WSL2-Linux-Kernel/Documentation/trace/ftrace.txt

		ftrace - Function Tracer
		========================

Copyright 2008 Red Hat Inc.
   Author:   Steven Rostedt <srostedt@redhat.com>
  License:   The GNU Free Documentation License, Version 1.2
               (dual licensed under the GPL v2)
Reviewers:   Elias Oltmanns, Randy Dunlap, Andrew Morton,
	     John Kacur, and David Teigland.
Written for: 2.6.28-rc2
Updated for: 3.10

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 debugfs file system to see what is
going on in certain parts of the kernel.


Implementation Details
----------------------

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


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

Ftrace uses the debugfs file system to hold the control files as
well as the files to display output.

When debugfs is configured into the kernel (which selecting any ftrace
option will do) the directory /sys/kernel/debug will be created. To mount
this directory, you can add to your /etc/fstab file:

 debugfs       /sys/kernel/debug          debugfs defaults        0       0

Or you can mount it at run time with:

 mount -t debugfs nodev /sys/kernel/debug

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

 ln -s /sys/kernel/debug /debug

Any selected ftrace option will also create a directory called tracing
within the debugfs. The rest of the document will assume that you are in
the ftrace directory (cd /sys/kernel/debug/tracing) and will only concentrate
on the files within that directory and not distract from the content with
the extended "/sys/kernel/debug/tracing" path name.

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

After mounting debugfs, you can see a directory called
"tracing".  This directory contains the control and output files
of ftrace. Here is a list of some of the key files:


 Note: all time values are in microseconds.

  current_tracer:

	This is used to set or display the current tracer
	that is configured.

  available_tracers:

	This holds the different types of tracers that
	have been compiled into the kernel. The
	tracers listed here can be configured by
	echoing their name into current_tracer.

  tracing_on:

	This sets or displays whether writing to the trace
	ring buffer is enabled. Echo 0 into this file to disable
	the tracer or 1 to enable it. Note, this only disables
	writing to the ring buffer, the tracing overhead may
	still be occurring.

  trace:

	This file holds the output of the trace in a human
	readable format (described below).

  trace_pipe:

	The output is the same as the "trace" file but this
	file is meant to be streamed with live tracing.
	Reads from this file will block until new data is
	retrieved.  Unlike the "trace" file, this file is a
	consumer. This means reading from this file causes
	sequential reads to display more current data. Once
	data is read from this file, it is consumed, and
	will not be read again with a sequential read. The
	"trace" file is static, and if the tracer is not
	adding more data,they will display the same
	information every time they are read.

  trace_options:

	This file lets the user control the amount of data
	that is displayed in one of the above output
	files. 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 time interrupts are disabled.
	This time is saved in this file. The max trace
	will also be stored, and displayed by "trace".
	A new max trace will only be recorded if the
	latency is greater than the value in this
	file. (in microseconds)

  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.
	( Note, the size may not be a multiple of the page size
	  due to buffer management meta-data. )

  buffer_total_size_kb:

	This displays the total combined size of all the trace buffers.

  free_buffer:

	If a process is performing the 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.

	This interface also allows for commands to be used. See the
	"Filter commands" section for more details.

  set_ftrace_notrace:

	This has an effect opposite to that of
	set_ftrace_filter. Any function that is added here will not
	be traced. If a function exists in both set_ftrace_filter
	and set_ftrace_notrace,	the function will _not_ be traced.

  set_ftrace_pid:

	Have the function tracer only trace a single thread.

  set_graph_function:

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

  available_filter_functions:

	This lists the functions that ftrace
	has processed and can trace. These are the function
	names that you can pass to "set_ftrace_filter" or
	"set_ftrace_notrace". (See the section "dynamic ftrace"
	below for more details.)

  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.

  function_profile_enabled:

	When set it will enable all functions with either the function
	tracer, or if enabled, the function graph tracer. It will
	keep a histogram of the number of functions that were called
	and if run with the function graph tracer, 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 (currently only trace_printk()
	does this), 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.

  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

	  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.

	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);

  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.

  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.

  "irqsoff"

	Traces the areas that disable interrupts and saves
	the trace with the longest max latency.
	See tracing_max_latency. When a new max is recorded,
	it replaces the old trace. It is best to view this
	trace with the latency-format option enabled.

