Documentation / trace / ftrace.txt

Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

			ftrace - Function Tracer
			========================
	
	Copyright 2008 Red Hat Inc.
	   Author:   Steven Rostedt <srostedt@redhat.com>
	  License:   The GNU Free Documentation License, Version 1.2
	               (dual licensed under the GPL v2)
	Original Reviewers:   Elias Oltmanns, Randy Dunlap, Andrew Morton,
			      John Kacur, and David Teigland.
	Written for: 2.6.28-rc2
	Updated for: 3.10
	Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt
	
	Introduction
	------------
	
	Ftrace is an internal tracer designed to help out developers and
	designers of systems to find what is going on inside the kernel.
	It can be used for debugging or analyzing latencies and
	performance issues that take place outside of user-space.
	
	Although ftrace is typically considered the function tracer, it
	is really a frame work of several assorted tracing utilities.
	There's latency tracing to examine what occurs between interrupts
	disabled and enabled, as well as for preemption and from a time
	a task is woken to the task is actually scheduled in.
	
	One of the most common uses of ftrace is the event tracing.
	Through out the kernel is hundreds of static event points that
	can be enabled via the tracefs file system to see what is
	going on in certain parts of the kernel.
	
	See events.txt for more information.
	
	
	Implementation Details
	----------------------
	
	See ftrace-design.txt for details for arch porters and such.
	
	
	The File System
	---------------
	
	Ftrace uses the tracefs file system to hold the control files as
	well as the files to display output.
	
	When tracefs is configured into the kernel (which selecting any ftrace
	option will do) the directory /sys/kernel/tracing will be created. To mount
	this directory, you can add to your /etc/fstab file:
	
	 tracefs       /sys/kernel/tracing       tracefs defaults        0       0
	
	Or you can mount it at run time with:
	
	 mount -t tracefs nodev /sys/kernel/tracing
	
	For quicker access to that directory you may want to make a soft link to
	it:
	
	 ln -s /sys/kernel/tracing /tracing
	
	     *** NOTICE ***
	
	Before 4.1, all ftrace tracing control files were within the debugfs
	file system, which is typically located at /sys/kernel/debug/tracing.
	For backward compatibility, when mounting the debugfs file system,
	the tracefs file system will be automatically mounted at:
	
	 /sys/kernel/debug/tracing
	
	All files located in the tracefs file system will be located in that
	debugfs file system directory as well.
	
	     *** NOTICE ***
	
	Any selected ftrace option will also create the tracefs file system.
	The rest of the document will assume that you are in the ftrace directory
	(cd /sys/kernel/tracing) and will only concentrate on the files within that
	directory and not distract from the content with the extended
	"/sys/kernel/tracing" path name.
	
	That's it! (assuming that you have ftrace configured into your kernel)
	
	After mounting tracefs you will have access to the control and output files
	of ftrace. Here is a list of some of the key files:
	
	
	 Note: all time values are in microseconds.
	
	  current_tracer:
	
		This is used to set or display the current tracer
		that is configured.
	
	  available_tracers:
	
		This holds the different types of tracers that
		have been compiled into the kernel. The
		tracers listed here can be configured by
		echoing their name into current_tracer.
	
	  tracing_on:
	
		This sets or displays whether writing to the trace
		ring buffer is enabled. Echo 0 into this file to disable
		the tracer or 1 to enable it. Note, this only disables
		writing to the ring buffer, the tracing overhead may
		still be occurring.
	
		The kernel function tracing_off() can be used within the
		kernel to disable writing to the ring buffer, which will
		set this file to "0". User space can re-enable tracing by
		echoing "1" into the file.
	
		Note, the function and event trigger "traceoff" will also
		set this file to zero and stop tracing. Which can also
		be re-enabled by user space using this file.
	
	  trace:
	
		This file holds the output of the trace in a human
		readable format (described below). Note, tracing is temporarily
		disabled while this file is being read (opened).
	
	  trace_pipe:
	
		The output is the same as the "trace" file but this
		file is meant to be streamed with live tracing.
		Reads from this file will block until new data is
		retrieved.  Unlike the "trace" file, this file is a
		consumer. This means reading from this file causes
		sequential reads to display more current data. Once
		data is read from this file, it is consumed, and
		will not be read again with a sequential read. The
		"trace" file is static, and if the tracer is not
		adding more data, it will display the same
		information every time it is read. This file will not
		disable tracing while being read.
	
	  trace_options:
	
		This file lets the user control the amount of data
		that is displayed in one of the above output
		files. Options also exist to modify how a tracer
		or events work (stack traces, timestamps, etc).
	
	  options:
	
		This is a directory that has a file for every available
		trace option (also in trace_options). Options may also be set
		or cleared by writing a "1" or "0" respectively into the
		corresponding file with the option name.
	
	  tracing_max_latency:
	
		Some of the tracers record the max latency.
		For example, the maximum time that interrupts are disabled.
		The maximum time is saved in this file. The max trace will also be
		stored,	and displayed by "trace". A new max trace will only be
		recorded if the latency is greater than the value in this file
		(in microseconds).
	
		By echoing in a time into this file, no latency will be recorded
		unless it is greater than the time in this file.
	
	  tracing_thresh:
	
		Some latency tracers will record a trace whenever the
		latency is greater than the number in this file.
		Only active when the file contains a number greater than 0.
		(in microseconds)
	
	  buffer_size_kb:
	
		This sets or displays the number of kilobytes each CPU
		buffer holds. By default, the trace buffers are the same size
		for each CPU. The displayed number is the size of the
		CPU buffer and not total size of all buffers. The
		trace buffers are allocated in pages (blocks of memory
		that the kernel uses for allocation, usually 4 KB in size).
		If the last page allocated has room for more bytes
		than requested, the rest of the page will be used,
		making the actual allocation bigger than requested or shown.
		( Note, the size may not be a multiple of the page size
		  due to buffer management meta-data. )
	
		Buffer sizes for individual CPUs may vary
		(see "per_cpu/cpu0/buffer_size_kb" below), and if they do
		this file will show "X".
	
	  buffer_total_size_kb:
	
		This displays the total combined size of all the trace buffers.
	
	  free_buffer:
	
		If a process is performing tracing, and the ring buffer	should be
		shrunk "freed" when the process is finished, even if it were to be
		killed by a signal, this file can be used for that purpose. On close
		of this file, the ring buffer will be resized to its minimum size.
		Having a process that is tracing also open this file, when the process
		exits its file descriptor for this file will be closed, and in doing so,
		the ring buffer will be "freed".
	
