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)

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

  Written for: 2.6.28-rc2

  Updated for: 3.10

  Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt

  
  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 typically considered the function tracer, it
  is really a frame work of several assorted tracing utilities.
  There's latency tracing to examine what occurs between interrupts
  disabled and enabled, as well as for preemption and from a time
  a task is woken to the task is actually scheduled in.
  
  One of the most common uses of ftrace is the event tracing.
  Through out the kernel is hundreds of static event points that

  can be enabled via the tracefs file system to see what is

  going on in certain parts of the kernel.

  See events.txt for more information.

  Implementation Details
  ----------------------
  
  See ftrace-design.txt for details for arch porters and such.

  The File System
  ---------------

  Ftrace uses the tracefs file system to hold the control files as

  well as the files to display output.

  When tracefs is configured into the kernel (which selecting any ftrace
  option will do) the directory /sys/kernel/tracing will be created. To mount

  this directory, you can add to your /etc/fstab file:

   tracefs       /sys/kernel/tracing       tracefs defaults        0       0

  
  Or you can mount it at run time with:

   mount -t tracefs nodev /sys/kernel/tracing

  For quicker access to that directory you may want to make a soft link to
  it:

   ln -s /sys/kernel/tracing /tracing
  
       *** NOTICE ***
  
  Before 4.1, all ftrace tracing control files were within the debugfs
  file system, which is typically located at /sys/kernel/debug/tracing.
  For backward compatibility, when mounting the debugfs file system,
  the tracefs file system will be automatically mounted at:
  
   /sys/kernel/debug/tracing

  All files located in the tracefs file system will be located in that
  debugfs file system directory as well.
  
       *** NOTICE ***
  
  Any selected ftrace option will also create the tracefs file system.
  The rest of the document will assume that you are in the ftrace directory
  (cd /sys/kernel/tracing) and will only concentrate on the files within that
  directory and not distract from the content with the extended
  "/sys/kernel/tracing" path name.

  
  That's it! (assuming that you have ftrace configured into your kernel)

  After mounting tracefs you will have access to 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. Note, this only disables
  	writing to the ring buffer, the tracing overhead may
  	still be occurring.

  	The kernel function tracing_off() can be used within the
  	kernel to disable writing to the ring buffer, which will
  	set this file to "0". User space can re-enable tracing by
  	echoing "1" into the file.
  
  	Note, the function and event trigger "traceoff" will also
  	set this file to zero and stop tracing. Which can also
  	be re-enabled by user space using this file.

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

  	readable format (described below). Note, tracing is temporarily
  	disabled while this file is being read (opened).

    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, it will display the same

  	information every time it is read. This file will not
  	disable tracing while being read.

  
    trace_options:
  
  	This file lets the user control the amount of data
  	that is displayed in one of the above output

  	files. Options also exist to modify how a tracer
  	or events work (stack traces, timestamps, etc).
  
    options:
  
  	This is a directory that has a file for every available
  	trace option (also in trace_options). Options may also be set
  	or cleared by writing a "1" or "0" respectively into the
  	corresponding file with the option name.

    tracing_max_latency:

  
  	Some of the tracers record the max latency.

  	For example, the maximum time that interrupts are disabled.
  	The maximum 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).
  
  	By echoing in a time into this file, no latency will be recorded
  	unless it is greater than the time in this file.

    tracing_thresh:
  
  	Some latency tracers will record a trace whenever the
  	latency is greater than the number in this file.
  	Only active when the file contains a number greater than 0.
  	(in microseconds)

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

  	buffer holds. By default, the trace 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 or shown.

  	( Note, the size may not be a multiple of the page size

  	  due to buffer management meta-data. )

  	Buffer sizes for individual CPUs may vary
  	(see "per_cpu/cpu0/buffer_size_kb" below), and if they do
  	this file will show "X".

    buffer_total_size_kb:
  
  	This displays the total combined size of all the trace buffers.
  
    free_buffer:

  	If a process is performing tracing, and the ring buffer	should be
  	shrunk "freed" when the process is finished, even if it were to be
  	killed by a signal, this file can be used for that purpose. On close
  	of this file, the ring buffer will be resized to its minimum size.
  	Having a process that is tracing also open this file, when the process
  	exits its file descriptor for this file will be closed, and in doing so,
  	the ring buffer will be "freed".

  
  	It may also stop tracing if disable_on_free option is set.

  
    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.

  	The functions listed in "available_filter_functions" are what
  	can be written into this file.

  	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 the threads whose PID are
  	listed in this file.
  
  	If the "function-fork" option is set, then when a task whose
  	PID is listed in this file forks, the child's PID will
  	automatically be added to this file, and the child will be
  	traced by the function tracer as well. This option will also
  	cause PIDs of tasks that exit to be removed from the file.

    set_event_pid:
  
  	Have the events only trace a task with a PID listed in this file.
  	Note, sched_switch and sched_wake_up will also trace events
  	listed in this file.

  	To have the PIDs of children of tasks with their PID in this file
  	added on fork, enable the "event-fork" option. That option will also
  	cause the PIDs of tasks to be removed from this file when the task
  	exits.

    set_graph_function:

  	Functions listed in this file will cause the function graph
  	tracer to only trace these functions and the functions that
  	they call. (See the section "dynamic ftrace" for more details).
  
    set_graph_notrace:
  
  	Similar to set_graph_function, but will disable function graph
  	tracing when the function is hit until it exits the function.
  	This makes it possible to ignore tracing functions that are called
  	by a specific function.

  
    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.)
  
    dyn_ftrace_total_info:
  
  	This file is for debugging purposes. The number of functions that
  	have been converted to nops and are available to be traced.

    enabled_functions:
  
  	This file is more for debugging ftrace, but can also be useful
  	in seeing if any function has a callback attached to it.
  	Not only does the trace infrastructure use ftrace function
  	trace utility, but other subsystems might too. This file
  	displays all functions that have a callback attached to them
  	as well as the number of callbacks that have been attached.
  	Note, a callback may also call multiple functions which will
  	not be listed in this count.
  
  	If the callback registered to be traced by a function with
  	the "save regs" attribute (thus even more overhead), a 'R'
  	will be displayed on the same line as the function that
  	is returning registers.

  	If the callback registered to be traced by a function with
  	the "ip modify" attribute (thus the regs->ip can be changed),
  	an 'I' will be displayed on the same line as the function that
  	can be overridden.

  	If the architecture supports it, it will also show what callback
  	is being directly called by the function. If the count is greater
  	than 1 it most likely will be ftrace_ops_list_func().
  
  	If the callback of the function jumps to a trampoline that is
  	specific to a the callback and not the standard trampoline,
  	its address will be printed as well as the function that the
  	trampoline calls.

    function_profile_enabled:
  
  	When set it will enable all functions with either the function

  	tracer, or if configured, the function graph tracer. It will

  	keep a histogram of the number of functions that were called

  	and if the function graph tracer was configured, it will also keep

  	track of the time spent in those functions. The histogram
  	content can be displayed in the files:
  
  	trace_stats/function<cpu> ( function0, function1, etc).
  
    trace_stats:
  
  	A directory that holds different tracing stats.
  
    kprobe_events:
   
  	Enable dynamic trace points. See kprobetrace.txt.
  
    kprobe_profile:
  
  	Dynamic trace points stats. See kprobetrace.txt.
  
    max_graph_depth:
  
  	Used with the function graph tracer. This is the max depth
  	it will trace into a function. Setting this to a value of
  	one will show only the first kernel function that is called
  	from user space.
  
    printk_formats:
  
  	This is for tools that read the raw format files. If an event in

  	the ring buffer references a string, only a pointer to the string
  	is recorded into the buffer and not the string itself. This prevents
  	tools from knowing what that string was. This file displays the string
  	and address for	the string allowing tools to map the pointers to what
  	the strings were.

  
    saved_cmdlines:
  
  	Only the pid of the task is recorded in a trace event unless
  	the event specifically saves the task comm as well. Ftrace
  	makes a cache of pid mappings to comms to try to display
  	comms for events. If a pid for a comm is not listed, then
  	"<...>" is displayed in the output.

  	If the option "record-cmd" is set to "0", then comms of tasks
  	will not be saved during recording. By default, it is enabled.
  
    saved_cmdlines_size:
  
  	By default, 128 comms are saved (see "saved_cmdlines" above). To
  	increase or decrease the amount of comms that are cached, echo
  	in a the number of comms to cache, into this file.
  
    saved_tgids:
  
  	If the option "record-tgid" is set, on each scheduling context switch
  	the Task Group ID of a task is saved in a table mapping the PID of
  	the thread to its TGID. By default, the "record-tgid" option is
  	disabled.

    snapshot:
  
  	This displays the "snapshot" buffer and also lets the user
  	take a snapshot of the current running trace.
  	See the "Snapshot" section below for more details.
  
    stack_max_size:
  
  	When the stack tracer is activated, this will display the
  	maximum stack size it has encountered.
  	See the "Stack Trace" section below.
  
    stack_trace:
  
  	This displays the stack back trace of the largest stack
  	that was encountered when the stack tracer is activated.
  	See the "Stack Trace" section below.
  
    stack_trace_filter:
  
  	This is similar to "set_ftrace_filter" but it limits what
  	functions the stack tracer will check.
  
    trace_clock:
  
  	Whenever an event is recorded into the ring buffer, a
  	"timestamp" is added. This stamp comes from a specified
  	clock. By default, ftrace uses the "local" clock. This
  	clock is very fast and strictly per cpu, but on some
  	systems it may not be monotonic with respect to other
  	CPUs. In other words, the local clocks may not be in sync
  	with local clocks on other CPUs.
  
