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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).
+
+
+The File System
+---------------
+
+Ftrace uses the debugfs file system to hold the control files as
+well as the files to display output.
+
+To mount the debugfs system:
+
+  # mkdir /debug
+  # mount -t debugfs nodev /debug
+
+( Note: it is more common to mount at /sys/kernel/debug, but for
+  simplicity this document will use /debug)
+
+That's it! (assuming that you have ftrace configured into your kernel)
+
+After mounting the debugfs, you can see a directory called
+"tracing".  This directory contains the control and output files
+of ftrace. Here is a list of some of the key files:
+
+
+ Note: all time values are in microseconds.
+
+  current_tracer:
+
+        This is used to set or display the current tracer
+        that is configured.
+
+  available_tracers:
+
+        This holds the different types of tracers that
+        have been compiled into the kernel. The
+        tracers listed here can be configured by
+        echoing their name into current_tracer.
+
+  tracing_enabled:
+
+        This sets or displays whether the current_tracer
+        is activated and tracing or not. Echo 0 into this
+        file to disable the tracer or 1 to enable it.
+
+  trace:
+
+        This file holds the output of the trace in a human
+        readable format (described below).
+
+  latency_trace:
+
+        This file shows the same trace but the information
+        is organized more to display possible latencies
+        in the system (described below).
+
+  trace_pipe:
+
+        The output is the same as the "trace" file but this
+        file is meant to be streamed with live tracing.
+        Reads from this file will block until new data
+        is retrieved. Unlike the "trace" and "latency_trace"
+        files, this file is a consumer. This means reading
+        from this file causes sequential reads to display
+        more current data. Once data is read from this
+        file, it is consumed, and will not be read
+        again with a sequential read. The "trace" and
+        "latency_trace" files are static, and if the
+        tracer is not adding more data, they will display
+        the same information every time they are read.
+
+  trace_options:
+
+        This file lets the user control the amount of data
+        that is displayed in one of the above output
+        files.
+
+  tracing_max_latency:
+
+        Some of the tracers record the max latency.
+        For example, the time interrupts are disabled.
+        This time is saved in this file. The max trace
+        will also be stored, and displayed by either
+        "trace" or "latency_trace".  A new max trace will
+        only be recorded if the latency is greater than
+        the value in this file. (in microseconds)
+
+  buffer_size_kb:
+
+        This sets or displays the number of kilobytes each CPU
+        buffer can hold. The tracer buffers are the same size
+        for each CPU. The displayed number is the size of the
+        CPU buffer and not total size of all buffers. The
+        trace buffers are allocated in pages (blocks of memory
+        that the kernel uses for allocation, usually 4 KB in size).
+        If the last page allocated has room for more bytes
+        than requested, the rest of the page will be used,
+        making the actual allocation bigger than requested.
+        ( Note, the size may not be a multiple of the page size
+          due to buffer managment overhead. )
+
+        This can only be updated when the current_tracer
+        is set to "nop".
+
+  tracing_cpumask:
+
+        This is a mask that lets the user only trace
+        on specified CPUS. The format is a hex string
+        representing the CPUS.
+
+  set_ftrace_filter:
+
+        When dynamic ftrace is configured in (see the
+        section below "dynamic ftrace"), the code is dynamically
+        modified (code text rewrite) to disable calling of the
+        function profiler (mcount). This lets tracing be configured
+        in with practically no overhead in performance.  This also
+        has a side effect of enabling or disabling specific functions
+        to be traced. Echoing names of functions into this file
+        will limit the trace to only those functions.
+
+  set_ftrace_notrace:
+
+        This has an effect opposite to that of
+        set_ftrace_filter. Any function that is added here will not
+        be traced. If a function exists in both set_ftrace_filter
+        and set_ftrace_notrace, the function will _not_ be traced.
+
+  set_ftrace_pid:
+
+        Have the function tracer only trace a single thread.
+
+  set_graph_function:
+
+        Set a "trigger" function where tracing should start
+        with the function graph tracer (See the section
+        "dynamic ftrace" for more details).
+
+  available_filter_functions:
+
+        This lists the functions that ftrace
+        has processed and can trace. These are the function
+        names that you can pass to "set_ftrace_filter" or
+        "set_ftrace_notrace". (See the section "dynamic ftrace"
+        below for more details.)
+
+
+The Tracers
+-----------
+
+Here is the list of current tracers that may be configured.
+
+  "function"
+
+        Function call tracer to trace all kernel functions.
+
+  "function_graph_tracer"
+
+        Similar to the function tracer except that the
+        function tracer probes the functions on their entry
+        whereas the function graph tracer traces on both entry
+        and exit of the functions. It then provides the ability
+        to draw a graph of function calls similar to C code
+        source.
+
+  "sched_switch"
+
+        Traces the context switches and wakeups between tasks.
+
+  "irqsoff"
+
+        Traces the areas that disable interrupts and saves
+        the trace with the longest max latency.
+        See tracing_max_latency. When a new max is recorded,
+        it replaces the old trace. It is best to view this
+        trace via the latency_trace file.
+
+  "preemptoff"
+
+        Similar to irqsoff but traces and records the amount of
+        time for which preemption is disabled.
+
+  "preemptirqsoff"
+
+        Similar to irqsoff and preemptoff, but traces and
+        records the largest time for which irqs and/or preemption
+        is disabled.
+
+  "wakeup"
+
+        Traces and records the max latency that it takes for
+        the highest priority task to get scheduled after
+        it has been woken up.
+
+  "hw-branch-tracer"
+
+        Uses the BTS CPU feature on x86 CPUs to traces all
+        branches executed.
+
+  "nop"
+
+        This is the "trace nothing" tracer. To remove all
+        tracers from tracing simply echo "nop" into
+        current_tracer.
+
+
+Examples of using the tracer
+----------------------------
+
+Here are typical examples of using the tracers when controlling
+them only with the debugfs interface (without using any
+user-land utilities).
+
+Output format:
+--------------
+
+Here is an example of the output format of the file "trace"
+
+                             --------
+# tracer: function
+#
+#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
+#              | |      |          |         |
+            bash-4251  [01] 10152.583854: path_put <-path_walk
+            bash-4251  [01] 10152.583855: dput <-path_put
+            bash-4251  [01] 10152.583855: _atomic_dec_and_lock <-dput
+                             --------
+
+A header is printed with the tracer name that is represented by
+the trace. In this case the tracer is "function". Then a header
+showing the format. Task name "bash", the task PID "4251", the
+CPU that it was running on "01", the timestamp in <secs>.<usecs>
+format, the function name that was traced "path_put" and the
+parent function that called this function "path_walk". The
+timestamp is the time at which the function was entered.
+
+The sched_switch tracer also includes tracing of task wakeups
+and context switches.
+
+     ksoftirqd/1-7     [01]  1453.070013:      7:115:R   +  2916:115:S
+     ksoftirqd/1-7     [01]  1453.070013:      7:115:R   +    10:115:S
+     ksoftirqd/1-7     [01]  1453.070013:      7:115:R ==>    10:115:R
+        events/1-10    [01]  1453.070013:     10:115:S ==>  2916:115:R
+     kondemand/1-2916  [01]  1453.070013:   2916:115:S ==>     7:115:R
+     ksoftirqd/1-7     [01]  1453.070013:      7:115:S ==>     0:140:R
+
+Wake ups are represented by a "+" and the context switches are
+shown as "==>".  The format is:
+
+ Context switches:
+
+       Previous task              Next Task
+
+  <pid>:<prio>:<state>  ==>  <pid>:<prio>:<state>
+
+ Wake ups:
+
+       Current task               Task waking up
+
+  <pid>:<prio>:<state>    +  <pid>:<prio>:<state>
+
+The prio is the internal kernel priority, which is the inverse
+of the priority that is usually displayed by user-space tools.
+Zero represents the highest priority (99). Prio 100 starts the
+"nice" priorities with 100 being equal to nice -20 and 139 being
+nice 19. The prio "140" is reserved for the idle task which is
+the lowest priority thread (pid 0).
+
+
+Latency trace format
+--------------------
+
+For traces that display latency times, the latency_trace file
+gives somewhat more information to see why a latency happened.
+Here is a typical trace.