		It may also stop tracing if disable_on_free option is set.
	
	  tracing_cpumask:
	
		This is a mask that lets the user only trace on specified CPUs.
		The format is a hex string representing the CPUs.
	
	  set_ftrace_filter:
	
		When dynamic ftrace is configured in (see the
		section below "dynamic ftrace"), the code is dynamically
		modified (code text rewrite) to disable calling of the
		function profiler (mcount). This lets tracing be configured
		in with practically no overhead in performance.  This also
		has a side effect of enabling or disabling specific functions
		to be traced. Echoing names of functions into this file
		will limit the trace to only those functions.
	
		The functions listed in "available_filter_functions" are what
		can be written into this file.
	
		This interface also allows for commands to be used. See the
		"Filter commands" section for more details.
	
	  set_ftrace_notrace:
	
		This has an effect opposite to that of
		set_ftrace_filter. Any function that is added here will not
		be traced. If a function exists in both set_ftrace_filter
		and set_ftrace_notrace,	the function will _not_ be traced.
	
	  set_ftrace_pid:
	
		Have the function tracer only trace the threads whose PID are
		listed in this file.
	
		If the "function-fork" option is set, then when a task whose
		PID is listed in this file forks, the child's PID will
		automatically be added to this file, and the child will be
		traced by the function tracer as well. This option will also
		cause PIDs of tasks that exit to be removed from the file.
	
	  set_event_pid:
	
		Have the events only trace a task with a PID listed in this file.
		Note, sched_switch and sched_wake_up will also trace events
		listed in this file.
	
		To have the PIDs of children of tasks with their PID in this file
		added on fork, enable the "event-fork" option. That option will also
		cause the PIDs of tasks to be removed from this file when the task
		exits.
	
	  set_graph_function:
	
		Functions listed in this file will cause the function graph
		tracer to only trace these functions and the functions that
		they call. (See the section "dynamic ftrace" for more details).
	
	  set_graph_notrace:
	
		Similar to set_graph_function, but will disable function graph
		tracing when the function is hit until it exits the function.
		This makes it possible to ignore tracing functions that are called
		by a specific function.
	
	  available_filter_functions:
	
		This lists the functions that ftrace has processed and can trace.
		These are the function names that you can pass to
		"set_ftrace_filter" or "set_ftrace_notrace".
		(See the section "dynamic ftrace" below for more details.)
	
	  dyn_ftrace_total_info:
	
		This file is for debugging purposes. The number of functions that
		have been converted to nops and are available to be traced.
	
	  enabled_functions:
	
		This file is more for debugging ftrace, but can also be useful
		in seeing if any function has a callback attached to it.
		Not only does the trace infrastructure use ftrace function
		trace utility, but other subsystems might too. This file
		displays all functions that have a callback attached to them
		as well as the number of callbacks that have been attached.
		Note, a callback may also call multiple functions which will
		not be listed in this count.
	
		If the callback registered to be traced by a function with
		the "save regs" attribute (thus even more overhead), a 'R'
		will be displayed on the same line as the function that
		is returning registers.
	
		If the callback registered to be traced by a function with
		the "ip modify" attribute (thus the regs->ip can be changed),
		an 'I' will be displayed on the same line as the function that
		can be overridden.
	
		If the architecture supports it, it will also show what callback
		is being directly called by the function. If the count is greater
		than 1 it most likely will be ftrace_ops_list_func().
	
		If the callback of the function jumps to a trampoline that is
		specific to a the callback and not the standard trampoline,
		its address will be printed as well as the function that the
		trampoline calls.
	
	  function_profile_enabled:
	
		When set it will enable all functions with either the function
		tracer, or if configured, the function graph tracer. It will
		keep a histogram of the number of functions that were called
		and if the function graph tracer was configured, it will also keep
		track of the time spent in those functions. The histogram
		content can be displayed in the files:
	
		trace_stats/function<cpu> ( function0, function1, etc).
	
	  trace_stats:
	
		A directory that holds different tracing stats.
	
	  kprobe_events:
	 
		Enable dynamic trace points. See kprobetrace.txt.
	
	  kprobe_profile:
	
		Dynamic trace points stats. See kprobetrace.txt.
	
	  max_graph_depth:
	
		Used with the function graph tracer. This is the max depth
		it will trace into a function. Setting this to a value of
		one will show only the first kernel function that is called
		from user space.
	
	  printk_formats:
	
		This is for tools that read the raw format files. If an event in
		the ring buffer references a string, only a pointer to the string
		is recorded into the buffer and not the string itself. This prevents
		tools from knowing what that string was. This file displays the string
		and address for	the string allowing tools to map the pointers to what
		the strings were.
	
	  saved_cmdlines:
	
		Only the pid of the task is recorded in a trace event unless
		the event specifically saves the task comm as well. Ftrace
		makes a cache of pid mappings to comms to try to display
		comms for events. If a pid for a comm is not listed, then
		"<...>" is displayed in the output.
	
		If the option "record-cmd" is set to "0", then comms of tasks
		will not be saved during recording. By default, it is enabled.
	
	  saved_cmdlines_size:
	
		By default, 128 comms are saved (see "saved_cmdlines" above). To
		increase or decrease the amount of comms that are cached, echo
		in a the number of comms to cache, into this file.
	
	  saved_tgids:
	
		If the option "record-tgid" is set, on each scheduling context switch
		the Task Group ID of a task is saved in a table mapping the PID of
		the thread to its TGID. By default, the "record-tgid" option is
		disabled.
	
	  snapshot:
	
		This displays the "snapshot" buffer and also lets the user
		take a snapshot of the current running trace.
		See the "Snapshot" section below for more details.
	
	  stack_max_size:
	
		When the stack tracer is activated, this will display the
		maximum stack size it has encountered.
		See the "Stack Trace" section below.
	
	  stack_trace:
	
		This displays the stack back trace of the largest stack
		that was encountered when the stack tracer is activated.
		See the "Stack Trace" section below.
	
	  stack_trace_filter:
	
		This is similar to "set_ftrace_filter" but it limits what
		functions the stack tracer will check.
	
	  trace_clock:
	
		Whenever an event is recorded into the ring buffer, a
		"timestamp" is added. This stamp comes from a specified
		clock. By default, ftrace uses the "local" clock. This
		clock is very fast and strictly per cpu, but on some
		systems it may not be monotonic with respect to other
		CPUs. In other words, the local clocks may not be in sync
		with local clocks on other CPUs.
	