  	Usual clocks for tracing:
  
  	  # cat trace_clock
  	  [local] global counter x86-tsc

  	  The clock with the square brackets around it is the one
  	  in effect.

  	  local: Default clock, but may not be in sync across CPUs
  
  	  global: This clock is in sync with all CPUs but may
  	  	  be a bit slower than the local clock.
  
  	  counter: This is not a clock at all, but literally an atomic
  	  	   counter. It counts up one by one, but is in sync
  		   with all CPUs. This is useful when you need to
  		   know exactly the order events occurred with respect to
  		   each other on different CPUs.
  
  	  uptime: This uses the jiffies counter and the time stamp
  	  	  is relative to the time since boot up.
  
  	  perf: This makes ftrace use the same clock that perf uses.
  	  	Eventually perf will be able to read ftrace buffers
  		and this will help out in interleaving the data.
  
  	  x86-tsc: Architectures may define their own clocks. For
  	  	   example, x86 uses its own TSC cycle clock here.

  	  ppc-tb: This uses the powerpc timebase register value.
  		  This is in sync across CPUs and can also be used
  		  to correlate events across hypervisor/guest if
  		  tb_offset is known.

  	  mono: This uses the fast monotonic clock (CLOCK_MONOTONIC)
  		which is monotonic and is subject to NTP rate adjustments.
  
  	  mono_raw:
  		This is the raw monotonic clock (CLOCK_MONOTONIC_RAW)
  		which is montonic but is not subject to any rate adjustments
  		and ticks at the same rate as the hardware clocksource.
  
  	  boot: This is the boot clock (CLOCK_BOOTTIME) and is based on the
  		fast monotonic clock, but also accounts for time spent in
  		suspend. Since the clock access is designed for use in
  		tracing in the suspend path, some side effects are possible
  		if clock is accessed after the suspend time is accounted before
  		the fast mono clock is updated. In this case, the clock update
  		appears to happen slightly sooner than it normally would have.
  		Also on 32-bit systems, it's possible that the 64-bit boot offset
  		sees a partial update. These effects are rare and post
  		processing should be able to handle them. See comments in the
  		ktime_get_boot_fast_ns() function for more information.

  	To set a clock, simply echo the clock name into this file.
  
  	  echo global > trace_clock
  
    trace_marker:
  
  	This is a very useful file for synchronizing user space
  	with events happening in the kernel. Writing strings into
  	this file will be written into the ftrace buffer.
  
  	It is useful in applications to open this file at the start
  	of the application and just reference the file descriptor
  	for the file.
  
  	void trace_write(const char *fmt, ...)
  	{
  		va_list ap;
  		char buf[256];
  		int n;
  
  		if (trace_fd < 0)
  			return;
  
  		va_start(ap, fmt);
  		n = vsnprintf(buf, 256, fmt, ap);
  		va_end(ap);
  
  		write(trace_fd, buf, n);
  	}
  
  	start:
  
  		trace_fd = open("trace_marker", WR_ONLY);

    trace_marker_raw:
  
  	This is similar to trace_marker above, but is meant for for binary data
  	to be written to it, where a tool can be used to parse the data
  	from trace_pipe_raw.

    uprobe_events:
   
  	Add dynamic tracepoints in programs.
  	See uprobetracer.txt
  
    uprobe_profile:
  
  	Uprobe statistics. See uprobetrace.txt
  
    instances:
  
  	This is a way to make multiple trace buffers where different
  	events can be recorded in different buffers.
  	See "Instances" section below.
  
    events:
  
  	This is the trace event directory. It holds event tracepoints
  	(also known as static tracepoints) that have been compiled
  	into the kernel. It shows what event tracepoints exist
  	and how they are grouped by system. There are "enable"
  	files at various levels that can enable the tracepoints
  	when a "1" is written to them.
  
  	See events.txt for more information.

    set_event:
  
  	By echoing in the event into this file, will enable that event.
  
  	See events.txt for more information.
  
    available_events:
  
  	A list of events that can be enabled in tracing.
  
  	See events.txt for more information.
  
    hwlat_detector:
  
  	Directory for the Hardware Latency Detector.
  	See "Hardware Latency Detector" section below.

    per_cpu:
  
  	This is a directory that contains the trace per_cpu information.
  
    per_cpu/cpu0/buffer_size_kb:
  
  	The ftrace buffer is defined per_cpu. That is, there's a separate
  	buffer for each CPU to allow writes to be done atomically,
  	and free from cache bouncing. These buffers may have different
  	size buffers. This file is similar to the buffer_size_kb
  	file, but it only displays or sets the buffer size for the
  	specific CPU. (here cpu0).
  
    per_cpu/cpu0/trace:
  
  	This is similar to the "trace" file, but it will only display
  	the data specific for the CPU. If written to, it only clears
  	the specific CPU buffer.
  
    per_cpu/cpu0/trace_pipe
  
  	This is similar to the "trace_pipe" file, and is a consuming
  	read, but it will only display (and consume) the data specific
  	for the CPU.
  
    per_cpu/cpu0/trace_pipe_raw
  
  	For tools that can parse the ftrace ring buffer binary format,
  	the trace_pipe_raw file can be used to extract the data
  	from the ring buffer directly. With the use of the splice()
  	system call, the buffer data can be quickly transferred to
  	a file or to the network where a server is collecting the
  	data.
  
  	Like trace_pipe, this is a consuming reader, where multiple
  	reads will always produce different data.
  
    per_cpu/cpu0/snapshot:
  
  	This is similar to the main "snapshot" file, but will only
  	snapshot the current CPU (if supported). It only displays
  	the content of the snapshot for a given CPU, and if
  	written to, only clears this CPU buffer.
  
    per_cpu/cpu0/snapshot_raw:
  
  	Similar to the trace_pipe_raw, but will read the binary format
  	from the snapshot buffer for the given CPU.
  
    per_cpu/cpu0/stats:
  
  	This displays certain stats about the ring buffer:
  
  	 entries: The number of events that are still in the buffer.
  
  	 overrun: The number of lost events due to overwriting when
  	 	  the buffer was full.
  
  	 commit overrun: Should always be zero.
  	 	This gets set if so many events happened within a nested
  		event (ring buffer is re-entrant), that it fills the
  		buffer and starts dropping events.
  
  	 bytes: Bytes actually read (not overwritten).
  
  	 oldest event ts: The oldest timestamp in the buffer
  
  	 now ts: The current timestamp
  
  	 dropped events: Events lost due to overwrite option being off.
  
  	 read events: The number of events read.

  
  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.

    "blk"
  
  	The block tracer. The tracer used by the blktrace user
  	application.
  
    "hwlat"
  
  	The Hardware Latency tracer is used to detect if the hardware
  	produces any latency. See "Hardware Latency Detector" section
  	below.

    "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, which
  	happens automatically when the tracer is selected.

    "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.

    "wakeup_dl"
  
  	Traces and records the max latency that it takes for
  	a SCHED_DEADLINE task to be woken (as the "wakeup" and
  	"wakeup_rt" does).
  
    "mmiotrace"
  
  	A special tracer that is used to trace binary module.
  	It will trace all the calls that a module makes to the
  	hardware. Everything it writes and reads from the I/O
  	as well.
  
    "branch"
  
  	This tracer can be configured when tracing likely/unlikely
  	calls within the kernel. It will trace when a likely and
  	unlikely branch is hit and if it was correct in its prediction
  	of being correct.

    "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 tracefs interface (without using any

  user-land utilities).

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

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

  
                               --------

  # tracer: function

  #

  # entries-in-buffer/entries-written: 140080/250280   #P:4
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
              bash-1977  [000] .... 17284.993652: sys_close <-system_call_fastpath
              bash-1977  [000] .... 17284.993653: __close_fd <-sys_close
              bash-1977  [000] .... 17284.993653: _raw_spin_lock <-__close_fd
              sshd-1974  [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
              bash-1977  [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
              bash-1977  [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
              bash-1977  [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
              bash-1977  [000] .... 17284.993657: filp_close <-__close_fd
              bash-1977  [000] .... 17284.993657: dnotify_flush <-filp_close
              sshd-1974  [003] .... 17284.993658: sys_select <-system_call_fastpath

                               --------

  A header is printed with the tracer name that is represented by

  the trace. In this case the tracer is "function". Then it shows the
  number of events in the buffer as well as the total number of entries
  that were written. The difference is the number of entries that were
  lost due to the buffer filling up (250280 - 140080 = 110200 events
  lost).
  
  The header explains the content of the events. Task name "bash", the task
  PID "1977", the CPU that it was running on "000", the latency format
  (explained below), the timestamp in <secs>.<usecs> format, the
  function name that was traced "sys_close" and the parent function that
  called this function "system_call_fastpath". The timestamp is the time
  at which the function was entered.

  Latency trace format
  --------------------

  When the latency-format option is enabled or when one of the latency
  tracers is set, 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 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: __lock_task_sighand
  #  => ended at:   _raw_spin_unlock_irqrestore
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
        ps-6143    2d...    0us!: trace_hardirqs_off <-__lock_task_sighand
        ps-6143    2d..1  259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
        ps-6143    2d..1  263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
        ps-6143    2d..1  306us : <stack trace>
   => trace_hardirqs_on_caller
   => trace_hardirqs_on
   => _raw_spin_unlock_irqrestore
   => do_task_stat
   => proc_tgid_stat
   => proc_single_show
   => seq_read
   => vfs_read
   => sys_read
   => system_call_fastpath

  This shows that the current tracer is "irqsoff" tracing the time

  for which interrupts were disabled. It gives the trace version (which
  never changes) and the version of the kernel upon which this was executed on

  (3.8). Then it displays the max latency in microseconds (259 us). The number

  of trace entries displayed and the total number (both are four: #4/4).
  VP, KP, SP, and HP are always zero and are reserved for later use.
  #P is the number of online CPUs (#P:4).