+
+# tracer: irqsoff
+#
+irqsoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+    -----------------
+    | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0)
+    -----------------
+ => started at: apic_timer_interrupt
+ => ended at:   do_softirq
+
+#                _------=> CPU#
+#               / _-----=> irqs-off
+#              | / _----=> need-resched
+#              || / _---=> hardirq/softirq
+#              ||| / _--=> preempt-depth
+#              |||| /
+#              |||||     delay
+#  cmd     pid ||||| time  |   caller
+#     \   /    |||||   \   |   /
+  <idle>-0     0d..1    0us+: trace_hardirqs_off_thunk (apic_timer_interrupt)
+  <idle>-0     0d.s.   97us : __do_softirq (do_softirq)
+  <idle>-0     0d.s1   98us : trace_hardirqs_on (do_softirq)
+
+
+This shows that the current tracer is "irqsoff" tracing the time
+for which interrupts were disabled. It gives the trace version
+and the version of the kernel upon which this was executed on
+(2.6.26-rc8). Then it displays the max latency in microsecs (97
+us). The number of trace entries displayed and the total number
+recorded (both are three: #3/3). The type of preemption that was
+used (PREEMPT). VP, KP, SP, and HP are always zero and are
+reserved for later use. #P is the number of online CPUS (#P:2).
+
+The task is the process that was running when the latency
+occurred. (swapper pid: 0).
+
+The start and stop (the functions in which the interrupts were
+disabled and enabled respectively) that caused the latencies:
+
+  apic_timer_interrupt is where the interrupts were disabled.
+  do_softirq is where they were enabled again.
+
+The next lines after the header are the trace itself. The header
+explains which is which.
+
+  cmd: The name of the process in the trace.
+
+  pid: The PID of that process.
+
+  CPU#: The CPU which the process was running on.
+
+  irqs-off: 'd' interrupts are disabled. '.' otherwise.
+            Note: If the architecture does not support a way to
+                  read the irq flags variable, an 'X' will always
+                  be printed here.
+
+  need-resched: 'N' task need_resched is set, '.' otherwise.
+
+  hardirq/softirq:
+        'H' - hard irq occurred inside a softirq.
+        'h' - hard irq is running
+        's' - soft irq is running
+        '.' - normal context.
+
+  preempt-depth: The level of preempt_disabled
+
+The above is mostly meaningful for kernel developers.
+
+  time: This differs from the trace file output. The trace file output
+        includes an absolute timestamp. The timestamp used by the
+        latency_trace file is relative to the start of the trace.
+
+  delay: This is just to help catch your eye a bit better. And
+         needs to be fixed to be only relative to the same CPU.
+         The marks are determined by the difference between this
+         current trace and the next trace.
+          '!' - greater than preempt_mark_thresh (default 100)
+          '+' - greater than 1 microsecond
+          ' ' - less than or equal to 1 microsecond.
+
+  The rest is the same as the 'trace' file.
+
+
+trace_options
+-------------
+
+The trace_options file is used to control what gets printed in
+the trace output. To see what is available, simply cat the file:
+
+  cat /debug/tracing/trace_options
+  print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \
+  noblock nostacktrace nosched-tree nouserstacktrace nosym-userobj
+
+To disable one of the options, echo in the option prepended with
+"no".
+
+  echo noprint-parent > /debug/tracing/trace_options
+
+To enable an option, leave off the "no".
+
+  echo sym-offset > /debug/tracing/trace_options
+
+Here are the available options:
+
+  print-parent - On function traces, display the calling (parent)
+                 function as well as the function being traced.
+
+  print-parent:
+   bash-4000  [01]  1477.606694: simple_strtoul <-strict_strtoul
+
+  noprint-parent:
+   bash-4000  [01]  1477.606694: simple_strtoul
+
+
+  sym-offset - Display not only the function name, but also the
+               offset in the function. For example, instead of
+               seeing just "ktime_get", you will see
+               "ktime_get+0xb/0x20".
+
+  sym-offset:
+   bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0
+
+  sym-addr - this will also display the function address as well
+             as the function name.
+
+  sym-addr:
+   bash-4000  [01]  1477.606694: simple_strtoul <c0339346>
+
+  verbose - This deals with the latency_trace file.
+
+    bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
+    (+0.000ms): simple_strtoul (strict_strtoul)
+
+  raw - This will display raw numbers. This option is best for
+        use with user applications that can translate the raw
+        numbers better than having it done in the kernel.
+
+  hex - Similar to raw, but the numbers will be in a hexadecimal
+        format.
+
+  bin - This will print out the formats in raw binary.
+
+  block - TBD (needs update)
+
+  stacktrace - This is one of the options that changes the trace
+               itself. When a trace is recorded, so is the stack
+               of functions. This allows for back traces of
+               trace sites.
+
+  userstacktrace - This option changes the trace. It records a
+                   stacktrace of the current userspace thread.
+
+  sym-userobj - when user stacktrace are enabled, look up which
+                object the address belongs to, and print a
+                relative address. This is especially useful when
+                ASLR is on, otherwise you don't get a chance to
+                resolve the address to object/file/line after
+                the app is no longer running
+
+                The lookup is performed when you read
+                trace,trace_pipe,latency_trace. Example:
+
+                a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
+x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
+
+  sched-tree - trace all tasks that are on the runqueue, at
+               every scheduling event. Will add overhead if
+               there's a lot of tasks running at once.
+
+
+sched_switch
+------------
+
+This tracer simply records schedule switches. Here is an example
+of how to use it.
+
+ # echo sched_switch > /debug/tracing/current_tracer
+ # echo 1 > /debug/tracing/tracing_enabled
+ # sleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/trace
+
+# tracer: sched_switch
+#
+#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
+#              | |      |          |         |
+            bash-3997  [01]   240.132281:   3997:120:R   +  4055:120:R
+            bash-3997  [01]   240.132284:   3997:120:R ==>  4055:120:R
+           sleep-4055  [01]   240.132371:   4055:120:S ==>  3997:120:R
+            bash-3997  [01]   240.132454:   3997:120:R   +  4055:120:S
+            bash-3997  [01]   240.132457:   3997:120:R ==>  4055:120:R
+           sleep-4055  [01]   240.132460:   4055:120:D ==>  3997:120:R
+            bash-3997  [01]   240.132463:   3997:120:R   +  4055:120:D
+            bash-3997  [01]   240.132465:   3997:120:R ==>  4055:120:R
+          <idle>-0     [00]   240.132589:      0:140:R   +     4:115:S
+          <idle>-0     [00]   240.132591:      0:140:R ==>     4:115:R
+     ksoftirqd/0-4     [00]   240.132595:      4:115:S ==>     0:140:R
+          <idle>-0     [00]   240.132598:      0:140:R   +     4:115:S
+          <idle>-0     [00]   240.132599:      0:140:R ==>     4:115:R
+     ksoftirqd/0-4     [00]   240.132603:      4:115:S ==>     0:140:R
+           sleep-4055  [01]   240.133058:   4055:120:S ==>  3997:120:R
+ [...]
+
+
+As we have discussed previously about this format, the header
+shows the name of the trace and points to the options. The
+"FUNCTION" is a misnomer since here it represents the wake ups
+and context switches.
+
+The sched_switch file only lists the wake ups (represented with
+'+') and context switches ('==>') with the previous task or
+current task first followed by the next task or task waking up.
+The format for both of these is PID:KERNEL-PRIO:TASK-STATE.
+Remember that the KERNEL-PRIO is the inverse of the actual
+priority with zero (0) being the highest priority and the nice
+values starting at 100 (nice -20). Below is a quick chart to map
+the kernel priority to user land priorities.
+
+  Kernel priority: 0 to 99    ==> user RT priority 99 to 0
+  Kernel priority: 100 to 139 ==> user nice -20 to 19
+  Kernel priority: 140        ==> idle task priority
+
+The task states are:
+
+ R - running : wants to run, may not actually be running
+ S - sleep   : process is waiting to be woken up (handles signals)
+ D - disk sleep (uninterruptible sleep) : process must be woken up
+                                        (ignores signals)
+ T - stopped : process suspended
+ t - traced  : process is being traced (with something like gdb)
+ Z - zombie  : process waiting to be cleaned up
+ X - unknown
+
+
+ftrace_enabled
+--------------
+
+The following tracers (listed below) give different output
+depending on whether or not the sysctl ftrace_enabled is set. To
+set ftrace_enabled, one can either use the sysctl function or
+set it via the proc file system interface.
+
+  sysctl kernel.ftrace_enabled=1
+
+ or
+
+  echo 1 > /proc/sys/kernel/ftrace_enabled
+
+To disable ftrace_enabled simply replace the '1' with '0' in the
+above commands.
+
+When ftrace_enabled is set the tracers will also record the
+functions that are within the trace. The descriptions of the
+tracers will also show an example with ftrace enabled.
+
+
+irqsoff
+-------
+
+When interrupts are disabled, the CPU can not react to any other
+external event (besides NMIs and SMIs). This prevents the timer
+interrupt from triggering or the mouse interrupt from letting
+the kernel know of a new mouse event. The result is a latency
+with the reaction time.