		Usual clocks for tracing:
	
		  # cat trace_clock
		  [local] global counter x86-tsc
	
		  The clock with the square brackets around it is the one
		  in effect.
	
		  local: Default clock, but may not be in sync across CPUs
	
		  global: This clock is in sync with all CPUs but may
		  	  be a bit slower than the local clock.
	
		  counter: This is not a clock at all, but literally an atomic
		  	   counter. It counts up one by one, but is in sync
			   with all CPUs. This is useful when you need to
			   know exactly the order events occurred with respect to
			   each other on different CPUs.
	
		  uptime: This uses the jiffies counter and the time stamp
		  	  is relative to the time since boot up.
	
		  perf: This makes ftrace use the same clock that perf uses.
		  	Eventually perf will be able to read ftrace buffers
			and this will help out in interleaving the data.
	
		  x86-tsc: Architectures may define their own clocks. For
		  	   example, x86 uses its own TSC cycle clock here.
	
		  ppc-tb: This uses the powerpc timebase register value.
			  This is in sync across CPUs and can also be used
			  to correlate events across hypervisor/guest if
			  tb_offset is known.
	
		  mono: This uses the fast monotonic clock (CLOCK_MONOTONIC)
			which is monotonic and is subject to NTP rate adjustments.
	
		  mono_raw:
			This is the raw monotonic clock (CLOCK_MONOTONIC_RAW)
			which is montonic but is not subject to any rate adjustments
			and ticks at the same rate as the hardware clocksource.
	
		  boot: This is the boot clock (CLOCK_BOOTTIME) and is based on the
			fast monotonic clock, but also accounts for time spent in
			suspend. Since the clock access is designed for use in
			tracing in the suspend path, some side effects are possible
			if clock is accessed after the suspend time is accounted before
			the fast mono clock is updated. In this case, the clock update
			appears to happen slightly sooner than it normally would have.
			Also on 32-bit systems, it's possible that the 64-bit boot offset
			sees a partial update. These effects are rare and post
			processing should be able to handle them. See comments in the
			ktime_get_boot_fast_ns() function for more information.
	
		To set a clock, simply echo the clock name into this file.
	
		  echo global > trace_clock
	
	  trace_marker:
	
		This is a very useful file for synchronizing user space
		with events happening in the kernel. Writing strings into
		this file will be written into the ftrace buffer.
	
		It is useful in applications to open this file at the start
		of the application and just reference the file descriptor
		for the file.
	
		void trace_write(const char *fmt, ...)
		{
			va_list ap;
			char buf[256];
			int n;
	
			if (trace_fd < 0)
				return;
	
			va_start(ap, fmt);
			n = vsnprintf(buf, 256, fmt, ap);
			va_end(ap);
	
			write(trace_fd, buf, n);
		}
	
		start:
	
			trace_fd = open("trace_marker", WR_ONLY);
	
	  trace_marker_raw:
	
		This is similar to trace_marker above, but is meant for for binary data
		to be written to it, where a tool can be used to parse the data
		from trace_pipe_raw.
	
	  uprobe_events:
	 
		Add dynamic tracepoints in programs.
		See uprobetracer.txt
	
	  uprobe_profile:
	
		Uprobe statistics. See uprobetrace.txt
	
	  instances:
	
		This is a way to make multiple trace buffers where different
		events can be recorded in different buffers.
		See "Instances" section below.
	
	  events:
	
		This is the trace event directory. It holds event tracepoints
		(also known as static tracepoints) that have been compiled
		into the kernel. It shows what event tracepoints exist
		and how they are grouped by system. There are "enable"
		files at various levels that can enable the tracepoints
		when a "1" is written to them.
	
		See events.txt for more information.
	
	  set_event:
	
		By echoing in the event into this file, will enable that event.
	
		See events.txt for more information.
	
	  available_events:
	
		A list of events that can be enabled in tracing.
	
		See events.txt for more information.
	
	  hwlat_detector:
	
		Directory for the Hardware Latency Detector.
		See "Hardware Latency Detector" section below.
	
	  per_cpu:
	
		This is a directory that contains the trace per_cpu information.
	
	  per_cpu/cpu0/buffer_size_kb:
	
		The ftrace buffer is defined per_cpu. That is, there's a separate
		buffer for each CPU to allow writes to be done atomically,
		and free from cache bouncing. These buffers may have different
		size buffers. This file is similar to the buffer_size_kb
		file, but it only displays or sets the buffer size for the
		specific CPU. (here cpu0).
	
	  per_cpu/cpu0/trace:
	
		This is similar to the "trace" file, but it will only display
		the data specific for the CPU. If written to, it only clears
		the specific CPU buffer.
	
	  per_cpu/cpu0/trace_pipe
	
		This is similar to the "trace_pipe" file, and is a consuming
		read, but it will only display (and consume) the data specific
		for the CPU.
	
	  per_cpu/cpu0/trace_pipe_raw
	
		For tools that can parse the ftrace ring buffer binary format,
		the trace_pipe_raw file can be used to extract the data
		from the ring buffer directly. With the use of the splice()
		system call, the buffer data can be quickly transferred to
		a file or to the network where a server is collecting the
		data.
	
		Like trace_pipe, this is a consuming reader, where multiple
		reads will always produce different data.
	
	  per_cpu/cpu0/snapshot:
	
		This is similar to the main "snapshot" file, but will only
		snapshot the current CPU (if supported). It only displays
		the content of the snapshot for a given CPU, and if
		written to, only clears this CPU buffer.
	
	  per_cpu/cpu0/snapshot_raw:
	
		Similar to the trace_pipe_raw, but will read the binary format
		from the snapshot buffer for the given CPU.
	
	  per_cpu/cpu0/stats:
	
		This displays certain stats about the ring buffer:
	
		 entries: The number of events that are still in the buffer.
	
		 overrun: The number of lost events due to overwriting when
		 	  the buffer was full.
	
		 commit overrun: Should always be zero.
		 	This gets set if so many events happened within a nested
			event (ring buffer is re-entrant), that it fills the
			buffer and starts dropping events.
	
		 bytes: Bytes actually read (not overwritten).
	
		 oldest event ts: The oldest timestamp in the buffer
	
		 now ts: The current timestamp
	
		 dropped events: Events lost due to overwrite option being off.
	
		 read events: The number of events read.
	
	The Tracers
	-----------
	
	Here is the list of current tracers that may be configured.
	
	  "function"
	
		Function call tracer to trace all kernel functions.
	
	  "function_graph"
	
		Similar to the function tracer except that the
		function tracer probes the functions on their entry
		whereas the function graph tracer traces on both entry
		and exit of the functions. It then provides the ability
		to draw a graph of function calls similar to C code
		source.
	
	  "blk"
	
		The block tracer. The tracer used by the blktrace user
		application.
	
	  "hwlat"
	
		The Hardware Latency tracer is used to detect if the hardware
		produces any latency. See "Hardware Latency Detector" section
		below.
	
	  "irqsoff"
	
		Traces the areas that disable interrupts and saves
		the trace with the longest max latency.
		See tracing_max_latency. When a new max is recorded,
		it replaces the old trace. It is best to view this
		trace with the latency-format option enabled, which
		happens automatically when the tracer is selected.
	