  The task is the process that was running when the latency

  occurred. (ps pid: 6143).

  The start and stop (the functions in which the interrupts were
  disabled and enabled respectively) that caused the latencies:

   __lock_task_sighand is where the interrupts were disabled.
   _raw_spin_unlock_irqrestore 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' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
  	'n' only TIF_NEED_RESCHED is set,
  	'p' only PREEMPT_NEED_RESCHED is set,
  	'.' otherwise.

  
    hardirq/softirq:

  	'Z' - NMI occurred inside a hardirq
  	'z' - NMI is running

  	'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 1 second

  	  '@' - greater than 100 milisecond
  	  '*' - greater than 10 milisecond

  	  '#' - greater than 1000 microsecond
  	  '!' - greater than 100 microsecond
  	  '+' - greater than 10 microsecond
  	  ' ' - less than or equal to 10 microsecond.

  
    The rest is the same as the 'trace' file.

    Note, the latency tracers will usually end with a back trace
    to easily find where the latency occurred.

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

  The trace_options file (or the options directory) is used to control
  what gets printed in the trace output, or manipulate the tracers.
  To see what is available, simply cat the file:

    cat trace_options

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

  trace_printk

  annotate
  nouserstacktrace
  nosym-userobj
  noprintk-msg-only
  context-info

  nolatency-format

  record-cmd

  norecord-tgid

  overwrite
  nodisable_on_free
  irq-info
  markers

  noevent-fork

  function-trace

  nofunction-fork

  nodisplay-graph
  nostacktrace

  nobranch

  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 <-kstrtoul

  
    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 (kstrtoul)

    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 - When set, reading trace_pipe will not block when polled.

    trace_printk - Can disable trace_printk() from writing into the buffer.

    annotate - It is sometimes confusing when the CPU buffers are full
    	     and one CPU buffer had a lot of events recently, thus
  	     a shorter time frame, were another CPU may have only had
  	     a few events, which lets it have older events. When
  	     the trace is reported, it shows the oldest events first,
  	     and it may look like only one CPU ran (the one with the
  	     oldest events). When the annotate option is set, it will
  	     display when a new CPU buffer started:
  
            <idle>-0     [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
            <idle>-0     [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
            <idle>-0     [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
  ##### CPU 2 buffer started ####
            <idle>-0     [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
            <idle>-0     [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
            <idle>-0     [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock

    userstacktrace - This option changes the trace. It records a

  		   stacktrace of the current user space thread after
  		   each trace event.

    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]

  
    printk-msg-only - When set, trace_printk()s will only show the format
    		    and not their parameters (if trace_bprintk() or
  		    trace_bputs() was used to save the trace_printk()).
  
    context-info - Show only the event data. Hides the comm, PID,
    	         timestamp, CPU, and other useful data.

    latency-format - This option changes the trace output. When it is enabled,
  		   the trace displays additional information about the
  		   latency, as described in "Latency trace format".

  
    record-cmd - When any event or tracer is enabled, a hook is enabled

  	       in the sched_switch trace point to fill comm cache

  	       with mapped pids and comms. But this may cause some
  	       overhead, and if you only care about pids, and not the
  	       name of the task, disabling this option can lower the

  	       impact of tracing. See "saved_cmdlines".
  
    record-tgid - When any event or tracer is enabled, a hook is enabled
  	        in the sched_switch trace point to fill the cache of
  		mapped Thread Group IDs (TGID) mapping to pids. See
  		"saved_tgids".

    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.

  	        (see per_cpu/cpu0/stats for overrun and dropped)

    disable_on_free - When the free_buffer is closed, tracing will
    		    stop (tracing_on set to 0).

    irq-info - Shows the interrupt, preempt count, need resched data.
    	     When disabled, the trace looks like:

  # tracer: function
  #
  # entries-in-buffer/entries-written: 144405/9452052   #P:4
  #
  #           TASK-PID   CPU#      TIMESTAMP  FUNCTION
  #              | |       |          |         |
            <idle>-0     [002]  23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
            <idle>-0     [002]  23636.756054: activate_task <-ttwu_do_activate.constprop.89
            <idle>-0     [002]  23636.756055: enqueue_task <-activate_task

    markers - When set, the trace_marker is writable (only by root).
    	    When disabled, the trace_marker will error with EINVAL
  	    on write.

    event-fork - When set, tasks with PIDs listed in set_event_pid will have
  	       the PIDs of their children added to set_event_pid when those
  	       tasks fork. Also, when tasks with PIDs in set_event_pid exit,
  	       their PIDs will be removed from the file.

  
    function-trace - The latency tracers will enable function tracing
    	    if this option is enabled (default it is). When
  	    it is disabled, the latency tracers do not trace
  	    functions. This keeps the overhead of the tracer down
  	    when performing latency tests.

    function-fork - When set, tasks with PIDs listed in set_ftrace_pid will
  		  have the PIDs of their children added to set_ftrace_pid
  		  when those tasks fork. Also, when tasks with PIDs in
  		  set_ftrace_pid exit, their PIDs will be removed from the
  		  file.

    display-graph - When set, the latency tracers (irqsoff, wakeup, etc) will
  	          use function graph tracing instead of function tracing.

    stacktrace - When set, a stack trace is recorded after any trace event
  	       is recorded.
  
    branch - Enable branch tracing with the tracer. This enables branch
  	   tracer along with the currently set tracer. Enabling this
  	   with the "nop" tracer is the same as just enabling the
  	   "branch" tracer.

  
   Note: Some tracers have their own options. They only appear in this
         file when the tracer is active. They always appear in the
         options directory.

  Here are the per tracer options:
  
  Options for function tracer:
  
    func_stack_trace - When set, a stack trace is recorded after every
  		     function that is recorded. NOTE! Limit the functions
  		     that are recorded before enabling this, with
  		     "set_ftrace_filter" otherwise the system performance
  		     will be critically degraded. Remember to disable
  		     this option before clearing the function filter.
  
  Options for function_graph tracer:
  
   Since the function_graph tracer has a slightly different output
   it has its own options to control what is displayed.
  
    funcgraph-overrun - When set, the "overrun" of the graph stack is
  		      displayed after each function traced. The
  		      overrun, is when the stack depth of the calls
  		      is greater than what is reserved for each task.
  		      Each task has a fixed array of functions to
  		      trace in the call graph. If the depth of the
  		      calls exceeds that, the function is not traced.
  		      The overrun is the number of functions missed
  		      due to exceeding this array.
  
    funcgraph-cpu - When set, the CPU number of the CPU where the trace
  		  occurred is displayed.
  
    funcgraph-overhead - When set, if the function takes longer than
  		       A certain amount, then a delay marker is
  		       displayed. See "delay" above, under the
  		       header description.
  
    funcgraph-proc - Unlike other tracers, the process' command line
  		   is not displayed by default, but instead only
  		   when a task is traced in and out during a context
  		   switch. Enabling this options has the command
  		   of each process displayed at every line.
  
    funcgraph-duration - At the end of each function (the return)
  		       the duration of the amount of time in the
  		       function is displayed in microseconds.
  
    funcgraph-abstime - When set, the timestamp is displayed at each
  		      line.
  
    funcgraph-irqs - When disabled, functions that happen inside an
  		   interrupt will not be traced.
  
    funcgraph-tail - When set, the return event will include the function
  		   that it represents. By default this is off, and
  		   only a closing curly bracket "}" is displayed for
  		   the return of a function.
  
    sleep-time - When running function graph tracer, to include
  	       the time a task schedules out in its function.
  	       When enabled, it will account time the task has been
  	       scheduled out as part of the function call.
  
    graph-time - When running function profiler with function graph tracer,
  	       to include the time to call nested functions. When this is
  	       not set, the time reported for the function will only
  	       include the time the function itself executed for, not the
  	       time for functions that it called.
  
  Options for blk tracer:
  
    blk_classic - Shows a more minimalistic output.