  "preemptoff"

	Similar to irqsoff but traces and records the amount of
	time for which preemption is disabled.

  "preemptirqsoff"

	Similar to irqsoff and preemptoff, but traces and
	records the largest time for which irqs and/or preemption
	is disabled.

  "wakeup"

	Traces and records the max latency that it takes for
	the highest priority task to get scheduled after
	it has been woken up.
        Traces all tasks as an average developer would expect.

  "wakeup_rt"

        Traces and records the max latency that it takes for just
        RT tasks (as the current "wakeup" does). This is useful
        for those interested in wake up timings of RT tasks.

  "nop"

	This is the "trace nothing" tracer. To remove all
	tracers from tracing simply echo "nop" into
	current_tracer.


Examples of using the tracer
----------------------------

Here are typical examples of using the tracers when controlling
them only with the debugfs interface (without using any
user-land utilities).

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

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

                             --------
# tracer: function
#
# 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
(3.10). 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:
	'H' - hard irq occurred inside a softirq.
	'h' - hard irq is running
	's' - soft irq is running
	'.' - normal context.

  preempt-depth: The level of preempt_disabled

The above is mostly meaningful for kernel developers.

  time: When the latency-format option is enabled, the trace file
	output includes a timestamp relative to the start of the
	trace. This differs from the output when latency-format
	is disabled, which includes an absolute timestamp.

  delay: This is just to help catch your eye a bit better. And
	 needs to be fixed to be only relative to the same CPU.
	 The marks are determined by the difference between this
	 current trace and the next trace.
	  '!' - greater than preempt_mark_thresh (default 100)
	  '+' - greater than 1 microsecond
	  ' ' - less than or equal to 1 microsecond.

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

  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
nostacktrace
trace_printk
noftrace_preempt
nobranch
annotate
nouserstacktrace
nosym-userobj
noprintk-msg-only
context-info
latency-format
sleep-time
graph-time
record-cmd
overwrite
nodisable_on_free
irq-info
markers
function-trace

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.

  stacktrace - This is one of the options that changes the trace
	       itself. When a trace is recorded, so is the stack
	       of functions. This allows for back traces of
	       trace sites.

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

  branch - Enable branch tracing with the tracer.

  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 userspace thread.

  sym-userobj - when user stacktrace are enabled, look up which
		object the address belongs to, and print a
		relative address. This is especially useful when
		ASLR is on, otherwise you don't get a chance to
		resolve the address to object/file/line after
		the app is no longer running

		The lookup is performed when you read
		trace,trace_pipe. Example:

		a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]


  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. When
                   it is enabled, the trace displays
                   additional information about the
                   latencies, as described in "Latency
                   trace format".

  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 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.

  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.

  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.


  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.

 Note: Some tracers have their own options. They only appear
       when the tracer is active.



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 - 20)
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_task <-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 : update_cpu_load_nohz <-tick_nohz_idle_exit
  <idle>-0       3dN.1   13us : _raw_spin_lock <-update_cpu_load_nohz
  <idle>-0       3dN.1   13us : add_preempt_count <-_raw_spin_lock
  <idle>-0       3dN.2   13us : __update_cpu_load <-update_cpu_load_nohz
  <idle>-0       3dN.2   14us : sched_avg_update <-__update_cpu_load
  <idle>-0       3dN.2   14us : _raw_spin_unlock <-update_cpu_load_nohz
  <idle>-0       3dN.2   14us : sub_preempt_count <-_raw_spin_unlock
  <idle>-0       3dN.1   15us : calc_load_exit_idle <-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


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 -1 > set_ftrace_pid
# cat trace |head
 # tracer: function
 #
 #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
 #              | |       |          |         |
 ##### CPU 3 buffer started ####
     yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
     yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
     yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
     yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
     yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit

If you want to trace a function when executing, you could use
something like this simple program:

