_on
  # cat 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 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_*' > 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 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 > set_ftrace_filter
  # cat set_ftrace_filter
  #
 
 Again, now we want to append.
 
  # echo sys_nanosleep > set_ftrace_filter
  # cat set_ftrace_filter
 sys_nanosleep
  # echo 'hrtimer_*' >> set_ftrace_filter
  # cat 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*' > 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
 
 
 Filter commands
 ---------------
 
 A few commands are supported by the set_ftrace_filter interface.
 Trace commands have the following format:
 
 <function>:<command>:<parameter>
 
 The following commands are supported:
 
 - mod
   This command enables function filtering per module. The
   parameter defines the module. For example, if only the write*
   functions in the ext3 module are desired, run:
 
    echo 'write*:mod:ext3' > set_ftrace_filter
 
   This command interacts with the filter in the same way as
   filtering based on function names. Thus, adding more functions
   in a different module is accomplished by appending (>>) to the
   filter file. Remove specific module functions by prepending
   '!':
 
    echo '!writeback*:mod:ext3' >> set_ftrace_filter
 
 - traceon/traceoff
   These commands turn tracing on and off when the specified
   functions are hit. The parameter determines how many times the
   tracing system is turned on and off. If unspecified, there is
   no limit. For example, to disable tracing when a schedule bug
   is hit the first 5 times, run:
 
    echo '__schedule_bug:traceoff:5' > set_ftrace_filter
 
   These commands are cumulative whether or not they are appended
   to set_ftrace_filter. To remove a command, prepend it by '!'
   and drop the parameter:
 
    echo '!__schedule_bug:traceoff' > set_ftrace_filter
 
 
 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 > current_tracer
  # cat trace_pipe > /tmp/trace.out &
 [1] 4153
  # echo 1 > tracing_on
  # usleep 1
  # echo 0 > tracing_on
  # cat 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 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 > current_tracer
  # echo 10000 > buffer_size_kb
  # cat 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 > buffer_size_kb
 -bash: echo: write error: Cannot allocate memory
  # cat buffer_size_kb
 85
 
 Snapshot
 --------
 CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
 available to all non latency tracers. (Latency tracers which
 record max latency, such as "irqsoff" or "wakeup", can't use
 this feature, since those are already using the snapshot
 mechanism internally.)
 
 Snapshot preserves a current trace buffer at a particular point
 in time without stopping tracing. Ftrace swaps the current
 buffer with a spare buffer, and tracing continues in the new
 current (=previous spare) buffer.
 
 The following debugfs files in "tracing" are related to this
 feature:
 
   snapshot:
 
         This is used to take a snapshot and to read the output
         of the snapshot. Echo 1 into this file to allocate a
         spare buffer and to take a snapshot (swap), then read
         the snapshot from this file in the same format as
         "trace" (described above in the section "The File
         System"). Both reads snapshot and tracing are executable
         in parallel. When the spare buffer is allocated, echoing
         0 frees it, and echoing else (positive) values clear the
         snapshot contents.
         More details are shown in the table below.
 
         status\input  |     0      |     1      |    else    |
         --------------+------------+------------+------------+
         not allocated |(do nothing)| alloc+swap |(do nothing)|
         --------------+------------+------------+------------+
         allocated     |    free    |    swap    |   clear    |
         --------------+------------+------------+------------+
 
 Here is an example of using the snapshot feature.
 
  # echo 1 > events/sched/enable
  # echo 1 > snapshot
  # cat snapshot
 # tracer: nop
 #
 # entries-in-buffer/entries-written: 71/71   #P:8
 #
 #                              _-----=> irqs-off
 #                             / _----=> need-resched
 #                            | / _---=> hardirq/softirq
 #                            || / _--=> preempt-depth
 #                            ||| /     delay
 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
 #              | |       |   ||||       |         |
           <idle>-0     [005] d...  2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120
            sleep-2242  [005] d...  2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120
 [...]
           <idle>-0     [002] d...  2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120
 
  # cat trace
 # tracer: nop
 #
 # entries-in-buffer/entries-written: 77/77   #P:8
 #
 #                              _-----=> irqs-off
 #                             / _----=> need-resched
 #                            | / _---=> hardirq/softirq
 #                            || / _--=> preempt-depth
 #                            ||| /     delay
 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
 #              | |       |   ||||       |         |
           <idle>-0     [007] d...  2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120
  snapshot-test-2-2229  [002] d...  2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120
 [...]
 
 
 If you try to use this snapshot feature when current tracer is
 one of the latency tracers, you will get the following results.
 
  # echo wakeup > current_tracer
  # echo 1 > snapshot
 bash: echo: write error: Device or resource busy
  # cat snapshot
 cat: snapshot: Device or resource busy
 
 -----------
 
 More details can be found in the source code, in the
 kernel/trace/*.c files.

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