+
+The irqsoff tracer tracks the time for which interrupts are
+disabled. When a new maximum latency is hit, the tracer saves
+the trace leading up to that latency point so that every time a
+new maximum is reached, the old saved trace is discarded and the
+new trace is saved.
+
+To reset the maximum, echo 0 into tracing_max_latency. Here is
+an example:
+
+ # echo irqsoff > /debug/tracing/current_tracer
+ # echo 0 > /debug/tracing/tracing_max_latency
+ # echo 1 > /debug/tracing/tracing_enabled
+ # ls -ltr
+ [...]
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/latency_trace
+# tracer: irqsoff
+#
+irqsoff latency trace v1.1.5 on 2.6.26
+--------------------------------------------------------------------
+ latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+    -----------------
+    | task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0)
+    -----------------
+ => started at: sys_setpgid
+ => ended at:   sys_setpgid
+
+#                _------=> CPU#
+#               / _-----=> irqs-off
+#              | / _----=> need-resched
+#              || / _---=> hardirq/softirq
+#              ||| / _--=> preempt-depth
+#              |||| /
+#              |||||     delay
+#  cmd     pid ||||| time  |   caller
+#     \   /    |||||   \   |   /
+    bash-3730  1d...    0us : _write_lock_irq (sys_setpgid)
+    bash-3730  1d..1    1us+: _write_unlock_irq (sys_setpgid)
+    bash-3730  1d..2   14us : trace_hardirqs_on (sys_setpgid)
+
+
+Here we see that that we had a latency of 12 microsecs (which is
+very good). The _write_lock_irq in sys_setpgid disabled
+interrupts. The difference between the 12 and the displayed
+timestamp 14us occurred because the clock was incremented
+between the time of recording the max latency and the time of
+recording the function that had that latency.
+
+Note the above example had ftrace_enabled not set. If we set the
+ftrace_enabled, we get a much larger output:
+
+# tracer: irqsoff
+#
+irqsoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+    -----------------
+    | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0)
+    -----------------
+ => started at: __alloc_pages_internal
+ => ended at:   __alloc_pages_internal
+
+#                _------=> CPU#
+#               / _-----=> irqs-off
+#              | / _----=> need-resched
+#              || / _---=> hardirq/softirq
+#              ||| / _--=> preempt-depth
+#              |||| /
+#              |||||     delay
+#  cmd     pid ||||| time  |   caller
+#     \   /    |||||   \   |   /
+      ls-4339  0...1    0us+: get_page_from_freelist (__alloc_pages_internal)
+      ls-4339  0d..1    3us : rmqueue_bulk (get_page_from_freelist)
+      ls-4339  0d..1    3us : _spin_lock (rmqueue_bulk)
+      ls-4339  0d..1    4us : add_preempt_count (_spin_lock)
+      ls-4339  0d..2    4us : __rmqueue (rmqueue_bulk)
+      ls-4339  0d..2    5us : __rmqueue_smallest (__rmqueue)
+      ls-4339  0d..2    5us : __mod_zone_page_state (__rmqueue_smallest)
+      ls-4339  0d..2    6us : __rmqueue (rmqueue_bulk)
+      ls-4339  0d..2    6us : __rmqueue_smallest (__rmqueue)
+      ls-4339  0d..2    7us : __mod_zone_page_state (__rmqueue_smallest)
+      ls-4339  0d..2    7us : __rmqueue (rmqueue_bulk)
+      ls-4339  0d..2    8us : __rmqueue_smallest (__rmqueue)
+[...]
+      ls-4339  0d..2   46us : __rmqueue_smallest (__rmqueue)
+      ls-4339  0d..2   47us : __mod_zone_page_state (__rmqueue_smallest)
+      ls-4339  0d..2   47us : __rmqueue (rmqueue_bulk)
+      ls-4339  0d..2   48us : __rmqueue_smallest (__rmqueue)
+      ls-4339  0d..2   48us : __mod_zone_page_state (__rmqueue_smallest)
+      ls-4339  0d..2   49us : _spin_unlock (rmqueue_bulk)
+      ls-4339  0d..2   49us : sub_preempt_count (_spin_unlock)
+      ls-4339  0d..1   50us : get_page_from_freelist (__alloc_pages_internal)
+      ls-4339  0d..2   51us : trace_hardirqs_on (__alloc_pages_internal)
+
+
+
+Here we traced a 50 microsecond latency. But we also see all the
+functions that were called during that time. Note that by
+enabling function tracing, we incur an added overhead. This
+overhead may extend the latency times. But nevertheless, this
+trace has provided some very helpful debugging information.
+
+
+preemptoff
+----------
+
+When preemption is disabled, we may be able to receive
+interrupts but the task cannot be preempted and a higher
+priority task must wait for preemption to be enabled again
+before it can preempt a lower priority task.
+
+The preemptoff tracer traces the places that disable preemption.
+Like the irqsoff tracer, it records the maximum latency for
+which preemption was disabled. The control of preemptoff tracer
+is much like the irqsoff tracer.
+
+ # echo preemptoff > /debug/tracing/current_tracer
+ # echo 0 > /debug/tracing/tracing_max_latency
+ # echo 1 > /debug/tracing/tracing_enabled
+ # ls -ltr
+ [...]
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/latency_trace
+# tracer: preemptoff
+#
+preemptoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+    -----------------
+    | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
+    -----------------
+ => started at: do_IRQ
+ => ended at:   __do_softirq
+
+#                _------=> CPU#
+#               / _-----=> irqs-off
+#              | / _----=> need-resched
+#              || / _---=> hardirq/softirq
+#              ||| / _--=> preempt-depth
+#              |||| /
+#              |||||     delay
+#  cmd     pid ||||| time  |   caller
+#     \   /    |||||   \   |   /
+    sshd-4261  0d.h.    0us+: irq_enter (do_IRQ)
+    sshd-4261  0d.s.   29us : _local_bh_enable (__do_softirq)
+    sshd-4261  0d.s1   30us : trace_preempt_on (__do_softirq)
+
+
+This has some more changes. Preemption was disabled when an
+interrupt came in (notice the 'h'), and was enabled while doing
+a softirq. (notice the 's'). But we also see that interrupts
+have been disabled when entering the preempt off section and
+leaving it (the 'd'). We do not know if interrupts were enabled
+in the mean time.
+
+# tracer: preemptoff
+#
+preemptoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+    -----------------
+    | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
+    -----------------
+ => started at: remove_wait_queue
+ => ended at:   __do_softirq
+
+#                _------=> CPU#
+#               / _-----=> irqs-off
+#              | / _----=> need-resched
+#              || / _---=> hardirq/softirq
+#              ||| / _--=> preempt-depth
+#              |||| /
+#              |||||     delay
+#  cmd     pid ||||| time  |   caller
+#     \   /    |||||   \   |   /
+    sshd-4261  0d..1    0us : _spin_lock_irqsave (remove_wait_queue)
+    sshd-4261  0d..1    1us : _spin_unlock_irqrestore (remove_wait_queue)
+    sshd-4261  0d..1    2us : do_IRQ (common_interrupt)
+    sshd-4261  0d..1    2us : irq_enter (do_IRQ)
+    sshd-4261  0d..1    2us : idle_cpu (irq_enter)
+    sshd-4261  0d..1    3us : add_preempt_count (irq_enter)
+    sshd-4261  0d.h1    3us : idle_cpu (irq_enter)
+    sshd-4261  0d.h.    4us : handle_fasteoi_irq (do_IRQ)
+[...]
+    sshd-4261  0d.h.   12us : add_preempt_count (_spin_lock)
+    sshd-4261  0d.h1   12us : ack_ioapic_quirk_irq (handle_fasteoi_irq)
+    sshd-4261  0d.h1   13us : move_native_irq (ack_ioapic_quirk_irq)
+    sshd-4261  0d.h1   13us : _spin_unlock (handle_fasteoi_irq)
+    sshd-4261  0d.h1   14us : sub_preempt_count (_spin_unlock)
+    sshd-4261  0d.h1   14us : irq_exit (do_IRQ)
+    sshd-4261  0d.h1   15us : sub_preempt_count (irq_exit)
+    sshd-4261  0d..2   15us : do_softirq (irq_exit)
+    sshd-4261  0d...   15us : __do_softirq (do_softirq)
+    sshd-4261  0d...   16us : __local_bh_disable (__do_softirq)
+    sshd-4261  0d...   16us+: add_preempt_count (__local_bh_disable)
+    sshd-4261  0d.s4   20us : add_preempt_count (__local_bh_disable)
+    sshd-4261  0d.s4   21us : sub_preempt_count (local_bh_enable)
+    sshd-4261  0d.s5   21us : sub_preempt_count (local_bh_enable)
+[...]