	  "preemptoff"
	
		Similar to irqsoff but traces and records the amount of
		time for which preemption is disabled.
	
	  "preemptirqsoff"
	
		Similar to irqsoff and preemptoff, but traces and
		records the largest time for which irqs and/or preemption
		is disabled.
	
	  "wakeup"
	
		Traces and records the max latency that it takes for
		the highest priority task to get scheduled after
		it has been woken up.
	        Traces all tasks as an average developer would expect.
	
	  "wakeup_rt"
	
	        Traces and records the max latency that it takes for just
	        RT tasks (as the current "wakeup" does). This is useful
	        for those interested in wake up timings of RT tasks.
	
	  "wakeup_dl"
	
		Traces and records the max latency that it takes for
		a SCHED_DEADLINE task to be woken (as the "wakeup" and
		"wakeup_rt" does).
	
	  "mmiotrace"
	
		A special tracer that is used to trace binary module.
		It will trace all the calls that a module makes to the
		hardware. Everything it writes and reads from the I/O
		as well.
	
	  "branch"
	
		This tracer can be configured when tracing likely/unlikely
		calls within the kernel. It will trace when a likely and
		unlikely branch is hit and if it was correct in its prediction
		of being correct.
	
	  "nop"
	
		This is the "trace nothing" tracer. To remove all
		tracers from tracing simply echo "nop" into
		current_tracer.
	
	
	Examples of using the tracer
	----------------------------
	
	Here are typical examples of using the tracers when controlling
	them only with the tracefs interface (without using any
	user-land utilities).
	
	Output format:
	--------------
	
	Here is an example of the output format of the file "trace"
	
	                             --------
	# tracer: function
	#
	# entries-in-buffer/entries-written: 140080/250280   #P:4
	#
	#                              _-----=> irqs-off
	#                             / _----=> need-resched
	#                            | / _---=> hardirq/softirq
	#                            || / _--=> preempt-depth
	#                            ||| /     delay
	#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
	#              | |       |   ||||       |         |
	            bash-1977  [000] .... 17284.993652: sys_close <-system_call_fastpath
	            bash-1977  [000] .... 17284.993653: __close_fd <-sys_close
	            bash-1977  [000] .... 17284.993653: _raw_spin_lock <-__close_fd
	            sshd-1974  [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
	            bash-1977  [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
	            bash-1977  [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
	            bash-1977  [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
	            bash-1977  [000] .... 17284.993657: filp_close <-__close_fd
	            bash-1977  [000] .... 17284.993657: dnotify_flush <-filp_close
	            sshd-1974  [003] .... 17284.993658: sys_select <-system_call_fastpath
	                             --------
	
	A header is printed with the tracer name that is represented by
	the trace. In this case the tracer is "function". Then it shows the
	number of events in the buffer as well as the total number of entries
	that were written. The difference is the number of entries that were
	lost due to the buffer filling up (250280 - 140080 = 110200 events
	lost).
	
	The header explains the content of the events. Task name "bash", the task
	PID "1977", the CPU that it was running on "000", the latency format
	(explained below), the timestamp in <secs>.<usecs> format, the
	function name that was traced "sys_close" and the parent function that
	called this function "system_call_fastpath". The timestamp is the time
	at which the function was entered.
	
	Latency trace format
	--------------------
	
	When the latency-format option is enabled or when one of the latency
	tracers is set, the trace file gives somewhat more information to see
	why a latency happened. Here is a typical trace.
	
	# tracer: irqsoff
	#
	# irqsoff latency trace v1.1.5 on 3.8.0-test+
	# --------------------------------------------------------------------
	# latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
	#    -----------------
	#    | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
	#    -----------------
	#  => started at: __lock_task_sighand
	#  => ended at:   _raw_spin_unlock_irqrestore
	#
	#
	#                  _------=> CPU#            
	#                 / _-----=> irqs-off        
	#                | / _----=> need-resched    
	#                || / _---=> hardirq/softirq 
	#                ||| / _--=> preempt-depth   
	#                |||| /     delay             
	#  cmd     pid   ||||| time  |   caller      
	#     \   /      |||||  \    |   /           
	      ps-6143    2d...    0us!: trace_hardirqs_off <-__lock_task_sighand
	      ps-6143    2d..1  259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
	      ps-6143    2d..1  263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
	      ps-6143    2d..1  306us : <stack trace>
	 => trace_hardirqs_on_caller
	 => trace_hardirqs_on
	 => _raw_spin_unlock_irqrestore
	 => do_task_stat
	 => proc_tgid_stat
	 => proc_single_show
	 => seq_read
	 => vfs_read
	 => sys_read
	 => system_call_fastpath
	
	
	This shows that the current tracer is "irqsoff" tracing the time
	for which interrupts were disabled. It gives the trace version (which
	never changes) and the version of the kernel upon which this was executed on
	(3.8). Then it displays the max latency in microseconds (259 us). The number
	of trace entries displayed and the total number (both are four: #4/4).
	VP, KP, SP, and HP are always zero and are reserved for later use.
	#P is the number of online CPUs (#P:4).
	
	The task is the process that was running when the latency
	occurred. (ps pid: 6143).
	
	The start and stop (the functions in which the interrupts were
	disabled and enabled respectively) that caused the latencies:
	
	 __lock_task_sighand is where the interrupts were disabled.
	 _raw_spin_unlock_irqrestore is where they were enabled again.
	
	The next lines after the header are the trace itself. The header
	explains which is which.
	
	  cmd: The name of the process in the trace.
	
	  pid: The PID of that process.
	
	  CPU#: The CPU which the process was running on.
	
	  irqs-off: 'd' interrupts are disabled. '.' otherwise.
		    Note: If the architecture does not support a way to
			  read the irq flags variable, an 'X' will always
			  be printed here.
	
	  need-resched:
		'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
		'n' only TIF_NEED_RESCHED is set,
		'p' only PREEMPT_NEED_RESCHED is set,
		'.' otherwise.
	
	  hardirq/softirq:
		'Z' - NMI occurred inside a hardirq
		'z' - NMI is running
		'H' - hard irq occurred inside a softirq.
		'h' - hard irq is running
		's' - soft irq is running
		'.' - normal context.
	
	  preempt-depth: The level of preempt_disabled
	
	The above is mostly meaningful for kernel developers.
	
	  time: When the latency-format option is enabled, the trace file
		output includes a timestamp relative to the start of the
		trace. This differs from the output when latency-format
		is disabled, which includes an absolute timestamp.
	
	  delay: This is just to help catch your eye a bit better. And
		 needs to be fixed to be only relative to the same CPU.
		 The marks are determined by the difference between this
		 current trace and the next trace.
		  '$' - greater than 1 second
		  '@' - greater than 100 milisecond
		  '*' - greater than 10 milisecond
		  '#' - greater than 1000 microsecond
		  '!' - greater than 100 microsecond
		  '+' - greater than 10 microsecond
		  ' ' - less than or equal to 10 microsecond.
	