  
  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 0 > options/function-trace

   # echo irqsoff > current_tracer

   # echo 1 > tracing_on

   # echo 0 > tracing_max_latency

   # ls -ltr
   [...]

   # echo 0 > tracing_on

   # cat trace

  # tracer: irqsoff
  #

  # irqsoff latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: run_timer_softirq
  #  => ended at:   run_timer_softirq
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
    <idle>-0       0d.s2    0us+: _raw_spin_lock_irq <-run_timer_softirq
    <idle>-0       0dNs3   17us : _raw_spin_unlock_irq <-run_timer_softirq
    <idle>-0       0dNs3   17us+: trace_hardirqs_on <-run_timer_softirq
    <idle>-0       0dNs3   25us : <stack trace>
   => _raw_spin_unlock_irq
   => run_timer_softirq
   => __do_softirq
   => call_softirq
   => do_softirq
   => irq_exit
   => smp_apic_timer_interrupt
   => apic_timer_interrupt
   => rcu_idle_exit
   => cpu_idle
   => rest_init
   => start_kernel
   => x86_64_start_reservations
   => x86_64_start_kernel
  
  Here we see that that we had a latency of 16 microseconds (which is
  very good). The _raw_spin_lock_irq in run_timer_softirq disabled
  interrupts. The difference between the 16 and the displayed
  timestamp 25us 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 function-trace not set. If we set
  function-trace, we get a much larger output:
  
   with echo 1 > options/function-trace

  
  # tracer: irqsoff
  #

  # irqsoff latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: ata_scsi_queuecmd
  #  => ended at:   ata_scsi_queuecmd
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
      bash-2042    3d...    0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
      bash-2042    3d...    0us : add_preempt_count <-_raw_spin_lock_irqsave
      bash-2042    3d..1    1us : ata_scsi_find_dev <-ata_scsi_queuecmd
      bash-2042    3d..1    1us : __ata_scsi_find_dev <-ata_scsi_find_dev
      bash-2042    3d..1    2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
      bash-2042    3d..1    2us : ata_qc_new_init <-__ata_scsi_queuecmd
      bash-2042    3d..1    3us : ata_sg_init <-__ata_scsi_queuecmd
      bash-2042    3d..1    4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
      bash-2042    3d..1    4us : ata_build_rw_tf <-ata_scsi_rw_xlat

  [...]

      bash-2042    3d..1   67us : delay_tsc <-__delay
      bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
      bash-2042    3d..2   67us : sub_preempt_count <-delay_tsc
      bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
      bash-2042    3d..2   68us : sub_preempt_count <-delay_tsc
      bash-2042    3d..1   68us+: ata_bmdma_start <-ata_bmdma_qc_issue
      bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
      bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
      bash-2042    3d..1   72us+: trace_hardirqs_on <-ata_scsi_queuecmd
      bash-2042    3d..1  120us : <stack trace>
   => _raw_spin_unlock_irqrestore
   => ata_scsi_queuecmd
   => scsi_dispatch_cmd
   => scsi_request_fn
   => __blk_run_queue_uncond
   => __blk_run_queue
   => blk_queue_bio
   => generic_make_request
   => submit_bio
   => submit_bh
   => __ext3_get_inode_loc
   => ext3_iget
   => ext3_lookup
   => lookup_real
   => __lookup_hash
   => walk_component
   => lookup_last
   => path_lookupat
   => filename_lookup
   => user_path_at_empty
   => user_path_at
   => vfs_fstatat
   => vfs_stat
   => sys_newstat
   => system_call_fastpath
  
  
  Here we traced a 71 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 0 > options/function-trace

   # echo preemptoff > current_tracer

   # echo 1 > tracing_on

   # echo 0 > tracing_max_latency

   # ls -ltr
   [...]

   # echo 0 > tracing_on

   # cat trace

  # tracer: preemptoff
  #

  # preemptoff latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: do_IRQ
  #  => ended at:   do_IRQ
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
      sshd-1991    1d.h.    0us+: irq_enter <-do_IRQ
      sshd-1991    1d..1   46us : irq_exit <-do_IRQ
      sshd-1991    1d..1   47us+: trace_preempt_on <-do_IRQ
      sshd-1991    1d..1   52us : <stack trace>
   => sub_preempt_count
   => irq_exit
   => do_IRQ
   => ret_from_intr

  This has some more changes. Preemption was disabled when an

  interrupt came in (notice the 'h'), and was enabled on exit.
  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 or shortly after this
  was over.

  
  # tracer: preemptoff
  #

  # preemptoff latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: wake_up_new_task
  #  => ended at:   task_rq_unlock
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
      bash-1994    1d..1    0us : _raw_spin_lock_irqsave <-wake_up_new_task
      bash-1994    1d..1    0us : select_task_rq_fair <-select_task_rq
      bash-1994    1d..1    1us : __rcu_read_lock <-select_task_rq_fair
      bash-1994    1d..1    1us : source_load <-select_task_rq_fair
      bash-1994    1d..1    1us : source_load <-select_task_rq_fair

  [...]

      bash-1994    1d..1   12us : irq_enter <-smp_apic_timer_interrupt
      bash-1994    1d..1   12us : rcu_irq_enter <-irq_enter
      bash-1994    1d..1   13us : add_preempt_count <-irq_enter
      bash-1994    1d.h1   13us : exit_idle <-smp_apic_timer_interrupt
      bash-1994    1d.h1   13us : hrtimer_interrupt <-smp_apic_timer_interrupt
      bash-1994    1d.h1   13us : _raw_spin_lock <-hrtimer_interrupt
      bash-1994    1d.h1   14us : add_preempt_count <-_raw_spin_lock
      bash-1994    1d.h2   14us : ktime_get_update_offsets <-hrtimer_interrupt

  [...]

      bash-1994    1d.h1   35us : lapic_next_event <-clockevents_program_event
      bash-1994    1d.h1   35us : irq_exit <-smp_apic_timer_interrupt
      bash-1994    1d.h1   36us : sub_preempt_count <-irq_exit
      bash-1994    1d..2   36us : do_softirq <-irq_exit
      bash-1994    1d..2   36us : __do_softirq <-call_softirq
      bash-1994    1d..2   36us : __local_bh_disable <-__do_softirq
      bash-1994    1d.s2   37us : add_preempt_count <-_raw_spin_lock_irq
      bash-1994    1d.s3   38us : _raw_spin_unlock <-run_timer_softirq
      bash-1994    1d.s3   39us : sub_preempt_count <-_raw_spin_unlock
      bash-1994    1d.s2   39us : call_timer_fn <-run_timer_softirq

  [...]

      bash-1994    1dNs2   81us : cpu_needs_another_gp <-rcu_process_callbacks
      bash-1994    1dNs2   82us : __local_bh_enable <-__do_softirq
      bash-1994    1dNs2   82us : sub_preempt_count <-__local_bh_enable
      bash-1994    1dN.2   82us : idle_cpu <-irq_exit
      bash-1994    1dN.2   83us : rcu_irq_exit <-irq_exit
      bash-1994    1dN.2   83us : sub_preempt_count <-irq_exit
      bash-1994    1.N.1   84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
      bash-1994    1.N.1   84us+: trace_preempt_on <-task_rq_unlock
      bash-1994    1.N.1  104us : <stack trace>
   => sub_preempt_count
   => _raw_spin_unlock_irqrestore
   => task_rq_unlock
   => wake_up_new_task
   => do_fork
   => sys_clone
   => stub_clone

  The above is an example of the preemptoff trace with

  function-trace set. Here we see that interrupts were not 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.

  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 0 > options/function-trace

   # echo preemptirqsoff > current_tracer

   # echo 1 > tracing_on

   # echo 0 > tracing_max_latency

   # ls -ltr
   [...]

   # echo 0 > tracing_on

   # cat trace

  # tracer: preemptirqsoff
  #

  # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: ata_scsi_queuecmd
  #  => ended at:   ata_scsi_queuecmd
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
        ls-2230    3d...    0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
        ls-2230    3...1  100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
        ls-2230    3...1  101us+: trace_preempt_on <-ata_scsi_queuecmd
        ls-2230    3...1  111us : <stack trace>
   => sub_preempt_count
   => _raw_spin_unlock_irqrestore
   => ata_scsi_queuecmd
   => scsi_dispatch_cmd
   => scsi_request_fn
   => __blk_run_queue_uncond
   => __blk_run_queue
   => blk_queue_bio
   => generic_make_request
   => submit_bio
   => submit_bh
   => ext3_bread
   => ext3_dir_bread
   => htree_dirblock_to_tree
   => ext3_htree_fill_tree
   => ext3_readdir
   => vfs_readdir
   => sys_getdents
   => system_call_fastpath

  
  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 function-trace set:

  
  # tracer: preemptirqsoff
  #

  # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: schedule
  #  => ended at:   mutex_unlock
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
  kworker/-59      3...1    0us : __schedule <-schedule
  kworker/-59      3d..1    0us : rcu_preempt_qs <-rcu_note_context_switch
  kworker/-59      3d..1    1us : add_preempt_count <-_raw_spin_lock_irq
  kworker/-59      3d..2    1us : deactivate_task <-__schedule
  kworker/-59      3d..2    1us : dequeue_task <-deactivate_task
  kworker/-59      3d..2    2us : update_rq_clock <-dequeue_task
  kworker/-59      3d..2    2us : dequeue_task_fair <-dequeue_task
  kworker/-59      3d..2    2us : update_curr <-dequeue_task_fair
  kworker/-59      3d..2    2us : update_min_vruntime <-update_curr
  kworker/-59      3d..2    3us : cpuacct_charge <-update_curr
  kworker/-59      3d..2    3us : __rcu_read_lock <-cpuacct_charge
  kworker/-59      3d..2    3us : __rcu_read_unlock <-cpuacct_charge
  kworker/-59      3d..2    3us : update_cfs_rq_blocked_load <-dequeue_task_fair
  kworker/-59      3d..2    4us : clear_buddies <-dequeue_task_fair
  kworker/-59      3d..2    4us : account_entity_dequeue <-dequeue_task_fair
  kworker/-59      3d..2    4us : update_min_vruntime <-dequeue_task_fair
  kworker/-59      3d..2    4us : update_cfs_shares <-dequeue_task_fair
  kworker/-59      3d..2    5us : hrtick_update <-dequeue_task_fair
  kworker/-59      3d..2    5us : wq_worker_sleeping <-__schedule
  kworker/-59      3d..2    5us : kthread_data <-wq_worker_sleeping
  kworker/-59      3d..2    5us : put_prev_task_fair <-__schedule
  kworker/-59      3d..2    6us : pick_next_task_fair <-pick_next_task
  kworker/-59      3d..2    6us : clear_buddies <-pick_next_task_fair
  kworker/-59      3d..2    6us : set_next_entity <-pick_next_task_fair
  kworker/-59      3d..2    6us : update_stats_wait_end <-set_next_entity
        ls-2269    3d..2    7us : finish_task_switch <-__schedule
        ls-2269    3d..2    7us : _raw_spin_unlock_irq <-finish_task_switch
        ls-2269    3d..2    8us : do_IRQ <-ret_from_intr
        ls-2269    3d..2    8us : irq_enter <-do_IRQ
        ls-2269    3d..2    8us : rcu_irq_enter <-irq_enter
        ls-2269    3d..2    9us : add_preempt_count <-irq_enter
        ls-2269    3d.h2    9us : exit_idle <-do_IRQ