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

#define _STR(x) #x
#define STR(x) _STR(x)
#define MAX_PATH 256

const char *find_debugfs(void)
{
       static char debugfs[MAX_PATH+1];
       static int debugfs_found;
       char type[100];
       FILE *fp;

       if (debugfs_found)
               return debugfs;

       if ((fp = fopen("/proc/mounts","r")) == NULL) {
               perror("/proc/mounts");
               return NULL;
       }

       while (fscanf(fp, "%*s %"
                     STR(MAX_PATH)
                     "s %99s %*s %*d %*d\n",
                     debugfs, type) == 2) {
               if (strcmp(type, "debugfs") == 0)
                       break;
       }
       fclose(fp);

       if (strcmp(type, "debugfs") != 0) {
               fprintf(stderr, "debugfs not mounted");
               return NULL;
       }

       strcat(debugfs, "/tracing/");
       debugfs_found = 1;

       return debugfs;
}

const char *tracing_file(const char *file_name)
{
       static char trace_file[MAX_PATH+1];
       snprintf(trace_file, MAX_PATH, "%s/%s", find_debugfs(), file_name);
       return trace_file;
}

int main (int argc, char **argv)
{
        if (argc < 1)
                exit(-1);

        if (fork() > 0) {
                int fd, ffd;
                char line[64];
                int s;

                ffd = open(tracing_file("current_tracer"), O_WRONLY);
                if (ffd < 0)
                        exit(-1);
                write(ffd, "nop", 3);

                fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
                s = sprintf(line, "%d\n", getpid());
                write(fd, line, s);

                write(ffd, "function", 8);

                close(fd);
                close(ffd);

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

        return 0;
}

Or this simple script!

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

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


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:

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

  [...]

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

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


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

	hide: echo nofuncgraph-proc > trace_options
	show: echo funcgraph-proc > trace_options

  ie:

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


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

	hide: echo nofuncgraph-abstime > trace_options
	show: echo funcgraph-abstime > trace_options

  ie:

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


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!\n")

will produce:

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


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

dynamic ftrace
--------------

If CONFIG_DYNAMIC_FTRACE is set, the system will run with
virtually no overhead when function tracing is disabled. The way
this works is the mcount function call (placed at the start of
every kernel function, produced by the -pg switch in gcc),
starts of pointing to a simple return. (Enabling FTRACE will
include the -pg switch in the compiling of the kernel.)

At compile time every C file object is run through the
recordmcount 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. (Note, only
white listed .text sections are processed, since processing other
sections like .init.text may cause races due to those sections
being freed unexpectedly).

A new section called "__mcount_loc" is created that holds
references to all the mcount 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 simple wild
cards. Only the following are currently available

  <match>*  - will match functions that begin with <match>
  *<match>  - will match functions that end with <match>
  *<match>* - will match functions that have <match> in it

These are the only wild cards which are supported.

  <match>*<match> will not work.

Note: It is better to use quotes to enclose the wild cards,
      otherwise the shell may expand the parameters into names
      of files in the local directory.

 # echo 'hrtimer_*' > set_ftrace_filter

Produces:

# tracer: 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

- 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 debugfs files in "tracing" are related to this
feature:

  snapshot:

	This is used to take a snapshot and to read the output
	of the snapshot. Echo 1 into this file to allocate a
	spare buffer and to take a snapshot (swap), then read
	the snapshot from this file in the same format as
	"trace" (described above in the section "The File
	System"). Both reads snapshot and tracing are executable
	in parallel. When the spare buffer is allocated, echoing
	0 frees it, and echoing else (positive) values clear the
	snapshot contents.
	More details are shown in the table below.

	status\input  |     0      |     1      |    else    |
	--------------+------------+------------+------------+
	not allocated |(do nothing)| alloc+swap |(do nothing)|
	--------------+------------+------------+------------+
	allocated     |    free    |    swap    |   clear    |
	--------------+------------+------------+------------+

Here is an example of using the snapshot feature.

 # echo 1 > events/sched/enable
 # echo 1 > snapshot
 # cat snapshot
# tracer: nop
#
# entries-in-buffer/entries-written: 71/71   #P:8
#
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
          <idle>-0     [005] d...  2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120
           sleep-2242  [005] d...  2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120
[...]
          <idle>-0     [002] d...  2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120

 # cat trace
# tracer: nop
#
# entries-in-buffer/entries-written: 77/77   #P:8
#
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
          <idle>-0     [007] d...  2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120
 snapshot-test-2-2229  [002] d...  2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120
[...]


If you try to use this snapshot feature when current tracer is
one of the latency tracers, you will get the following results.

 # echo wakeup > current_tracer
 # echo 1 > snapshot
bash: echo: write error: Device or resource busy
 # cat snapshot
cat: snapshot: Device or resource busy


Instances
---------
In the debugfs 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.