	  The rest is the same as the 'trace' file.
	
	  Note, the latency tracers will usually end with a back trace
	  to easily find where the latency occurred.
	
	trace_options
	-------------
	
	The trace_options file (or the options directory) is used to control
	what gets printed in the trace output, or manipulate the tracers.
	To see what is available, simply cat the file:
	
	  cat trace_options
	print-parent
	nosym-offset
	nosym-addr
	noverbose
	noraw
	nohex
	nobin
	noblock
	trace_printk
	annotate
	nouserstacktrace
	nosym-userobj
	noprintk-msg-only
	context-info
	nolatency-format
	record-cmd
	norecord-tgid
	overwrite
	nodisable_on_free
	irq-info
	markers
	noevent-fork
	function-trace
	nofunction-fork
	nodisplay-graph
	nostacktrace
	nobranch
	
	To disable one of the options, echo in the option prepended with
	"no".
	
	  echo noprint-parent > trace_options
	
	To enable an option, leave off the "no".
	
	  echo sym-offset > trace_options
	
	Here are the available options:
	
	  print-parent - On function traces, display the calling (parent)
			 function as well as the function being traced.
	
	  print-parent:
	   bash-4000  [01]  1477.606694: simple_strtoul <-kstrtoul
	
	  noprint-parent:
	   bash-4000  [01]  1477.606694: simple_strtoul
	
	
	  sym-offset - Display not only the function name, but also the
		       offset in the function. For example, instead of
		       seeing just "ktime_get", you will see
		       "ktime_get+0xb/0x20".
	
	  sym-offset:
	   bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0
	
	  sym-addr - this will also display the function address as well
		     as the function name.
	
	  sym-addr:
	   bash-4000  [01]  1477.606694: simple_strtoul <c0339346>
	
	  verbose - This deals with the trace file when the
	            latency-format option is enabled.
	
	    bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
	    (+0.000ms): simple_strtoul (kstrtoul)
	
	  raw - This will display raw numbers. This option is best for
		use with user applications that can translate the raw
		numbers better than having it done in the kernel.
	
	  hex - Similar to raw, but the numbers will be in a hexadecimal
		format.
	
	  bin - This will print out the formats in raw binary.
	
	  block - When set, reading trace_pipe will not block when polled.
	
	  trace_printk - Can disable trace_printk() from writing into the buffer.
	
	  annotate - It is sometimes confusing when the CPU buffers are full
	  	     and one CPU buffer had a lot of events recently, thus
		     a shorter time frame, were another CPU may have only had
		     a few events, which lets it have older events. When
		     the trace is reported, it shows the oldest events first,
		     and it may look like only one CPU ran (the one with the
		     oldest events). When the annotate option is set, it will
		     display when a new CPU buffer started:
	
	          <idle>-0     [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
	          <idle>-0     [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
	          <idle>-0     [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
	##### CPU 2 buffer started ####
	          <idle>-0     [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
	          <idle>-0     [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
	          <idle>-0     [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
	
	  userstacktrace - This option changes the trace. It records a
			   stacktrace of the current user space thread after
			   each trace event.
	
	  sym-userobj - when user stacktrace are enabled, look up which
			object the address belongs to, and print a
			relative address. This is especially useful when
			ASLR is on, otherwise you don't get a chance to
			resolve the address to object/file/line after
			the app is no longer running
	
			The lookup is performed when you read
			trace,trace_pipe. Example:
	
			a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
	x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
	
	
	  printk-msg-only - When set, trace_printk()s will only show the format
	  		    and not their parameters (if trace_bprintk() or
			    trace_bputs() was used to save the trace_printk()).
	
	  context-info - Show only the event data. Hides the comm, PID,
	  	         timestamp, CPU, and other useful data.
	
	  latency-format - This option changes the trace output. When it is enabled,
			   the trace displays additional information about the
			   latency, as described in "Latency trace format".
	
	  record-cmd - When any event or tracer is enabled, a hook is enabled
		       in the sched_switch trace point to fill comm cache
		       with mapped pids and comms. But this may cause some
		       overhead, and if you only care about pids, and not the
		       name of the task, disabling this option can lower the
		       impact of tracing. See "saved_cmdlines".
	
	  record-tgid - When any event or tracer is enabled, a hook is enabled
		        in the sched_switch trace point to fill the cache of
			mapped Thread Group IDs (TGID) mapping to pids. See
			"saved_tgids".
	
	  overwrite - This controls what happens when the trace buffer is
	              full. If "1" (default), the oldest events are
	              discarded and overwritten. If "0", then the newest
	              events are discarded.
		        (see per_cpu/cpu0/stats for overrun and dropped)
	
	  disable_on_free - When the free_buffer is closed, tracing will
	  		    stop (tracing_on set to 0).
	
	  irq-info - Shows the interrupt, preempt count, need resched data.
	  	     When disabled, the trace looks like:
	
	# tracer: function
	#
	# entries-in-buffer/entries-written: 144405/9452052   #P:4
	#
	#           TASK-PID   CPU#      TIMESTAMP  FUNCTION
	#              | |       |          |         |
	          <idle>-0     [002]  23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
	          <idle>-0     [002]  23636.756054: activate_task <-ttwu_do_activate.constprop.89
	          <idle>-0     [002]  23636.756055: enqueue_task <-activate_task
	
	
	  markers - When set, the trace_marker is writable (only by root).
	  	    When disabled, the trace_marker will error with EINVAL
		    on write.
	
	  event-fork - When set, tasks with PIDs listed in set_event_pid will have
		       the PIDs of their children added to set_event_pid when those
		       tasks fork. Also, when tasks with PIDs in set_event_pid exit,
		       their PIDs will be removed from the file.
	
	  function-trace - The latency tracers will enable function tracing
	  	    if this option is enabled (default it is). When
		    it is disabled, the latency tracers do not trace
		    functions. This keeps the overhead of the tracer down
		    when performing latency tests.
	
	  function-fork - When set, tasks with PIDs listed in set_ftrace_pid will
			  have the PIDs of their children added to set_ftrace_pid
			  when those tasks fork. Also, when tasks with PIDs in
			  set_ftrace_pid exit, their PIDs will be removed from the
			  file.
	
	  display-graph - When set, the latency tracers (irqsoff, wakeup, etc) will
		          use function graph tracing instead of function tracing.
	
	  stacktrace - When set, a stack trace is recorded after any trace event
		       is recorded.
	
	  branch - Enable branch tracing with the tracer. This enables branch
		   tracer along with the currently set tracer. Enabling this
		   with the "nop" tracer is the same as just enabling the
		   "branch" tracer.
	