+    sshd-4261  0d.s6   41us : add_preempt_count (__local_bh_disable)
+    sshd-4261  0d.s6   42us : sub_preempt_count (local_bh_enable)
+    sshd-4261  0d.s7   42us : sub_preempt_count (local_bh_enable)
+    sshd-4261  0d.s5   43us : add_preempt_count (__local_bh_disable)
+    sshd-4261  0d.s5   43us : sub_preempt_count (local_bh_enable_ip)
+    sshd-4261  0d.s6   44us : sub_preempt_count (local_bh_enable_ip)
+    sshd-4261  0d.s5   44us : add_preempt_count (__local_bh_disable)
+    sshd-4261  0d.s5   45us : sub_preempt_count (local_bh_enable)
+[...]
+    sshd-4261  0d.s.   63us : _local_bh_enable (__do_softirq)
+    sshd-4261  0d.s1   64us : trace_preempt_on (__do_softirq)
+
+
+The above is an example of the preemptoff trace with
+ftrace_enabled set. Here we see that interrupts were disabled
+the entire time. The irq_enter code lets us know that we entered
+an interrupt 'h'. Before that, the functions being traced still
+show that it is not in an interrupt, but we can see from the
+functions themselves that this is not the case.
+
+Notice that __do_softirq when called does not have a
+preempt_count. It may seem that we missed a preempt enabling.
+What really happened is that the preempt count is held on the
+thread's stack and we switched to the softirq stack (4K stacks
+in effect). The code does not copy the preempt count, but
+because interrupts are disabled, we do not need to worry about
+it. Having a tracer like this is good for letting people know
+what really happens inside the kernel.
+
+
+preemptirqsoff
+--------------
+
+Knowing the locations that have interrupts disabled or
+preemption disabled for the longest times is helpful. But
+sometimes we would like to know when either preemption and/or
+interrupts are disabled.
+
+Consider the following code:
+
+    local_irq_disable();
+    call_function_with_irqs_off();
+    preempt_disable();
+    call_function_with_irqs_and_preemption_off();
+    local_irq_enable();
+    call_function_with_preemption_off();
+    preempt_enable();
+
+The irqsoff tracer will record the total length of
+call_function_with_irqs_off() and
+call_function_with_irqs_and_preemption_off().
+
+The preemptoff tracer will record the total length of
+call_function_with_irqs_and_preemption_off() and
+call_function_with_preemption_off().
+
+But neither will trace the time that interrupts and/or
+preemption is disabled. This total time is the time that we can
+not schedule. To record this time, use the preemptirqsoff
+tracer.
+
+Again, using this trace is much like the irqsoff and preemptoff
+tracers.
+
+ # echo preemptirqsoff > /debug/tracing/current_tracer
+ # echo 0 > /debug/tracing/tracing_max_latency
+ # echo 1 > /debug/tracing/tracing_enabled
+ # ls -ltr
+ [...]
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/latency_trace
+# tracer: preemptirqsoff
+#
+preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+    -----------------
+    | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0)
+    -----------------
+ => started at: apic_timer_interrupt
+ => ended at:   __do_softirq
+
+#                _------=> CPU#
+#               / _-----=> irqs-off
+#              | / _----=> need-resched
+#              || / _---=> hardirq/softirq
+#              ||| / _--=> preempt-depth
+#              |||| /
+#              |||||     delay
+#  cmd     pid ||||| time  |   caller
+#     \   /    |||||   \   |   /
+      ls-4860  0d...    0us!: trace_hardirqs_off_thunk (apic_timer_interrupt)
+      ls-4860  0d.s.  294us : _local_bh_enable (__do_softirq)
+      ls-4860  0d.s1  294us : trace_preempt_on (__do_softirq)
+
+
+
+The trace_hardirqs_off_thunk is called from assembly on x86 when
+interrupts are disabled in the assembly code. Without the
+function tracing, we do not know if interrupts were enabled
+within the preemption points. We do see that it started with
+preemption enabled.
+
+Here is a trace with ftrace_enabled set:
+
+
+# tracer: preemptirqsoff
+#
+preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+    -----------------
+    | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
+    -----------------
+ => started at: write_chan
+ => ended at:   __do_softirq
+
+#                _------=> CPU#
+#               / _-----=> irqs-off
+#              | / _----=> need-resched
+#              || / _---=> hardirq/softirq
+#              ||| / _--=> preempt-depth
+#              |||| /
+#              |||||     delay
+#  cmd     pid ||||| time  |   caller
+#     \   /    |||||   \   |   /
+      ls-4473  0.N..    0us : preempt_schedule (write_chan)
+      ls-4473  0dN.1    1us : _spin_lock (schedule)
+      ls-4473  0dN.1    2us : add_preempt_count (_spin_lock)
+      ls-4473  0d..2    2us : put_prev_task_fair (schedule)
+[...]
+      ls-4473  0d..2   13us : set_normalized_timespec (ktime_get_ts)
+      ls-4473  0d..2   13us : __switch_to (schedule)
+    sshd-4261  0d..2   14us : finish_task_switch (schedule)
+    sshd-4261  0d..2   14us : _spin_unlock_irq (finish_task_switch)
+    sshd-4261  0d..1   15us : add_preempt_count (_spin_lock_irqsave)
+    sshd-4261  0d..2   16us : _spin_unlock_irqrestore (hrtick_set)
+    sshd-4261  0d..2   16us : do_IRQ (common_interrupt)
+    sshd-4261  0d..2   17us : irq_enter (do_IRQ)
+    sshd-4261  0d..2   17us : idle_cpu (irq_enter)
+    sshd-4261  0d..2   18us : add_preempt_count (irq_enter)
+    sshd-4261  0d.h2   18us : idle_cpu (irq_enter)
+    sshd-4261  0d.h.   18us : handle_fasteoi_irq (do_IRQ)
+    sshd-4261  0d.h.   19us : _spin_lock (handle_fasteoi_irq)
+    sshd-4261  0d.h.   19us : add_preempt_count (_spin_lock)
+    sshd-4261  0d.h1   20us : _spin_unlock (handle_fasteoi_irq)
+    sshd-4261  0d.h1   20us : sub_preempt_count (_spin_unlock)
+[...]
+    sshd-4261  0d.h1   28us : _spin_unlock (handle_fasteoi_irq)
+    sshd-4261  0d.h1   29us : sub_preempt_count (_spin_unlock)
+    sshd-4261  0d.h2   29us : irq_exit (do_IRQ)
+    sshd-4261  0d.h2   29us : sub_preempt_count (irq_exit)
+    sshd-4261  0d..3   30us : do_softirq (irq_exit)
+    sshd-4261  0d...   30us : __do_softirq (do_softirq)
+    sshd-4261  0d...   31us : __local_bh_disable (__do_softirq)
+    sshd-4261  0d...   31us+: add_preempt_count (__local_bh_disable)
+    sshd-4261  0d.s4   34us : add_preempt_count (__local_bh_disable)
+[...]
+    sshd-4261  0d.s3   43us : sub_preempt_count (local_bh_enable_ip)
+    sshd-4261  0d.s4   44us : sub_preempt_count (local_bh_enable_ip)
+    sshd-4261  0d.s3   44us : smp_apic_timer_interrupt (apic_timer_interrupt)
+    sshd-4261  0d.s3   45us : irq_enter (smp_apic_timer_interrupt)
+    sshd-4261  0d.s3   45us : idle_cpu (irq_enter)
+    sshd-4261  0d.s3   46us : add_preempt_count (irq_enter)
+    sshd-4261  0d.H3   46us : idle_cpu (irq_enter)
+    sshd-4261  0d.H3   47us : hrtimer_interrupt (smp_apic_timer_interrupt)
+    sshd-4261  0d.H3   47us : ktime_get (hrtimer_interrupt)
+[...]
+    sshd-4261  0d.H3   81us : tick_program_event (hrtimer_interrupt)
+    sshd-4261  0d.H3   82us : ktime_get (tick_program_event)
+    sshd-4261  0d.H3   82us : ktime_get_ts (ktime_get)
+    sshd-4261  0d.H3   83us : getnstimeofday (ktime_get_ts)
+    sshd-4261  0d.H3   83us : set_normalized_timespec (ktime_get_ts)
+    sshd-4261  0d.H3   84us : clockevents_program_event (tick_program_event)
+    sshd-4261  0d.H3   84us : lapic_next_event (clockevents_program_event)
+    sshd-4261  0d.H3   85us : irq_exit (smp_apic_timer_interrupt)
+    sshd-4261  0d.H3   85us : sub_preempt_count (irq_exit)
+    sshd-4261  0d.s4   86us : sub_preempt_count (irq_exit)
+    sshd-4261  0d.s3   86us : add_preempt_count (__local_bh_disable)
+[...]