  [...]

        ls-2269    3d.h3   20us : sub_preempt_count <-_raw_spin_unlock
        ls-2269    3d.h2   20us : irq_exit <-do_IRQ
        ls-2269    3d.h2   21us : sub_preempt_count <-irq_exit
        ls-2269    3d..3   21us : do_softirq <-irq_exit
        ls-2269    3d..3   21us : __do_softirq <-call_softirq
        ls-2269    3d..3   21us+: __local_bh_disable <-__do_softirq
        ls-2269    3d.s4   29us : sub_preempt_count <-_local_bh_enable_ip
        ls-2269    3d.s5   29us : sub_preempt_count <-_local_bh_enable_ip
        ls-2269    3d.s5   31us : do_IRQ <-ret_from_intr
        ls-2269    3d.s5   31us : irq_enter <-do_IRQ
        ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter

  [...]

        ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
        ls-2269    3d.s5   32us : add_preempt_count <-irq_enter
        ls-2269    3d.H5   32us : exit_idle <-do_IRQ
        ls-2269    3d.H5   32us : handle_irq <-do_IRQ
        ls-2269    3d.H5   32us : irq_to_desc <-handle_irq
        ls-2269    3d.H5   33us : handle_fasteoi_irq <-handle_irq

  [...]

        ls-2269    3d.s5  158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
        ls-2269    3d.s3  158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
        ls-2269    3d.s3  159us : __local_bh_enable <-__do_softirq
        ls-2269    3d.s3  159us : sub_preempt_count <-__local_bh_enable
        ls-2269    3d..3  159us : idle_cpu <-irq_exit
        ls-2269    3d..3  159us : rcu_irq_exit <-irq_exit
        ls-2269    3d..3  160us : sub_preempt_count <-irq_exit
        ls-2269    3d...  161us : __mutex_unlock_slowpath <-mutex_unlock
        ls-2269    3d...  162us+: trace_hardirqs_on <-mutex_unlock
        ls-2269    3d...  186us : <stack trace>
   => __mutex_unlock_slowpath
   => mutex_unlock
   => process_output
   => n_tty_write
   => tty_write
   => vfs_write
   => sys_write
   => system_call_fastpath
  
  This is an interesting trace. It started with kworker running and
  scheduling out and ls taking over. But as soon as ls released the
  rq lock and enabled interrupts (but not preemption) an interrupt
  triggered. When the interrupt finished, it started running softirqs.
  But while the softirq was running, another interrupt triggered.
  When an interrupt is running inside a softirq, the annotation is 'H'.

  
  
  wakeup
  ------

  One common case that people are interested in tracing is the
  time it takes for a task that is woken to actually wake up.
  Now for non Real-Time tasks, this can be arbitrary. But tracing
  it none the less can be interesting. 
  
  Without function tracing:
  
   # echo 0 > options/function-trace
   # echo wakeup > current_tracer
   # echo 1 > tracing_on
   # echo 0 > tracing_max_latency
   # chrt -f 5 sleep 1
   # echo 0 > tracing_on
   # cat trace
  # tracer: wakeup
  #
  # wakeup latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
  #    -----------------
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
    <idle>-0       3dNs7    0us :      0:120:R   + [003]   312:100:R kworker/3:1H
    <idle>-0       3dNs7    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
    <idle>-0       3d..3   15us : __schedule <-schedule
    <idle>-0       3d..3   15us :      0:120:R ==> [003]   312:100:R kworker/3:1H
  
  The tracer only traces the highest priority task in the system
  to avoid tracing the normal circumstances. Here we see that
  the kworker with a nice priority of -20 (not very nice), took
  just 15 microseconds from the time it woke up, to the time it
  ran.
  
  Non Real-Time tasks are not that interesting. A more interesting
  trace is to concentrate only on Real-Time tasks.
  
  wakeup_rt
  ---------

  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_rt 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 (just run the normal wakeup
  tracer for a while to see that effect).

  
  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 0 > options/function-trace
   # echo wakeup_rt > current_tracer

   # echo 1 > tracing_on

   # echo 0 > tracing_max_latency

   # chrt -f 5 sleep 1

   # echo 0 > tracing_on

   # cat trace

  # tracer: wakeup
  #

  # tracer: wakeup_rt
  #
  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: sleep-2389 (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       3d.h4    0us :      0:120:R   + [003]  2389: 94:R sleep
    <idle>-0       3d.h4    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
    <idle>-0       3d..3    5us : __schedule <-schedule
    <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
  
  
  Running this on an idle system, we see that it only took 5 microseconds
  to perform the task switch.  Note, since the trace point 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 2389

  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.

  Note, that the trace data shows the internal priority (99 - rtprio).

    <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep

  The 0:120:R means idle was running with a nice priority of 0 (120 - 120)

  and in the running state 'R'. The sleep task was scheduled in with
  2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
  and it too is in the running state.
  
  Doing the same with chrt -r 5 and function-trace set.
  
    echo 1 > options/function-trace
  
  # tracer: wakeup_rt

  #

  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: sleep-2448 (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       3d.h4    1us+:      0:120:R   + [003]  2448: 94:R sleep
    <idle>-0       3d.h4    2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
    <idle>-0       3d.h3    3us : check_preempt_curr <-ttwu_do_wakeup

    <idle>-0       3d.h3    3us : resched_curr <-check_preempt_curr

    <idle>-0       3dNh3    4us : task_woken_rt <-ttwu_do_wakeup
    <idle>-0       3dNh3    4us : _raw_spin_unlock <-try_to_wake_up
    <idle>-0       3dNh3    4us : sub_preempt_count <-_raw_spin_unlock
    <idle>-0       3dNh2    5us : ttwu_stat <-try_to_wake_up
    <idle>-0       3dNh2    5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
    <idle>-0       3dNh2    6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
    <idle>-0       3dNh1    6us : _raw_spin_lock <-__run_hrtimer
    <idle>-0       3dNh1    6us : add_preempt_count <-_raw_spin_lock
    <idle>-0       3dNh2    7us : _raw_spin_unlock <-hrtimer_interrupt
    <idle>-0       3dNh2    7us : sub_preempt_count <-_raw_spin_unlock
    <idle>-0       3dNh1    7us : tick_program_event <-hrtimer_interrupt
    <idle>-0       3dNh1    7us : clockevents_program_event <-tick_program_event
    <idle>-0       3dNh1    8us : ktime_get <-clockevents_program_event
    <idle>-0       3dNh1    8us : lapic_next_event <-clockevents_program_event
    <idle>-0       3dNh1    8us : irq_exit <-smp_apic_timer_interrupt
    <idle>-0       3dNh1    9us : sub_preempt_count <-irq_exit
    <idle>-0       3dN.2    9us : idle_cpu <-irq_exit
    <idle>-0       3dN.2    9us : rcu_irq_exit <-irq_exit
    <idle>-0       3dN.2   10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
    <idle>-0       3dN.2   10us : sub_preempt_count <-irq_exit
    <idle>-0       3.N.1   11us : rcu_idle_exit <-cpu_idle
    <idle>-0       3dN.1   11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
    <idle>-0       3.N.1   11us : tick_nohz_idle_exit <-cpu_idle
    <idle>-0       3dN.1   12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
    <idle>-0       3dN.1   12us : ktime_get <-tick_nohz_idle_exit
    <idle>-0       3dN.1   12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit

    <idle>-0       3dN.1   13us : cpu_load_update_nohz <-tick_nohz_idle_exit
    <idle>-0       3dN.1   13us : _raw_spin_lock <-cpu_load_update_nohz

    <idle>-0       3dN.1   13us : add_preempt_count <-_raw_spin_lock

    <idle>-0       3dN.2   13us : __cpu_load_update <-cpu_load_update_nohz
    <idle>-0       3dN.2   14us : sched_avg_update <-__cpu_load_update
    <idle>-0       3dN.2   14us : _raw_spin_unlock <-cpu_load_update_nohz