	 Note: Some tracers have their own options. They only appear in this
	       file when the tracer is active. They always appear in the
	       options directory.
	
	
	Here are the per tracer options:
	
	Options for function tracer:
	
	  func_stack_trace - When set, a stack trace is recorded after every
			     function that is recorded. NOTE! Limit the functions
			     that are recorded before enabling this, with
			     "set_ftrace_filter" otherwise the system performance
			     will be critically degraded. Remember to disable
			     this option before clearing the function filter.
	
	Options for function_graph tracer:
	
	 Since the function_graph tracer has a slightly different output
	 it has its own options to control what is displayed.
	
	  funcgraph-overrun - When set, the "overrun" of the graph stack is
			      displayed after each function traced. The
			      overrun, is when the stack depth of the calls
			      is greater than what is reserved for each task.
			      Each task has a fixed array of functions to
			      trace in the call graph. If the depth of the
			      calls exceeds that, the function is not traced.
			      The overrun is the number of functions missed
			      due to exceeding this array.
	
	  funcgraph-cpu - When set, the CPU number of the CPU where the trace
			  occurred is displayed.
	
	  funcgraph-overhead - When set, if the function takes longer than
			       A certain amount, then a delay marker is
			       displayed. See "delay" above, under the
			       header description.
	
	  funcgraph-proc - Unlike other tracers, the process' command line
			   is not displayed by default, but instead only
			   when a task is traced in and out during a context
			   switch. Enabling this options has the command
			   of each process displayed at every line.
	
	  funcgraph-duration - At the end of each function (the return)
			       the duration of the amount of time in the
			       function is displayed in microseconds.
	
	  funcgraph-abstime - When set, the timestamp is displayed at each
			      line.
	
	  funcgraph-irqs - When disabled, functions that happen inside an
			   interrupt will not be traced.
	
	  funcgraph-tail - When set, the return event will include the function
			   that it represents. By default this is off, and
			   only a closing curly bracket "}" is displayed for
			   the return of a function.
	
	  sleep-time - When running function graph tracer, to include
		       the time a task schedules out in its function.
		       When enabled, it will account time the task has been
		       scheduled out as part of the function call.
	
	  graph-time - When running function profiler with function graph tracer,
		       to include the time to call nested functions. When this is
		       not set, the time reported for the function will only
		       include the time the function itself executed for, not the
		       time for functions that it called.
	
	Options for blk tracer:
	
	  blk_classic - Shows a more minimalistic output.
	
	
	irqsoff
	-------
	
	When interrupts are disabled, the CPU can not react to any other
	external event (besides NMIs and SMIs). This prevents the timer
	interrupt from triggering or the mouse interrupt from letting
	the kernel know of a new mouse event. The result is a latency
	with the reaction time.
	
	The irqsoff tracer tracks the time for which interrupts are
	disabled. When a new maximum latency is hit, the tracer saves
	the trace leading up to that latency point so that every time a
	new maximum is reached, the old saved trace is discarded and the
	new trace is saved.
	
	To reset the maximum, echo 0 into tracing_max_latency. Here is
	an example:
	
	 # echo 0 > options/function-trace
	 # echo irqsoff > current_tracer
	 # echo 1 > tracing_on
	 # echo 0 > tracing_max_latency
	 # ls -ltr
	 [...]
	 # echo 0 > tracing_on
	 # cat trace
	# tracer: irqsoff
	#
	# irqsoff latency trace v1.1.5 on 3.8.0-test+
	# --------------------------------------------------------------------
	# latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
	#    -----------------
	#    | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
	#    -----------------
	#  => started at: run_timer_softirq
	#  => ended at:   run_timer_softirq
	#
	#
	#                  _------=> CPU#            
	#                 / _-----=> irqs-off        
	#                | / _----=> need-resched    
	#                || / _---=> hardirq/softirq 
	#                ||| / _--=> preempt-depth   
	#                |||| /     delay             
	#  cmd     pid   ||||| time  |   caller      
	#     \   /      |||||  \    |   /           
	  <idle>-0       0d.s2    0us+: _raw_spin_lock_irq <-run_timer_softirq
	  <idle>-0       0dNs3   17us : _raw_spin_unlock_irq <-run_timer_softirq
	  <idle>-0       0dNs3   17us+: trace_hardirqs_on <-run_timer_softirq
	  <idle>-0       0dNs3   25us : <stack trace>
	 => _raw_spin_unlock_irq
	 => run_timer_softirq
	 => __do_softirq
	 => call_softirq
	 => do_softirq
	 => irq_exit
	 => smp_apic_timer_interrupt
	 => apic_timer_interrupt
	 => rcu_idle_exit
	 => cpu_idle
	 => rest_init
	 => start_kernel
	 => x86_64_start_reservations
	 => x86_64_start_kernel
	
	Here we see that that we had a latency of 16 microseconds (which is
	very good). The _raw_spin_lock_irq in run_timer_softirq disabled
	interrupts. The difference between the 16 and the displayed
	timestamp 25us occurred because the clock was incremented
	between the time of recording the max latency and the time of
	recording the function that had that latency.
	
	Note the above example had function-trace not set. If we set
	function-trace, we get a much larger output:
	
	 with echo 1 > options/function-trace
	
	# tracer: irqsoff
	#
	# irqsoff latency trace v1.1.5 on 3.8.0-test+
	# --------------------------------------------------------------------
	# latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
	#    -----------------
	#    | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
	#    -----------------
	#  => started at: ata_scsi_queuecmd
	#  => ended at:   ata_scsi_queuecmd
	#
	#
	#                  _------=> CPU#            
	#                 / _-----=> irqs-off        
	#                | / _----=> need-resched    
	#                || / _---=> hardirq/softirq 
	#                ||| / _--=> preempt-depth   
	#                |||| /     delay             
	#  cmd     pid   ||||| time  |   caller      
	#     \   /      |||||  \    |   /           
	    bash-2042    3d...    0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
	    bash-2042    3d...    0us : add_preempt_count <-_raw_spin_lock_irqsave
	    bash-2042    3d..1    1us : ata_scsi_find_dev <-ata_scsi_queuecmd
	    bash-2042    3d..1    1us : __ata_scsi_find_dev <-ata_scsi_find_dev
	    bash-2042    3d..1    2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
	    bash-2042    3d..1    2us : ata_qc_new_init <-__ata_scsi_queuecmd
	    bash-2042    3d..1    3us : ata_sg_init <-__ata_scsi_queuecmd
	    bash-2042    3d..1    4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
	    bash-2042    3d..1    4us : ata_build_rw_tf <-ata_scsi_rw_xlat
	[...]
	    bash-2042    3d..1   67us : delay_tsc <-__delay
	    bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
	    bash-2042    3d..2   67us : sub_preempt_count <-delay_tsc
	    bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
	    bash-2042    3d..2   68us : sub_preempt_count <-delay_tsc
	    bash-2042    3d..1   68us+: ata_bmdma_start <-ata_bmdma_qc_issue
	    bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
	    bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
	    bash-2042    3d..1   72us+: trace_hardirqs_on <-ata_scsi_queuecmd
	    bash-2042    3d..1  120us : <stack trace>
	 => _raw_spin_unlock_irqrestore
	 => ata_scsi_queuecmd
	 => scsi_dispatch_cmd
	 => scsi_request_fn
	 => __blk_run_queue_uncond
	 => __blk_run_queue
	 => blk_queue_bio
	 => generic_make_request
	 => submit_bio
	 => submit_bh
	 => __ext3_get_inode_loc
	 => ext3_iget
	 => ext3_lookup
	 => lookup_real
	 => __lookup_hash
	 => walk_component
	 => lookup_last
	 => path_lookupat
	 => filename_lookup
	 => user_path_at_empty
	 => user_path_at
	 => vfs_fstatat
	 => vfs_stat
	 => sys_newstat
	 => system_call_fastpath
	