+    sshd-4261  0d.s1   98us : sub_preempt_count (net_rx_action)
+    sshd-4261  0d.s.   99us : add_preempt_count (_spin_lock_irq)
+    sshd-4261  0d.s1   99us+: _spin_unlock_irq (run_timer_softirq)
+    sshd-4261  0d.s.  104us : _local_bh_enable (__do_softirq)
+    sshd-4261  0d.s.  104us : sub_preempt_count (_local_bh_enable)
+    sshd-4261  0d.s.  105us : _local_bh_enable (__do_softirq)
+    sshd-4261  0d.s1  105us : trace_preempt_on (__do_softirq)
+
+
+This is a very interesting trace. It started with the preemption
+of the ls task. We see that the task had the "need_resched" bit
+set via the 'N' in the trace.  Interrupts were disabled before
+the spin_lock at the beginning of the trace. We see that a
+schedule took place to run sshd.  When the interrupts were
+enabled, we took an interrupt. On return from the interrupt
+handler, the softirq ran. We took another interrupt while
+running the softirq as we see from the capital 'H'.
+
+
+wakeup
+------
+
+In a Real-Time environment it is very important to know the
+wakeup time it takes for the highest priority task that is woken
+up to the time that it executes. This is also known as "schedule
+latency". I stress the point that this is about RT tasks. It is
+also important to know the scheduling latency of non-RT tasks,
+but the average schedule latency is better for non-RT tasks.
+Tools like LatencyTop are more appropriate for such
+measurements.
+
+Real-Time environments are interested in the worst case latency.
+That is the longest latency it takes for something to happen,
+and not the average. We can have a very fast scheduler that may
+only have a large latency once in a while, but that would not
+work well with Real-Time tasks.  The wakeup tracer was designed
+to record the worst case wakeups of RT tasks. Non-RT tasks are
+not recorded because the tracer only records one worst case and
+tracing non-RT tasks that are unpredictable will overwrite the
+worst case latency of RT tasks.
+
+Since this tracer only deals with RT tasks, we will run this
+slightly differently than we did with the previous tracers.
+Instead of performing an 'ls', we will run 'sleep 1' under
+'chrt' which changes the priority of the task.
+
+ # echo wakeup > /debug/tracing/current_tracer
+ # echo 0 > /debug/tracing/tracing_max_latency
+ # echo 1 > /debug/tracing/tracing_enabled
+ # chrt -f 5 sleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/latency_trace
+# tracer: wakeup
+#
+wakeup latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+    -----------------
+    | task: sleep-4901 (uid:0 nice:0 policy:1 rt_prio:5)
+    -----------------
+
+#                _------=> CPU#
+#               / _-----=> irqs-off
+#              | / _----=> need-resched
+#              || / _---=> hardirq/softirq
+#              ||| / _--=> preempt-depth
+#              |||| /
+#              |||||     delay
+#  cmd     pid ||||| time  |   caller
+#     \   /    |||||   \   |   /
+  <idle>-0     1d.h4    0us+: try_to_wake_up (wake_up_process)
+  <idle>-0     1d..4    4us : schedule (cpu_idle)
+
+
+Running this on an idle system, we see that it only took 4
+microseconds to perform the task switch.  Note, since the trace
+marker in the schedule is before the actual "switch", we stop
+the tracing when the recorded task is about to schedule in. This
+may change if we add a new marker at the end of the scheduler.
+
+Notice that the recorded task is 'sleep' with the PID of 4901
+and it has an rt_prio of 5. This priority is user-space priority
+and not the internal kernel priority. The policy is 1 for
+SCHED_FIFO and 2 for SCHED_RR.
+
+Doing the same with chrt -r 5 and ftrace_enabled set.
+
+# tracer: wakeup
+#
+wakeup latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+    -----------------
+    | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5)
+    -----------------
+
+#                _------=> CPU#
+#               / _-----=> irqs-off
+#              | / _----=> need-resched
+#              || / _---=> hardirq/softirq
+#              ||| / _--=> preempt-depth
+#              |||| /
+#              |||||     delay
+#  cmd     pid ||||| time  |   caller
+#     \   /    |||||   \   |   /
+ksoftirq-7     1d.H3    0us : try_to_wake_up (wake_up_process)
+ksoftirq-7     1d.H4    1us : sub_preempt_count (marker_probe_cb)
+ksoftirq-7     1d.H3    2us : check_preempt_wakeup (try_to_wake_up)
+ksoftirq-7     1d.H3    3us : update_curr (check_preempt_wakeup)
+ksoftirq-7     1d.H3    4us : calc_delta_mine (update_curr)
+ksoftirq-7     1d.H3    5us : __resched_task (check_preempt_wakeup)
+ksoftirq-7     1d.H3    6us : task_wake_up_rt (try_to_wake_up)
+ksoftirq-7     1d.H3    7us : _spin_unlock_irqrestore (try_to_wake_up)
+[...]
+ksoftirq-7     1d.H2   17us : irq_exit (smp_apic_timer_interrupt)
+ksoftirq-7     1d.H2   18us : sub_preempt_count (irq_exit)
+ksoftirq-7     1d.s3   19us : sub_preempt_count (irq_exit)
+ksoftirq-7     1..s2   20us : rcu_process_callbacks (__do_softirq)
+[...]
+ksoftirq-7     1..s2   26us : __rcu_process_callbacks (rcu_process_callbacks)
+ksoftirq-7     1d.s2   27us : _local_bh_enable (__do_softirq)
+ksoftirq-7     1d.s2   28us : sub_preempt_count (_local_bh_enable)
+ksoftirq-7     1.N.3   29us : sub_preempt_count (ksoftirqd)
+ksoftirq-7     1.N.2   30us : _cond_resched (ksoftirqd)
+ksoftirq-7     1.N.2   31us : __cond_resched (_cond_resched)
+ksoftirq-7     1.N.2   32us : add_preempt_count (__cond_resched)
+ksoftirq-7     1.N.2   33us : schedule (__cond_resched)
+ksoftirq-7     1.N.2   33us : add_preempt_count (schedule)
+ksoftirq-7     1.N.3   34us : hrtick_clear (schedule)
+ksoftirq-7     1dN.3   35us : _spin_lock (schedule)
+ksoftirq-7     1dN.3   36us : add_preempt_count (_spin_lock)
+ksoftirq-7     1d..4   37us : put_prev_task_fair (schedule)
+ksoftirq-7     1d..4   38us : update_curr (put_prev_task_fair)
+[...]
+ksoftirq-7     1d..5   47us : _spin_trylock (tracing_record_cmdline)
+ksoftirq-7     1d..5   48us : add_preempt_count (_spin_trylock)
+ksoftirq-7     1d..6   49us : _spin_unlock (tracing_record_cmdline)
+ksoftirq-7     1d..6   49us : sub_preempt_count (_spin_unlock)
+ksoftirq-7     1d..4   50us : schedule (__cond_resched)
+
+The interrupt went off while running ksoftirqd. This task runs
+at SCHED_OTHER. Why did not we see the 'N' set early? This may
+be a harmless bug with x86_32 and 4K stacks. On x86_32 with 4K
+stacks configured, the interrupt and softirq run with their own
+stack. Some information is held on the top of the task's stack
+(need_resched and preempt_count are both stored there). The
+setting of the NEED_RESCHED bit is done directly to the task's
+stack, but the reading of the NEED_RESCHED is done by looking at
+the current stack, which in this case is the stack for the hard
+interrupt. This hides the fact that NEED_RESCHED has been set.
+We do not see the 'N' until we switch back to the task's
+assigned stack.
+
+function
+--------
+
+This tracer is the function tracer. Enabling the function tracer
+can be done from the debug file system. Make sure the
+ftrace_enabled is set; otherwise this tracer is a nop.
+
+ # sysctl kernel.ftrace_enabled=1
+ # echo function > /debug/tracing/current_tracer
+ # echo 1 > /debug/tracing/tracing_enabled
+ # usleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/trace
+# tracer: function
+#
+#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
+#              | |      |          |         |
+            bash-4003  [00]   123.638713: finish_task_switch <-schedule
+            bash-4003  [00]   123.638714: _spin_unlock_irq <-finish_task_switch
+            bash-4003  [00]   123.638714: sub_preempt_count <-_spin_unlock_irq
+            bash-4003  [00]   123.638715: hrtick_set <-schedule
+            bash-4003  [00]   123.638715: _spin_lock_irqsave <-hrtick_set
+            bash-4003  [00]   123.638716: add_preempt_count <-_spin_lock_irqsave
+            bash-4003  [00]   123.638716: _spin_unlock_irqrestore <-hrtick_set
+            bash-4003  [00]   123.638717: sub_preempt_count <-_spin_unlock_irqrestore
+            bash-4003  [00]   123.638717: hrtick_clear <-hrtick_set
+            bash-4003  [00]   123.638718: sub_preempt_count <-schedule
+            bash-4003  [00]   123.638718: sub_preempt_count <-preempt_schedule
+            bash-4003  [00]   123.638719: wait_for_completion <-__stop_machine_run
+            bash-4003  [00]   123.638719: wait_for_common <-wait_for_completion
+            bash-4003  [00]   123.638720: _spin_lock_irq <-wait_for_common
+            bash-4003  [00]   123.638720: add_preempt_count <-_spin_lock_irq
+[...]