    <idle>-0       3dN.2   14us : sub_preempt_count <-_raw_spin_unlock

    <idle>-0       3dN.1   15us : calc_load_nohz_stop <-tick_nohz_idle_exit

    <idle>-0       3dN.1   15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
    <idle>-0       3dN.1   15us : hrtimer_cancel <-tick_nohz_idle_exit
    <idle>-0       3dN.1   15us : hrtimer_try_to_cancel <-hrtimer_cancel
    <idle>-0       3dN.1   16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
    <idle>-0       3dN.1   16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
    <idle>-0       3dN.1   16us : add_preempt_count <-_raw_spin_lock_irqsave
    <idle>-0       3dN.2   17us : __remove_hrtimer <-remove_hrtimer.part.16
    <idle>-0       3dN.2   17us : hrtimer_force_reprogram <-__remove_hrtimer
    <idle>-0       3dN.2   17us : tick_program_event <-hrtimer_force_reprogram
    <idle>-0       3dN.2   18us : clockevents_program_event <-tick_program_event
    <idle>-0       3dN.2   18us : ktime_get <-clockevents_program_event
    <idle>-0       3dN.2   18us : lapic_next_event <-clockevents_program_event
    <idle>-0       3dN.2   19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
    <idle>-0       3dN.2   19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
    <idle>-0       3dN.1   19us : hrtimer_forward <-tick_nohz_idle_exit
    <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
    <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
    <idle>-0       3dN.1   20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
    <idle>-0       3dN.1   20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
    <idle>-0       3dN.1   21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
    <idle>-0       3dN.1   21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
    <idle>-0       3dN.1   21us : add_preempt_count <-_raw_spin_lock_irqsave
    <idle>-0       3dN.2   22us : ktime_add_safe <-__hrtimer_start_range_ns
    <idle>-0       3dN.2   22us : enqueue_hrtimer <-__hrtimer_start_range_ns
    <idle>-0       3dN.2   22us : tick_program_event <-__hrtimer_start_range_ns
    <idle>-0       3dN.2   23us : clockevents_program_event <-tick_program_event
    <idle>-0       3dN.2   23us : ktime_get <-clockevents_program_event
    <idle>-0       3dN.2   23us : lapic_next_event <-clockevents_program_event
    <idle>-0       3dN.2   24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
    <idle>-0       3dN.2   24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
    <idle>-0       3dN.1   24us : account_idle_ticks <-tick_nohz_idle_exit
    <idle>-0       3dN.1   24us : account_idle_time <-account_idle_ticks
    <idle>-0       3.N.1   25us : sub_preempt_count <-cpu_idle
    <idle>-0       3.N..   25us : schedule <-cpu_idle
    <idle>-0       3.N..   25us : __schedule <-preempt_schedule
    <idle>-0       3.N..   26us : add_preempt_count <-__schedule
    <idle>-0       3.N.1   26us : rcu_note_context_switch <-__schedule
    <idle>-0       3.N.1   26us : rcu_sched_qs <-rcu_note_context_switch
    <idle>-0       3dN.1   27us : rcu_preempt_qs <-rcu_note_context_switch
    <idle>-0       3.N.1   27us : _raw_spin_lock_irq <-__schedule
    <idle>-0       3dN.1   27us : add_preempt_count <-_raw_spin_lock_irq
    <idle>-0       3dN.2   28us : put_prev_task_idle <-__schedule
    <idle>-0       3dN.2   28us : pick_next_task_stop <-pick_next_task
    <idle>-0       3dN.2   28us : pick_next_task_rt <-pick_next_task
    <idle>-0       3dN.2   29us : dequeue_pushable_task <-pick_next_task_rt
    <idle>-0       3d..3   29us : __schedule <-preempt_schedule
    <idle>-0       3d..3   30us :      0:120:R ==> [003]  2448: 94:R sleep
  
  This isn't that big of a trace, even with function tracing enabled,
  so I included the entire trace.
  
  The interrupt went off while when the system was idle. Somewhere
  before task_woken_rt() was called, the NEED_RESCHED flag was set,
  this is indicated by the first occurrence of the 'N' flag.
  
  Latency tracing and events
  --------------------------
  As function tracing can induce a much larger latency, but without
  seeing what happens within the latency it is hard to know what
  caused it. There is a middle ground, and that is with enabling
  events.
  
   # echo 0 > options/function-trace
   # echo wakeup_rt > current_tracer
   # echo 1 > events/enable
   # echo 1 > tracing_on
   # echo 0 > tracing_max_latency
   # chrt -f 5 sleep 1
   # echo 0 > tracing_on
   # cat trace
  # tracer: wakeup_rt
  #
  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: sleep-5882 (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       2d.h4    0us :      0:120:R   + [002]  5882: 94:R sleep
    <idle>-0       2d.h4    0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
    <idle>-0       2d.h4    1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
    <idle>-0       2dNh2    1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
    <idle>-0       2.N.2    2us : power_end: cpu_id=2
    <idle>-0       2.N.2    3us : cpu_idle: state=4294967295 cpu_id=2
    <idle>-0       2dN.3    4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
    <idle>-0       2dN.3    4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
    <idle>-0       2.N.2    5us : rcu_utilization: Start context switch
    <idle>-0       2.N.2    5us : rcu_utilization: End context switch
    <idle>-0       2d..3    6us : __schedule <-schedule
    <idle>-0       2d..3    6us :      0:120:R ==> [002]  5882: 94:R sleep

  Hardware Latency Detector
  -------------------------
  
  The hardware latency detector is executed by enabling the "hwlat" tracer.
  
  NOTE, this tracer will affect the performance of the system as it will
  periodically make a CPU constantly busy with interrupts disabled.
  
   # echo hwlat > current_tracer
   # sleep 100
   # cat trace
  # tracer: hwlat
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
             <...>-3638  [001] d... 19452.055471: #1     inner/outer(us):   12/14    ts:1499801089.066141940
             <...>-3638  [003] d... 19454.071354: #2     inner/outer(us):   11/9     ts:1499801091.082164365
             <...>-3638  [002] dn.. 19461.126852: #3     inner/outer(us):   12/9     ts:1499801098.138150062
             <...>-3638  [001] d... 19488.340960: #4     inner/outer(us):    8/12    ts:1499801125.354139633
             <...>-3638  [003] d... 19494.388553: #5     inner/outer(us):    8/12    ts:1499801131.402150961
             <...>-3638  [003] d... 19501.283419: #6     inner/outer(us):    0/12    ts:1499801138.297435289 nmi-total:4 nmi-count:1
  
  
  The above output is somewhat the same in the header. All events will have
  interrupts disabled 'd'. Under the FUNCTION title there is:
  
   #1 - This is the count of events recorded that were greater than the
        tracing_threshold (See below).
  
   inner/outer(us):   12/14
  
        This shows two numbers as "inner latency" and "outer latency". The test
        runs in a loop checking a timestamp twice. The latency detected within
        the two timestamps is the "inner latency" and the latency detected
        after the previous timestamp and the next timestamp in the loop is
        the "outer latency".
  
   ts:1499801089.066141940
  
        The absolute timestamp that the event happened.
  
   nmi-total:4 nmi-count:1
  
        On architectures that support it, if an NMI comes in during the
        test, the time spent in NMI is reported in "nmi-total" (in
        microseconds).
  
        All architectures that have NMIs will show the "nmi-count" if an
        NMI comes in during the test.
  
  hwlat files:
  
    tracing_threshold - This gets automatically set to "10" to represent 10
  		      microseconds. This is the threshold of latency that
  		      needs to be detected before the trace will be recorded.
  
  		      Note, when hwlat tracer is finished (another tracer is
  		      written into "current_tracer"), the original value for
  		      tracing_threshold is placed back into this file.
  
    hwlat_detector/width - The length of time the test runs with interrupts
  			 disabled.
  
    hwlat_detector/window - The length of time of the window which the test
  			  runs. That is, the test will run for "width"
  			  microseconds per "window" microseconds
  
    tracing_cpumask - When the test is started. A kernel thread is created that
  		    runs the test. This thread will alternate between CPUs
  		    listed in the tracing_cpumask between each period
  		    (one "window"). To limit the test to specific CPUs
  		    set the mask in this file to only the CPUs that the test
  		    should run on.

  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.

  See the "ftrace_enabled" section below.

  
   # sysctl kernel.ftrace_enabled=1

   # echo function > current_tracer

   # echo 1 > tracing_on

   # usleep 1

   # echo 0 > tracing_on

   # cat trace

  # tracer: function

  #

  # entries-in-buffer/entries-written: 24799/24799   #P:4
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
              bash-1994  [002] ....  3082.063030: mutex_unlock <-rb_simple_write
              bash-1994  [002] ....  3082.063031: __mutex_unlock_slowpath <-mutex_unlock
              bash-1994  [002] ....  3082.063031: __fsnotify_parent <-fsnotify_modify
              bash-1994  [002] ....  3082.063032: fsnotify <-fsnotify_modify
              bash-1994  [002] ....  3082.063032: __srcu_read_lock <-fsnotify
              bash-1994  [002] ....  3082.063032: add_preempt_count <-__srcu_read_lock
              bash-1994  [002] ...1  3082.063032: sub_preempt_count <-__srcu_read_lock
              bash-1994  [002] ....  3082.063033: __srcu_read_unlock <-fsnotify

  [...]

  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 > 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_tracefs(void)

  {

         static char tracefs[MAX_PATH+1];
         static int tracefs_found;

         char type[100];
         FILE *fp;

         if (tracefs_found)
                 return tracefs;

  
         if ((fp = fopen("/proc/mounts","r")) == NULL) {
                 perror("/proc/mounts");
                 return NULL;
         }
  
         while (fscanf(fp, "%*s %"
                       STR(MAX_PATH)
                       "s %99s %*s %*d %*d
  ",

                       tracefs, type) == 2) {
                 if (strcmp(type, "tracefs") == 0)

                         break;
         }
         fclose(fp);

         if (strcmp(type, "tracefs") != 0) {
                 fprintf(stderr, "tracefs not mounted");

                 return NULL;
         }

         strcat(tracefs, "/tracing/");
         tracefs_found = 1;

         return tracefs;

  }
  
  const char *tracing_file(const char *file_name)
  {
         static char trace_file[MAX_PATH+1];

         snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), 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
  ", getpid());
                  write(fd, line, s);
  
                  write(ffd, "function", 8);
  
                  close(fd);
                  close(ffd);
  
                  execvp(argv[1], argv+1);
          }
  
          return 0;
  }

  Or this simple script!