	
	Here we traced a 71 microsecond latency. But we also see all the
	functions that were called during that time. Note that by
	enabling function tracing, we incur an added overhead. This
	overhead may extend the latency times. But nevertheless, this
	trace has provided some very helpful debugging information.
	
	
	preemptoff
	----------
	
	When preemption is disabled, we may be able to receive
	interrupts but the task cannot be preempted and a higher
	priority task must wait for preemption to be enabled again
	before it can preempt a lower priority task.
	
	The preemptoff tracer traces the places that disable preemption.
	Like the irqsoff tracer, it records the maximum latency for
	which preemption was disabled. The control of preemptoff tracer
	is much like the irqsoff tracer.
	
	 # echo 0 > options/function-trace
	 # echo preemptoff > current_tracer
	 # echo 1 > tracing_on
	 # echo 0 > tracing_max_latency
	 # ls -ltr
	 [...]
	 # echo 0 > tracing_on
	 # cat trace
	# tracer: preemptoff
	#
	# preemptoff latency trace v1.1.5 on 3.8.0-test+
	# --------------------------------------------------------------------
	# latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
	#    -----------------
	#    | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
	#    -----------------
	#  => started at: do_IRQ
	#  => ended at:   do_IRQ
	#
	#
	#                  _------=> CPU#            
	#                 / _-----=> irqs-off        
	#                | / _----=> need-resched    
	#                || / _---=> hardirq/softirq 
	#                ||| / _--=> preempt-depth   
	#                |||| /     delay             
	#  cmd     pid   ||||| time  |   caller      
	#     \   /      |||||  \    |   /           
	    sshd-1991    1d.h.    0us+: irq_enter <-do_IRQ
	    sshd-1991    1d..1   46us : irq_exit <-do_IRQ
	    sshd-1991    1d..1   47us+: trace_preempt_on <-do_IRQ
	    sshd-1991    1d..1   52us : <stack trace>
	 => sub_preempt_count
	 => irq_exit
	 => do_IRQ
	 => ret_from_intr
	
	
	This has some more changes. Preemption was disabled when an
	interrupt came in (notice the 'h'), and was enabled on exit.
	But we also see that interrupts have been disabled when entering
	the preempt off section and leaving it (the 'd'). We do not know if
	interrupts were enabled in the mean time or shortly after this
	was over.
	
	# tracer: preemptoff
	#
	# preemptoff latency trace v1.1.5 on 3.8.0-test+
	# --------------------------------------------------------------------
	# latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
	#    -----------------
	#    | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
	#    -----------------
	#  => started at: wake_up_new_task
	#  => ended at:   task_rq_unlock
	#
	#
	#                  _------=> CPU#            
	#                 / _-----=> irqs-off        
	#                | / _----=> need-resched    
	#                || / _---=> hardirq/softirq 
	#                ||| / _--=> preempt-depth   
	#                |||| /     delay             
	#  cmd     pid   ||||| time  |   caller      
	#     \   /      |||||  \    |   /           
	    bash-1994    1d..1    0us : _raw_spin_lock_irqsave <-wake_up_new_task
	    bash-1994    1d..1    0us : select_task_rq_fair <-select_task_rq
	    bash-1994    1d..1    1us : __rcu_read_lock <-select_task_rq_fair
	    bash-1994    1d..1    1us : source_load <-select_task_rq_fair
	    bash-1994    1d..1    1us : source_load <-select_task_rq_fair
	[...]
	    bash-1994    1d..1   12us : irq_enter <-smp_apic_timer_interrupt
	    bash-1994    1d..1   12us : rcu_irq_enter <-irq_enter
	    bash-1994    1d..1   13us : add_preempt_count <-irq_enter
	    bash-1994    1d.h1   13us : exit_idle <-smp_apic_timer_interrupt
	    bash-1994    1d.h1   13us : hrtimer_interrupt <-smp_apic_timer_interrupt
	    bash-1994    1d.h1   13us : _raw_spin_lock <-hrtimer_interrupt
	    bash-1994    1d.h1   14us : add_preempt_count <-_raw_spin_lock
	    bash-1994    1d.h2   14us : ktime_get_update_offsets <-hrtimer_interrupt
	[...]
	    bash-1994    1d.h1   35us : lapic_next_event <-clockevents_program_event
	    bash-1994    1d.h1   35us : irq_exit <-smp_apic_timer_interrupt
	    bash-1994    1d.h1   36us : sub_preempt_count <-irq_exit
	    bash-1994    1d..2   36us : do_softirq <-irq_exit
	    bash-1994    1d..2   36us : __do_softirq <-call_softirq
	    bash-1994    1d..2   36us : __local_bh_disable <-__do_softirq
	    bash-1994    1d.s2   37us : add_preempt_count <-_raw_spin_lock_irq
	    bash-1994    1d.s3   38us : _raw_spin_unlock <-run_timer_softirq
	    bash-1994    1d.s3   39us : sub_preempt_count <-_raw_spin_unlock
	    bash-1994    1d.s2   39us : call_timer_fn <-run_timer_softirq
	[...]
	    bash-1994    1dNs2   81us : cpu_needs_another_gp <-rcu_process_callbacks
	    bash-1994    1dNs2   82us : __local_bh_enable <-__do_softirq
	    bash-1994    1dNs2   82us : sub_preempt_count <-__local_bh_enable
	    bash-1994    1dN.2   82us : idle_cpu <-irq_exit
	    bash-1994    1dN.2   83us : rcu_irq_exit <-irq_exit
	    bash-1994    1dN.2   83us : sub_preempt_count <-irq_exit
	    bash-1994    1.N.1   84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
	    bash-1994    1.N.1   84us+: trace_preempt_on <-task_rq_unlock
	    bash-1994    1.N.1  104us : <stack trace>
	 => sub_preempt_count
	 => _raw_spin_unlock_irqrestore
	 => task_rq_unlock
	 => wake_up_new_task
	 => do_fork
	 => sys_clone
	 => stub_clone
	