+
+
+Note: function tracer uses ring buffers to store the above
+entries. The newest data may overwrite the oldest data.
+Sometimes using echo to stop the trace is not sufficient because
+the tracing could have overwritten the data that you wanted to
+record. For this reason, it is sometimes better to disable
+tracing directly from a program. This allows you to stop the
+tracing at the point that you hit the part that you are
+interested in. To disable the tracing directly from a C program,
+something like following code snippet can be used:
+
+int trace_fd;
+[...]
+int main(int argc, char *argv[]) {
+        [...]
+        trace_fd = open("/debug/tracing/tracing_enabled", O_WRONLY);
+        [...]
+        if (condition_hit()) {
+                write(trace_fd, "0", 1);
+        }
+        [...]
+}
+
+Note: Here we hard coded the path name. The debugfs mount is not
+guaranteed to be at /debug (and is more commonly at
+/sys/kernel/debug). For simple one time traces, the above is
+sufficent. For anything else, a search through /proc/mounts may
+be needed to find where the debugfs file-system is mounted.
+
+
+Single thread tracing
+---------------------
+
+By writing into /debug/tracing/set_ftrace_pid you can trace a
+single thread. For example:
+
+# cat /debug/tracing/set_ftrace_pid
+no pid
+# echo 3111 > /debug/tracing/set_ftrace_pid
+# cat /debug/tracing/set_ftrace_pid
+3111
+# echo function > /debug/tracing/current_tracer
+# cat /debug/tracing/trace | head
+ # tracer: function
+ #
+ #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
+ #              | |       |          |         |
+     yum-updatesd-3111  [003]  1637.254676: finish_task_switch <-thread_return
+     yum-updatesd-3111  [003]  1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
+     yum-updatesd-3111  [003]  1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
+     yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
+     yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
+     yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll
+# echo -1 > /debug/tracing/set_ftrace_pid
+# cat /debug/tracing/trace |head
+ # tracer: function
+ #
+ #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
+ #              | |       |          |         |
+ ##### CPU 3 buffer started ####
+     yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
+     yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
+     yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
+     yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
+     yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit
+
+If you want to trace a function when executing, you could use
+something like this simple program:
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+int main (int argc, char **argv)
+{
+        if (argc < 1)
+                exit(-1);
+
+        if (fork() > 0) {
+                int fd, ffd;
+                char line[64];
+                int s;
+
+                ffd = open("/debug/tracing/current_tracer", O_WRONLY);
+                if (ffd < 0)
+                        exit(-1);
+                write(ffd, "nop", 3);
+
+                fd = open("/debug/tracing/set_ftrace_pid", O_WRONLY);
+                s = sprintf(line, "%d\n", getpid());
+                write(fd, line, s);
+
+                write(ffd, "function", 8);
+
+                close(fd);
+                close(ffd);
+
+                execvp(argv[1], argv+1);
+        }
+
+        return 0;
+}
+
+
+hw-branch-tracer (x86 only)
+---------------------------
+
+This tracer uses the x86 last branch tracing hardware feature to
+collect a branch trace on all cpus with relatively low overhead.
+
+The tracer uses a fixed-size circular buffer per cpu and only
+traces ring 0 branches. The trace file dumps that buffer in the
+following format:
+
+# tracer: hw-branch-tracer
+#
+# CPU#        TO  <-  FROM
+   0  scheduler_tick+0xb5/0x1bf   <-  task_tick_idle+0x5/0x6
+   2  run_posix_cpu_timers+0x2b/0x72a     <-  run_posix_cpu_timers+0x25/0x72a
+   0  scheduler_tick+0x139/0x1bf          <-  scheduler_tick+0xed/0x1bf
+   0  scheduler_tick+0x17c/0x1bf          <-  scheduler_tick+0x148/0x1bf
+   2  run_posix_cpu_timers+0x9e/0x72a     <-  run_posix_cpu_timers+0x5e/0x72a
+   0  scheduler_tick+0x1b6/0x1bf          <-  scheduler_tick+0x1aa/0x1bf
+
+
+The tracer may be used to dump the trace for the oops'ing cpu on
+a kernel oops into the system log. To enable this,
+ftrace_dump_on_oops must be set. To set ftrace_dump_on_oops, one
+can either use the sysctl function or set it via the proc system
+interface.
+
+  sysctl kernel.ftrace_dump_on_oops=1
+
+or
+
+  echo 1 > /proc/sys/kernel/ftrace_dump_on_oops
+
+
+Here's an example of such a dump after a null pointer
+dereference in a kernel module:
+
+[57848.105921] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
+[57848.106019] IP: [<ffffffffa0000006>] open+0x6/0x14 [oops]
+[57848.106019] PGD 2354e9067 PUD 2375e7067 PMD 0
+[57848.106019] Oops: 0002 [#1] SMP
+[57848.106019] last sysfs file: /sys/devices/pci0000:00/0000:00:1e.0/0000:20:05.0/local_cpus
+[57848.106019] Dumping ftrace buffer:
+[57848.106019] ---------------------------------
+[...]
+[57848.106019]    0  chrdev_open+0xe6/0x165       <-  cdev_put+0x23/0x24
+[57848.106019]    0  chrdev_open+0x117/0x165      <-  chrdev_open+0xfa/0x165
+[57848.106019]    0  chrdev_open+0x120/0x165      <-  chrdev_open+0x11c/0x165
+[57848.106019]    0  chrdev_open+0x134/0x165      <-  chrdev_open+0x12b/0x165
+[57848.106019]    0  open+0x0/0x14 [oops]         <-  chrdev_open+0x144/0x165
+[57848.106019]    0  page_fault+0x0/0x30          <-  open+0x6/0x14 [oops]
+[57848.106019]    0  error_entry+0x0/0x5b         <-  page_fault+0x4/0x30
+[57848.106019]    0  error_kernelspace+0x0/0x31   <-  error_entry+0x59/0x5b
+[57848.106019]    0  error_sti+0x0/0x1    <-  error_kernelspace+0x2d/0x31
+[57848.106019]    0  page_fault+0x9/0x30          <-  error_sti+0x0/0x1
+[57848.106019]    0  do_page_fault+0x0/0x881      <-  page_fault+0x1a/0x30
+[...]
+[57848.106019]    0  do_page_fault+0x66b/0x881    <-  is_prefetch+0x1ee/0x1f2
+[57848.106019]    0  do_page_fault+0x6e0/0x881    <-  do_page_fault+0x67a/0x881
+[57848.106019]    0  oops_begin+0x0/0x96          <-  do_page_fault+0x6e0/0x881
+[57848.106019]    0  trace_hw_branch_oops+0x0/0x2d        <-  oops_begin+0x9/0x96
+[...]
+[57848.106019]    0  ds_suspend_bts+0x2a/0xe3     <-  ds_suspend_bts+0x1a/0xe3
+[57848.106019] ---------------------------------
+[57848.106019] CPU 0
+[57848.106019] Modules linked in: oops
+[57848.106019] Pid: 5542, comm: cat Tainted: G        W  2.6.28 #23
+[57848.106019] RIP: 0010:[<ffffffffa0000006>]  [<ffffffffa0000006>] open+0x6/0x14 [oops]
+[57848.106019] RSP: 0018:ffff880235457d48  EFLAGS: 00010246
+[...]
+
+
+function graph tracer
+---------------------------
+
+This tracer is similar to the function tracer except that it
+probes a function on its entry and its exit. This is done by
+using a dynamically allocated stack of return addresses in each
+task_struct. On function entry the tracer overwrites the return
+address of each function traced to set a custom probe. Thus the
+original return address is stored on the stack of return address
+in the task_struct.
+
+Probing on both ends of a function leads to special features
+such as:
+
+- measure of a function's time execution
+- having a reliable call stack to draw function calls graph
+
+This tracer is useful in several situations:
+
+- you want to find the reason of a strange kernel behavior and
+  need to see what happens in detail on any areas (or specific
+  ones).
+
+- you are experiencing weird latencies but it's difficult to
+  find its origin.
+
+- you want to find quickly which path is taken by a specific
+  function
+
+- you just want to peek inside a working kernel and want to see
+  what happens there.