  ------
  #!/bin/bash

  tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts`
  echo nop > $tracefs/tracing/current_tracer
  echo 0 > $tracefs/tracing/tracing_on
  echo $$ > $tracefs/tracing/set_ftrace_pid
  echo function > $tracefs/tracing/current_tracer
  echo 1 > $tracefs/tracing/tracing_on

  exec "$@"
  ------

  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:

    3) # 1837.709 us |          } /* __switch_to */
    3)               |          finish_task_switch() {
    3)   0.313 us    |            _raw_spin_unlock_irq();
    3)   3.177 us    |          }
    3) # 1889.063 us |        } /* __schedule */
    3) ! 140.417 us  |      } /* __schedule */
    3) # 2034.948 us |    } /* schedule */
    3) * 33998.59 us |  } /* schedule_preempt_disabled */

  
    [...]

    1)   0.260 us    |              msecs_to_jiffies();
    1)   0.313 us    |              __rcu_read_unlock();
    1) + 61.770 us   |            }
    1) + 64.479 us   |          }
    1)   0.313 us    |          rcu_bh_qs();
    1)   0.313 us    |          __local_bh_enable();
    1) ! 217.240 us  |        }
    1)   0.365 us    |        idle_cpu();
    1)               |        rcu_irq_exit() {
    1)   0.417 us    |          rcu_eqs_enter_common.isra.47();
    1)   3.125 us    |        }
    1) ! 227.812 us  |      }
    1) ! 457.395 us  |    }
    1) @ 119760.2 us |  }
  
    [...]
  
    2)               |    handle_IPI() {
    1)   6.979 us    |                  }
    2)   0.417 us    |      scheduler_ipi();
    1)   9.791 us    |                }
    1) + 12.917 us   |              }
    2)   3.490 us    |    }
    1) + 15.729 us   |            }
    1) + 18.542 us   |          }
    2) $ 3594274 us  |  }

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

    # means that the function exceeded 1000 usecs.

    * means that the function exceeded 10 msecs.
    @ means that the function exceeded 100 msecs.

    $ means that the function exceeded 1 sec.

  - 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();

  The function name is always displayed after the closing bracket
  for a function if the start of that function is not in the
  trace buffer.
  
  Display of the function name after the closing bracket may be
  enabled for functions whose start is in the trace buffer,
  allowing easier searching with grep for function durations.
  It is default disabled.
  
  	hide: echo nofuncgraph-tail > trace_options
  	show: echo funcgraph-tail > trace_options
  
    Example with nofuncgraph-tail (default):
    0)               |      putname() {
    0)               |        kmem_cache_free() {
    0)   0.518 us    |          __phys_addr();
    0)   1.757 us    |        }
    0)   2.861 us    |      }
  
    Example with funcgraph-tail:
    0)               |      putname() {
    0)               |        kmem_cache_free() {
    0)   0.518 us    |          __phys_addr();
    0)   1.757 us    |        } /* kmem_cache_free() */
    0)   2.861 us    |      } /* putname() */

  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!
  ")

  
  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 program (located in the scripts directory). This
  program will parse the ELF headers in the C object to find all

  the locations in the .text section that call mcount. Starting
  with gcc verson 4.6, the -mfentry has been added for x86, which
  calls "__fentry__" instead of "mcount". Which is called before
  the creation of the stack frame.
  
  Note, not all sections are traced. They may be prevented by either
  a notrace, or blocked another way and all inline functions are not
  traced. Check the "available_filter_functions" file to see what functions
  can be traced.
  
  A section called "__mcount_loc" is created that holds
  references to all the mcount/fentry call sites in the .text section.

  The recordmcount program re-links this section back into the
  original object. The final linking stage of the kernel 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, the process of modifying the function
  tracepoints is dependent on architecture. The old method is to use
  kstop_machine to prevent races with the CPUs executing code being
  modified (which can cause the CPU to do undesirable things, especially
  if the modified code crosses cache (or page) boundaries), 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.

  The new method of modifying the function tracepoints is to place
  a breakpoint at the location to be modified, sync all CPUs, modify
  the rest of the instruction not covered by the breakpoint. Sync
  all CPUs again, and then remove the breakpoint with the finished
  version to the ftrace call site.
  
  Some archs do not even need to monkey around with the synchronization,
  and can just slap the new code on top of the old without any
  problems with other CPUs executing it at the same time.

  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: function
  #
  # entries-in-buffer/entries-written: 5/5   #P:4

  #

  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
            usleep-2665  [001] ....  4186.475355: sys_nanosleep <-system_call_fastpath
            <idle>-0     [001] d.h1  4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
            usleep-2665  [001] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
            <idle>-0     [003] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
            <idle>-0     [002] d.h1  4186.475427: 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 glob(7) matching.

    <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

    <match1>*<match2> - will match functions that begin with
                        <match1> and end with <match2>

  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: function

  #

  # entries-in-buffer/entries-written: 897/897   #P:4
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
            <idle>-0     [003] dN.1  4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
            <idle>-0     [003] dN.1  4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
            <idle>-0     [003] dN.2  4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
            <idle>-0     [003] dN.1  4228.547805: hrtimer_forward <-tick_nohz_idle_exit
            <idle>-0     [003] dN.1  4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
            <idle>-0     [003] d..1  4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
            <idle>-0     [003] d..1  4228.547859: hrtimer_start <-__tick_nohz_idle_enter
            <idle>-0     [003] d..2  4228.547860: hrtimer_force_reprogram <-__rem

  
  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: function
  #
  # entries-in-buffer/entries-written: 39608/39608   #P:4

  #

  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
              bash-1994  [000] ....  4342.324896: file_ra_state_init <-do_dentry_open
              bash-1994  [000] ....  4342.324897: open_check_o_direct <-do_last
              bash-1994  [000] ....  4342.324897: ima_file_check <-do_last
              bash-1994  [000] ....  4342.324898: process_measurement <-ima_file_check
              bash-1994  [000] ....  4342.324898: ima_get_action <-process_measurement
              bash-1994  [000] ....  4342.324898: ima_match_policy <-ima_get_action
              bash-1994  [000] ....  4342.324899: do_truncate <-do_last
              bash-1994  [000] ....  4342.324899: should_remove_suid <-do_truncate
              bash-1994  [000] ....  4342.324899: notify_change <-do_truncate
              bash-1994  [000] ....  4342.324900: current_fs_time <-notify_change
              bash-1994  [000] ....  4342.324900: current_kernel_time <-current_fs_time
              bash-1994  [000] ....  4342.324900: timespec_trunc <-current_fs_time

  
  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

  ftrace_enabled
  --------------
  
  Note, the proc sysctl ftrace_enable is a big on/off switch for the
  function tracer. By default it is enabled (when function tracing is
  enabled in the kernel). If it is disabled, all function tracing is
  disabled. This includes not only the function tracers for ftrace, but
  also for any other uses (perf, kprobes, stack tracing, profiling, etc).
  
  Please disable this with care.
  
  This can be disable (and enabled) with:
  
    sysctl kernel.ftrace_enabled=0
    sysctl kernel.ftrace_enabled=1
  
   or
  
    echo 0 > /proc/sys/kernel/ftrace_enabled
    echo 1 > /proc/sys/kernel/ftrace_enabled

  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

    Mod command supports module globbing. Disable tracing for all
    functions except a specific module:
  
     echo '!*:mod:!ext3' >> set_ftrace_filter
  
    Disable tracing for all modules, but still trace kernel:
  
     echo '!*:mod:*' >> set_ftrace_filter
  
    Enable filter only for kernel:
  
     echo '*write*:mod:!*' >> set_ftrace_filter
  
    Enable filter for module globbing:
  
     echo '*write*:mod:*snd*' >> 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

    To always disable tracing when __schedule_bug is hit:
  
     echo '__schedule_bug:traceoff' > 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:0' > set_ftrace_filter
  
      The above removes the traceoff command for __schedule_bug
      that have a counter. To remove commands without counters:

     echo '!__schedule_bug:traceoff' > set_ftrace_filter

  - snapshot
    Will cause a snapshot to be triggered when the function is hit.
  
     echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
  
    To only snapshot once:
  
     echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
  
    To remove the above commands:
  
     echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
     echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
  
  - enable_event/disable_event
    These commands can enable or disable a trace event. Note, because
    function tracing callbacks are very sensitive, when these commands
    are registered, the trace point is activated, but disabled in
    a "soft" mode. That is, the tracepoint will be called, but
    just will not be traced. The event tracepoint stays in this mode
    as long as there's a command that triggers it.
  
     echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
     	 set_ftrace_filter
  
    The format is:
  
      <function>:enable_event:<system>:<event>[:count]
      <function>:disable_event:<system>:<event>[:count]
  
    To remove the events commands:
  
  
     echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
     	 set_ftrace_filter
     echo '!schedule:disable_event:sched:sched_switch' > \
     	 set_ftrace_filter

  - dump
    When the function is hit, it will dump the contents of the ftrace
    ring buffer to the console. This is useful if you need to debug
    something, and want to dump the trace when a certain function
    is hit. Perhaps its a function that is called before a tripple
    fault happens and does not allow you to get a regular dump.

  - cpudump
    When the function is hit, it will dump the contents of the ftrace
    ring buffer for the current CPU to the console. Unlike the "dump"
    command, it only prints out the contents of the ring buffer for the
    CPU that executed the function that triggered the dump.

  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

  #

  # entries-in-buffer/entries-written: 0/0   #P:4
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |

  
   #
   # cat /tmp/trace.out

              bash-1994  [000] ....  5281.568961: mutex_unlock <-rb_simple_write
              bash-1994  [000] ....  5281.568963: __mutex_unlock_slowpath <-mutex_unlock
              bash-1994  [000] ....  5281.568963: __fsnotify_parent <-fsnotify_modify
              bash-1994  [000] ....  5281.568964: fsnotify <-fsnotify_modify
              bash-1994  [000] ....  5281.568964: __srcu_read_lock <-fsnotify
              bash-1994  [000] ....  5281.568964: add_preempt_count <-__srcu_read_lock
              bash-1994  [000] ...1  5281.568965: sub_preempt_count <-__srcu_read_lock
              bash-1994  [000] ....  5281.568965: __srcu_read_unlock <-fsnotify
              bash-1994  [000] ....  5281.568967: sys_dup2 <-system_call_fastpath

  Note, reading the trace_pipe file will block until more input is

  added.

  
  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)

  Or simply read buffer_total_size_kb
  
   # cat buffer_total_size_kb 
  5632
  
  To modify the buffer, simple echo in a number (in 1024 byte segments).

   # echo 10000 > buffer_size_kb
   # cat buffer_size_kb

  10000 (units kilobytes)

  It will try to allocate as much as possible. If you allocate too
  much, it can cause Out-Of-Memory to trigger.

   # echo 1000000000000 > buffer_size_kb

  -bash: echo: write error: Cannot allocate memory

   # cat buffer_size_kb

  85

  The per_cpu buffers can be changed individually as well:
  
   # echo 10000 > per_cpu/cpu0/buffer_size_kb
   # echo 100 > per_cpu/cpu1/buffer_size_kb
  
  When the per_cpu buffers are not the same, the buffer_size_kb
  at the top level will just show an X
  
   # cat buffer_size_kb
  X
  
  This is where the buffer_total_size_kb is useful:
  
   # cat buffer_total_size_kb 
  12916
  
  Writing to the top level buffer_size_kb will reset all the buffers
  to be the same again.

  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 tracefs 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

  
  Instances
  ---------

  In the tracefs tracing directory is a directory called "instances".

  This directory can have new directories created inside of it using
  mkdir, and removing directories with rmdir. The directory created
  with mkdir in this directory will already contain files and other
  directories after it is created.
  
   # mkdir instances/foo
   # ls instances/foo
  buffer_size_kb  buffer_total_size_kb  events  free_buffer  per_cpu
  set_event  snapshot  trace  trace_clock  trace_marker  trace_options
  trace_pipe  tracing_on
  
  As you can see, the new directory looks similar to the tracing directory
  itself. In fact, it is very similar, except that the buffer and
  events are agnostic from the main director, or from any other
  instances that are created.
  
  The files in the new directory work just like the files with the
  same name in the tracing directory except the buffer that is used
  is a separate and new buffer. The files affect that buffer but do not
  affect the main buffer with the exception of trace_options. Currently,
  the trace_options affect all instances and the top level buffer
  the same, but this may change in future releases. That is, options
  may become specific to the instance they reside in.
  
  Notice that none of the function tracer files are there, nor is
  current_tracer and available_tracers. This is because the buffers
  can currently only have events enabled for them.
  
   # mkdir instances/foo
   # mkdir instances/bar
   # mkdir instances/zoot
   # echo 100000 > buffer_size_kb
   # echo 1000 > instances/foo/buffer_size_kb
   # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
   # echo function > current_trace
   # echo 1 > instances/foo/events/sched/sched_wakeup/enable
   # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
   # echo 1 > instances/foo/events/sched/sched_switch/enable
   # echo 1 > instances/bar/events/irq/enable
   # echo 1 > instances/zoot/events/syscalls/enable
   # cat trace_pipe
  CPU:2 [LOST 11745 EVENTS]
              bash-2044  [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
              bash-2044  [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
              bash-2044  [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
              bash-2044  [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
              bash-2044  [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
              bash-2044  [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
              bash-2044  [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
              bash-2044  [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
              bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
              bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
              bash-2044  [002] .... 10594.481035: arch_dup_task_struct <-copy_process
  [...]
  
   # cat instances/foo/trace_pipe
              bash-1998  [000] d..4   136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
              bash-1998  [000] dN.4   136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
            <idle>-0     [003] d.h3   136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
            <idle>-0     [003] d..3   136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120
       rcu_preempt-9     [003] d..3   136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120
              bash-1998  [000] d..4   136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
              bash-1998  [000] dN.4   136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
              bash-1998  [000] d..3   136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120
       kworker/0:1-59    [000] d..4   136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
       kworker/0:1-59    [000] d..3   136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120
  [...]
  
   # cat instances/bar/trace_pipe
       migration/1-14    [001] d.h3   138.732674: softirq_raise: vec=3 [action=NET_RX]
            <idle>-0     [001] dNh3   138.732725: softirq_raise: vec=3 [action=NET_RX]
              bash-1998  [000] d.h1   138.733101: softirq_raise: vec=1 [action=TIMER]
              bash-1998  [000] d.h1   138.733102: softirq_raise: vec=9 [action=RCU]
              bash-1998  [000] ..s2   138.733105: softirq_entry: vec=1 [action=TIMER]
              bash-1998  [000] ..s2   138.733106: softirq_exit: vec=1 [action=TIMER]
              bash-1998  [000] ..s2   138.733106: softirq_entry: vec=9 [action=RCU]
              bash-1998  [000] ..s2   138.733109: softirq_exit: vec=9 [action=RCU]
              sshd-1995  [001] d.h1   138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
              sshd-1995  [001] d.h1   138.733280: irq_handler_exit: irq=21 ret=unhandled
              sshd-1995  [001] d.h1   138.733281: irq_handler_entry: irq=21 name=eth0
              sshd-1995  [001] d.h1   138.733283: irq_handler_exit: irq=21 ret=handled
  [...]
  
   # cat instances/zoot/trace
  # tracer: nop
  #
  # entries-in-buffer/entries-written: 18996/18996   #P:4
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
              bash-1998  [000] d...   140.733501: sys_write -> 0x2
              bash-1998  [000] d...   140.733504: sys_dup2(oldfd: a, newfd: 1)
              bash-1998  [000] d...   140.733506: sys_dup2 -> 0x1
              bash-1998  [000] d...   140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
              bash-1998  [000] d...   140.733509: sys_fcntl -> 0x1
              bash-1998  [000] d...   140.733510: sys_close(fd: a)
              bash-1998  [000] d...   140.733510: sys_close -> 0x0
              bash-1998  [000] d...   140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
              bash-1998  [000] d...   140.733515: sys_rt_sigprocmask -> 0x0
              bash-1998  [000] d...   140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
              bash-1998  [000] d...   140.733516: sys_rt_sigaction -> 0x0
  
  You can see that the trace of the top most trace buffer shows only
  the function tracing. The foo instance displays wakeups and task
  switches.
  
  To remove the instances, simply delete their directories:
  
   # rmdir instances/foo
   # rmdir instances/bar
   # rmdir instances/zoot
  
  Note, if a process has a trace file open in one of the instance
  directories, the rmdir will fail with EBUSY.
  
  
  Stack trace

  -----------

  Since the kernel has a fixed sized stack, it is important not to
  waste it in functions. A kernel developer must be conscience of
  what they allocate on the stack. If they add too much, the system
  can be in danger of a stack overflow, and corruption will occur,
  usually leading to a system panic.
  
  There are some tools that check this, usually with interrupts
  periodically checking usage. But if you can perform a check
  at every function call that will become very useful. As ftrace provides
  a function tracer, it makes it convenient to check the stack size
  at every function call. This is enabled via the stack tracer.
  
  CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
  To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
  
   # echo 1 > /proc/sys/kernel/stack_tracer_enabled
  
  You can also enable it from the kernel command line to trace
  the stack size of the kernel during boot up, by adding "stacktrace"
  to the kernel command line parameter.
  
  After running it for a few minutes, the output looks like:
  
   # cat stack_max_size
  2928
  
   # cat stack_trace
          Depth    Size   Location    (18 entries)
          -----    ----   --------
    0)     2928     224   update_sd_lb_stats+0xbc/0x4ac
    1)     2704     160   find_busiest_group+0x31/0x1f1
    2)     2544     256   load_balance+0xd9/0x662
    3)     2288      80   idle_balance+0xbb/0x130
    4)     2208     128   __schedule+0x26e/0x5b9
    5)     2080      16   schedule+0x64/0x66
    6)     2064     128   schedule_timeout+0x34/0xe0
    7)     1936     112   wait_for_common+0x97/0xf1
    8)     1824      16   wait_for_completion+0x1d/0x1f
    9)     1808     128   flush_work+0xfe/0x119
   10)     1680      16   tty_flush_to_ldisc+0x1e/0x20
   11)     1664      48   input_available_p+0x1d/0x5c
   12)     1616      48   n_tty_poll+0x6d/0x134
   13)     1568      64   tty_poll+0x64/0x7f
   14)     1504     880   do_select+0x31e/0x511
   15)      624     400   core_sys_select+0x177/0x216
   16)      224      96   sys_select+0x91/0xb9
   17)      128     128   system_call_fastpath+0x16/0x1b
  
  Note, if -mfentry is being used by gcc, functions get traced before
  they set up the stack frame. This means that leaf level functions
  are not tested by the stack tracer when -mfentry is used.
  
  Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
  
  ---------

  
  More details can be found in the source code, in the

  kernel/trace/*.c files.