+
+# tracer: function_graph
+#
+# CPU  DURATION                  FUNCTION CALLS
+# |     |   |                     |   |   |   |
+
+ 0)               |  sys_open() {
+ 0)               |    do_sys_open() {
+ 0)               |      getname() {
+ 0)               |        kmem_cache_alloc() {
+ 0)   1.382 us    |          __might_sleep();
+ 0)   2.478 us    |        }
+ 0)               |        strncpy_from_user() {
+ 0)               |          might_fault() {
+ 0)   1.389 us    |            __might_sleep();
+ 0)   2.553 us    |          }
+ 0)   3.807 us    |        }
+ 0)   7.876 us    |      }
+ 0)               |      alloc_fd() {
+ 0)   0.668 us    |        _spin_lock();
+ 0)   0.570 us    |        expand_files();
+ 0)   0.586 us    |        _spin_unlock();
+
+
+There are several columns that can be dynamically
+enabled/disabled. You can use every combination of options you
+want, depending on your needs.
+
+- The cpu number on which the function executed is default
+  enabled.  It is sometimes better to only trace one cpu (see
+  tracing_cpu_mask file) or you might sometimes see unordered
+  function calls while cpu tracing switch.
+
+        hide: echo nofuncgraph-cpu > /debug/tracing/trace_options
+        show: echo funcgraph-cpu > /debug/tracing/trace_options
+
+- The duration (function's time of execution) is displayed on
+  the closing bracket line of a function or on the same line
+  than the current function in case of a leaf one. It is default
+  enabled.
+
+        hide: echo nofuncgraph-duration > /debug/tracing/trace_options
+        show: echo funcgraph-duration > /debug/tracing/trace_options
+
+- The overhead field precedes the duration field in case of
+  reached duration thresholds.
+
+        hide: echo nofuncgraph-overhead > /debug/tracing/trace_options
+        show: echo funcgraph-overhead > /debug/tracing/trace_options
+        depends on: funcgraph-duration
+
+  ie:
+
+  0)               |    up_write() {
+  0)   0.646 us    |      _spin_lock_irqsave();
+  0)   0.684 us    |      _spin_unlock_irqrestore();
+  0)   3.123 us    |    }
+  0)   0.548 us    |    fput();
+  0) + 58.628 us   |  }
+
+  [...]
+
+  0)               |      putname() {
+  0)               |        kmem_cache_free() {
+  0)   0.518 us    |          __phys_addr();
+  0)   1.757 us    |        }
+  0)   2.861 us    |      }
+  0) ! 115.305 us  |    }
+  0) ! 116.402 us  |  }
+
+  + means that the function exceeded 10 usecs.
+  ! means that the function exceeded 100 usecs.
+
+
+- The task/pid field displays the thread cmdline and pid which
+  executed the function. It is default disabled.
+
+        hide: echo nofuncgraph-proc > /debug/tracing/trace_options
+        show: echo funcgraph-proc > /debug/tracing/trace_options
+
+  ie:
+
+  # tracer: function_graph
+  #
+  # CPU  TASK/PID        DURATION                  FUNCTION CALLS
+  # |    |    |           |   |                     |   |   |   |
+  0)    sh-4802     |               |                  d_free() {
+  0)    sh-4802     |               |                    call_rcu() {
+  0)    sh-4802     |               |                      __call_rcu() {
+  0)    sh-4802     |   0.616 us    |                        rcu_process_gp_end();
+  0)    sh-4802     |   0.586 us    |                        check_for_new_grace_period();
+  0)    sh-4802     |   2.899 us    |                      }
+  0)    sh-4802     |   4.040 us    |                    }
+  0)    sh-4802     |   5.151 us    |                  }
+  0)    sh-4802     | + 49.370 us   |                }
+
+
+- The absolute time field is an absolute timestamp given by the
+  system clock since it started. A snapshot of this time is
+  given on each entry/exit of functions
+
+        hide: echo nofuncgraph-abstime > /debug/tracing/trace_options
+        show: echo funcgraph-abstime > /debug/tracing/trace_options
+
+  ie:
+
+  #
+  #      TIME       CPU  DURATION                  FUNCTION CALLS
+  #       |         |     |   |                     |   |   |   |
+  360.774522 |   1)   0.541 us    |                                          }
+  360.774522 |   1)   4.663 us    |                                        }
+  360.774523 |   1)   0.541 us    |                                        __wake_up_bit();
+  360.774524 |   1)   6.796 us    |                                      }
+  360.774524 |   1)   7.952 us    |                                    }
+  360.774525 |   1)   9.063 us    |                                  }
+  360.774525 |   1)   0.615 us    |                                  journal_mark_dirty();
+  360.774527 |   1)   0.578 us    |                                  __brelse();
+  360.774528 |   1)               |                                  reiserfs_prepare_for_journal() {
+  360.774528 |   1)               |                                    unlock_buffer() {
+  360.774529 |   1)               |                                      wake_up_bit() {
+  360.774529 |   1)               |                                        bit_waitqueue() {
+  360.774530 |   1)   0.594 us    |                                          __phys_addr();
+
+
+You can put some comments on specific functions by using
+trace_printk() For example, if you want to put a comment inside
+the __might_sleep() function, you just have to include
+<linux/ftrace.h> and call trace_printk() inside __might_sleep()
+
+trace_printk("I'm a comment!\n")
+
+will produce:
+
+ 1)               |             __might_sleep() {
+ 1)               |                /* I'm a comment! */
+ 1)   1.449 us    |             }
+
+
+You might find other useful features for this tracer in the
+following "dynamic ftrace" section such as tracing only specific
+functions or tasks.
+
+dynamic ftrace
+--------------
+
+If CONFIG_DYNAMIC_FTRACE is set, the system will run with
+virtually no overhead when function tracing is disabled. The way
+this works is the mcount function call (placed at the start of
+every kernel function, produced by the -pg switch in gcc),
+starts of pointing to a simple return. (Enabling FTRACE will
+include the -pg switch in the compiling of the kernel.)
+
+At compile time every C file object is run through the
+recordmcount.pl script (located in the scripts directory). This
+script will process the C object using objdump to find all the
+locations in the .text section that call mcount. (Note, only the
+.text section is processed, since processing other sections like
+.init.text may cause races due to those sections being freed).
+
+A new section called "__mcount_loc" is created that holds
+references to all the mcount call sites in the .text section.
+This section is compiled back into the original object. The
+final linker will add all these references into a single table.
+
+On boot up, before SMP is initialized, the dynamic ftrace code
+scans this table and updates all the locations into nops. It
+also records the locations, which are added to the
+available_filter_functions list.  Modules are processed as they
+are loaded and before they are executed.  When a module is
+unloaded, it also removes its functions from the ftrace function
+list. This is automatic in the module unload code, and the
+module author does not need to worry about it.
+
+When tracing is enabled, kstop_machine is called to prevent
+races with the CPUS executing code being modified (which can
+cause the CPU to do undesireable things), and the nops are
+patched back to calls. But this time, they do not call mcount
+(which is just a function stub). They now call into the ftrace
+infrastructure.
+
+One special side-effect to the recording of the functions being
+traced is that we can now selectively choose which functions we
+wish to trace and which ones we want the mcount calls to remain
+as nops.
+
+Two files are used, one for enabling and one for disabling the
+tracing of specified functions. They are:
+
+  set_ftrace_filter
+
+and
+
+  set_ftrace_notrace
+
+A list of available functions that you can add to these files is
+listed in:
+
+   available_filter_functions
+
+ # cat /debug/tracing/available_filter_functions
+put_prev_task_idle
+kmem_cache_create
+pick_next_task_rt
+get_online_cpus
+pick_next_task_fair
+mutex_lock
+[...]
+
+If I am only interested in sys_nanosleep and hrtimer_interrupt:
+
+ # echo sys_nanosleep hrtimer_interrupt \
+                > /debug/tracing/set_ftrace_filter
+ # echo ftrace > /debug/tracing/current_tracer
+ # echo 1 > /debug/tracing/tracing_enabled
+ # usleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/trace
+# tracer: ftrace
+#
+#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
+#              | |      |          |         |
+          usleep-4134  [00]  1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt
+          usleep-4134  [00]  1317.070111: sys_nanosleep <-syscall_call
+          <idle>-0     [00]  1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt
+
+To see which functions are being traced, you can cat the file:
+
+ # cat /debug/tracing/set_ftrace_filter
+hrtimer_interrupt
+sys_nanosleep
+
+
+Perhaps this is not enough. The filters also allow simple wild
+cards. Only the following are currently available
+
+  <match>*  - will match functions that begin with <match>
+  *<match>  - will match functions that end with <match>
+  *<match>* - will match functions that have <match> in it
+
+These are the only wild cards which are supported.
+
+  <match>*<match> will not work.
+
+Note: It is better to use quotes to enclose the wild cards,
+      otherwise the shell may expand the parameters into names
+      of files in the local directory.
+
+ # echo 'hrtimer_*' > /debug/tracing/set_ftrace_filter
+
+Produces:
+
+# tracer: ftrace
+#
+#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
+#              | |      |          |         |
+            bash-4003  [00]  1480.611794: hrtimer_init <-copy_process
+            bash-4003  [00]  1480.611941: hrtimer_start <-hrtick_set
+            bash-4003  [00]  1480.611956: hrtimer_cancel <-hrtick_clear
+            bash-4003  [00]  1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel
+          <idle>-0     [00]  1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt
+          <idle>-0     [00]  1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt
+          <idle>-0     [00]  1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt
+          <idle>-0     [00]  1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt
+          <idle>-0     [00]  1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt
+
+
+Notice that we lost the sys_nanosleep.
+
+ # cat /debug/tracing/set_ftrace_filter
+hrtimer_run_queues
+hrtimer_run_pending
+hrtimer_init
+hrtimer_cancel
+hrtimer_try_to_cancel
+hrtimer_forward
+hrtimer_start
+hrtimer_reprogram
+hrtimer_force_reprogram
+hrtimer_get_next_event
+hrtimer_interrupt
+hrtimer_nanosleep
+hrtimer_wakeup
+hrtimer_get_remaining
+hrtimer_get_res
+hrtimer_init_sleeper
+
+
+This is because the '>' and '>>' act just like they do in bash.
+To rewrite the filters, use '>'
+To append to the filters, use '>>'
+
+To clear out a filter so that all functions will be recorded
+again:
+
+ # echo > /debug/tracing/set_ftrace_filter
+ # cat /debug/tracing/set_ftrace_filter
+ #
+
+Again, now we want to append.
+
+ # echo sys_nanosleep > /debug/tracing/set_ftrace_filter
+ # cat /debug/tracing/set_ftrace_filter
+sys_nanosleep
+ # echo 'hrtimer_*' >> /debug/tracing/set_ftrace_filter
+ # cat /debug/tracing/set_ftrace_filter
+hrtimer_run_queues
+hrtimer_run_pending
+hrtimer_init
+hrtimer_cancel
+hrtimer_try_to_cancel
+hrtimer_forward
+hrtimer_start
+hrtimer_reprogram
+hrtimer_force_reprogram
+hrtimer_get_next_event
+hrtimer_interrupt
+sys_nanosleep
+hrtimer_nanosleep
+hrtimer_wakeup
+hrtimer_get_remaining
+hrtimer_get_res
+hrtimer_init_sleeper
+
+
+The set_ftrace_notrace prevents those functions from being
+traced.
+
+ # echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace
+
+Produces:
+
+# tracer: ftrace
+#
+#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
+#              | |      |          |         |
+            bash-4043  [01]   115.281644: finish_task_switch <-schedule
+            bash-4043  [01]   115.281645: hrtick_set <-schedule
+            bash-4043  [01]   115.281645: hrtick_clear <-hrtick_set
+            bash-4043  [01]   115.281646: wait_for_completion <-__stop_machine_run
+            bash-4043  [01]   115.281647: wait_for_common <-wait_for_completion
+            bash-4043  [01]   115.281647: kthread_stop <-stop_machine_run
+            bash-4043  [01]   115.281648: init_waitqueue_head <-kthread_stop
+            bash-4043  [01]   115.281648: wake_up_process <-kthread_stop
+            bash-4043  [01]   115.281649: try_to_wake_up <-wake_up_process
+
+We can see that there's no more lock or preempt tracing.
+
+
+Dynamic ftrace with the function graph tracer
+---------------------------------------------
+
+Although what has been explained above concerns both the
+function tracer and the function-graph-tracer, there are some
+special features only available in the function-graph tracer.
+
+If you want to trace only one function and all of its children,
+you just have to echo its name into set_graph_function:
+
+ echo __do_fault > set_graph_function
+
+will produce the following "expanded" trace of the __do_fault()
+function:
+
+ 0)               |  __do_fault() {
+ 0)               |    filemap_fault() {
+ 0)               |      find_lock_page() {
+ 0)   0.804 us    |        find_get_page();
+ 0)               |        __might_sleep() {
+ 0)   1.329 us    |        }
+ 0)   3.904 us    |      }
+ 0)   4.979 us    |    }
+ 0)   0.653 us    |    _spin_lock();
+ 0)   0.578 us    |    page_add_file_rmap();
+ 0)   0.525 us    |    native_set_pte_at();
+ 0)   0.585 us    |    _spin_unlock();
+ 0)               |    unlock_page() {
+ 0)   0.541 us    |      page_waitqueue();
+ 0)   0.639 us    |      __wake_up_bit();
+ 0)   2.786 us    |    }
+ 0) + 14.237 us   |  }
+ 0)               |  __do_fault() {
+ 0)               |    filemap_fault() {
+ 0)               |      find_lock_page() {
+ 0)   0.698 us    |        find_get_page();
+ 0)               |        __might_sleep() {
+ 0)   1.412 us    |        }
+ 0)   3.950 us    |      }
+ 0)   5.098 us    |    }
+ 0)   0.631 us    |    _spin_lock();
+ 0)   0.571 us    |    page_add_file_rmap();
+ 0)   0.526 us    |    native_set_pte_at();
+ 0)   0.586 us    |    _spin_unlock();
+ 0)               |    unlock_page() {
+ 0)   0.533 us    |      page_waitqueue();
+ 0)   0.638 us    |      __wake_up_bit();
+ 0)   2.793 us    |    }
+ 0) + 14.012 us   |  }
+
+You can also expand several functions at once:
+
+ echo sys_open > set_graph_function
+ echo sys_close >> set_graph_function
+
+Now if you want to go back to trace all functions you can clear
+this special filter via:
+
+ echo > set_graph_function
+
+
+trace_pipe
+----------
+
+The trace_pipe outputs the same content as the trace file, but
+the effect on the tracing is different. Every read from
+trace_pipe is consumed. This means that subsequent reads will be
+different. The trace is live.
+
+ # echo function > /debug/tracing/current_tracer
+ # cat /debug/tracing/trace_pipe > /tmp/trace.out &
+[1] 4153
+ # echo 1 > /debug/tracing/tracing_enabled
+ # usleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/trace
+# tracer: function
+#
+#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
+#              | |      |          |         |
+
+ #
+ # cat /tmp/trace.out
+            bash-4043  [00] 41.267106: finish_task_switch <-schedule
+            bash-4043  [00] 41.267106: hrtick_set <-schedule
+            bash-4043  [00] 41.267107: hrtick_clear <-hrtick_set
+            bash-4043  [00] 41.267108: wait_for_completion <-__stop_machine_run
+            bash-4043  [00] 41.267108: wait_for_common <-wait_for_completion
+            bash-4043  [00] 41.267109: kthread_stop <-stop_machine_run
+            bash-4043  [00] 41.267109: init_waitqueue_head <-kthread_stop
+            bash-4043  [00] 41.267110: wake_up_process <-kthread_stop
+            bash-4043  [00] 41.267110: try_to_wake_up <-wake_up_process
+            bash-4043  [00] 41.267111: select_task_rq_rt <-try_to_wake_up
+
+
+Note, reading the trace_pipe file will block until more input is
+added. By changing the tracer, trace_pipe will issue an EOF. We
+needed to set the function tracer _before_ we "cat" the
+trace_pipe file.
+
+
+trace entries
+-------------
+
+Having too much or not enough data can be troublesome in
+diagnosing an issue in the kernel. The file buffer_size_kb is
+used to modify the size of the internal trace buffers. The
+number listed is the number of entries that can be recorded per
+CPU. To know the full size, multiply the number of possible CPUS
+with the number of entries.
+
+ # cat /debug/tracing/buffer_size_kb
+1408 (units kilobytes)
+
+Note, to modify this, you must have tracing completely disabled.
+To do that, echo "nop" into the current_tracer. If the
+current_tracer is not set to "nop", an EINVAL error will be
+returned.
+
+ # echo nop > /debug/tracing/current_tracer
+ # echo 10000 > /debug/tracing/buffer_size_kb
+ # cat /debug/tracing/buffer_size_kb
+10000 (units kilobytes)
+
+The number of pages which will be allocated is limited to a
+percentage of available memory. Allocating too much will produce
+an error.
+
+ # echo 1000000000000 > /debug/tracing/buffer_size_kb
+-bash: echo: write error: Cannot allocate memory
+ # cat /debug/tracing/buffer_size_kb
+85
+
+-----------
+
+More details can be found in the source code, in the
+kernel/tracing/*.c files.