	
	The above is an example of the preemptoff trace with
	function-trace set. Here we see that interrupts were not disabled
	the entire time. The irq_enter code lets us know that we entered
	an interrupt 'h'. Before that, the functions being traced still
	show that it is not in an interrupt, but we can see from the
	functions themselves that this is not the case.
	
	preemptirqsoff
	--------------
	
	Knowing the locations that have interrupts disabled or
	preemption disabled for the longest times is helpful. But
	sometimes we would like to know when either preemption and/or
	interrupts are disabled.
	
	Consider the following code:
	
	    local_irq_disable();
	    call_function_with_irqs_off();
	    preempt_disable();
	    call_function_with_irqs_and_preemption_off();
	    local_irq_enable();
	    call_function_with_preemption_off();
	    preempt_enable();
	
	The irqsoff tracer will record the total length of
	call_function_with_irqs_off() and
	call_function_with_irqs_and_preemption_off().
	
	The preemptoff tracer will record the total length of
	call_function_with_irqs_and_preemption_off() and
	call_function_with_preemption_off().
	
	But neither will trace the time that interrupts and/or
	preemption is disabled. This total time is the time that we can
	not schedule. To record this time, use the preemptirqsoff
	tracer.
	
	Again, using this trace is much like the irqsoff and preemptoff
	tracers.
	
	 # echo 0 > options/function-trace
	 # echo preemptirqsoff > current_tracer
	 # echo 1 > tracing_on
	 # echo 0 > tracing_max_latency
	 # ls -ltr
	 [...]
	 # echo 0 > tracing_on
	 # cat trace
	# tracer: preemptirqsoff
	#
	# preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
	# --------------------------------------------------------------------
	# latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
	#    -----------------
	#    | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
	#    -----------------
	#  => started at: ata_scsi_queuecmd
	#  => ended at:   ata_scsi_queuecmd
	#
	#
	#                  _------=> CPU#            
	#                 / _-----=> irqs-off        
	#                | / _----=> need-resched    
	#                || / _---=> hardirq/softirq 
	#                ||| / _--=> preempt-depth   
	#                |||| /     delay             
	#  cmd     pid   ||||| time  |   caller      
	#     \   /      |||||  \    |   /           
	      ls-2230    3d...    0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
	      ls-2230    3...1  100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
	      ls-2230    3...1  101us+: trace_preempt_on <-ata_scsi_queuecmd
	      ls-2230    3...1  111us : <stack trace>
	 => sub_preempt_count
	 => _raw_spin_unlock_irqrestore
	 => ata_scsi_queuecmd
	 => scsi_dispatch_cmd
	 => scsi_request_fn
	 => __blk_run_queue_uncond
	 => __blk_run_queue
	 => blk_queue_bio
	 => generic_make_request
	 => submit_bio
	 => submit_bh
	 => ext3_bread
	 => ext3_dir_bread
	 => htree_dirblock_to_tree
	 => ext3_htree_fill_tree
	 => ext3_readdir
	 => vfs_readdir
	 => sys_getdents
	 => system_call_fastpath
	
	
	The trace_hardirqs_off_thunk is called from assembly on x86 when
	interrupts are disabled in the assembly code. Without the
	function tracing, we do not know if interrupts were enabled
	within the preemption points. We do see that it started with
	preemption enabled.
	
	Here is a trace with function-trace set:
	
	# tracer: preemptirqsoff
	#
	# preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
	# --------------------------------------------------------------------
	# latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
	#    -----------------
	#    | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
	#    -----------------
	#  => started at: schedule
	#  => ended at:   mutex_unlock
	#
	#
	#                  _------=> CPU#            
	#                 / _-----=> irqs-off        
	#                | / _----=> need-resched    
	#                || / _---=> hardirq/softirq 
	#                ||| / _--=> preempt-depth   
	#                |||| /     delay             
	#  cmd     pid   ||||| time  |   caller      
	#     \   /      |||||  \    |   /           
	kworker/-59      3...1    0us : __schedule <-schedule
	kworker/-59      3d..1    0us : rcu_preempt_qs <-rcu_note_context_switch
	kworker/-59      3d..1    1us : add_preempt_count <-_raw_spin_lock_irq
	kworker/-59      3d..2    1us : deactivate_task <-__schedule
	kworker/-59      3d..2    1us : dequeue_task <-deactivate_task
	kworker/-59      3d..2    2us : update_rq_clock <-dequeue_task
	kworker/-59      3d..2    2us : dequeue_task_fair <-dequeue_task
	kworker/-59      3d..2    2us : update_curr <-dequeue_task_fair
	kworker/-59      3d..2    2us : update_min_vruntime <-update_curr
	kworker/-59      3d..2    3us : cpuacct_charge <-update_curr
	kworker/-59      3d..2    3us : __rcu_read_lock <-cpuacct_charge
	kworker/-59      3d..2    3us : __rcu_read_unlock <-cpuacct_charge
	kworker/-59      3d..2    3us : update_cfs_rq_blocked_load <-dequeue_task_fair
	kworker/-59      3d..2    4us : clear_buddies <-dequeue_task_fair
	kworker/-59      3d..2    4us : account_entity_dequeue <-dequeue_task_fair
	kworker/-59      3d..2    4us : update_min_vruntime <-dequeue_task_fair
	kworker/-59      3d..2    4us : update_cfs_shares <-dequeue_task_fair
	kworker/-59      3d..2    5us : hrtick_update <-dequeue_task_fair
	kworker/-59      3d..2    5us : wq_worker_sleeping <-__schedule
	kworker/-59      3d..2    5us : kthread_data <-wq_worker_sleeping
	kworker/-59      3d..2    5us : put_prev_task_fair <-__schedule
	kworker/-59      3d..2    6us : pick_next_task_fair <-pick_next_task
	kworker/-59      3d..2    6us : clear_buddies <-pick_next_task_fair
	kworker/-59      3d..2    6us : set_next_entity <-pick_next_task_fair
	kworker/-59      3d..2    6us : update_stats_wait_end <-set_next_entity
	      ls-2269    3d..2    7us : finish_task_switch <-__schedule
	      ls-2269    3d..2    7us : _raw_spin_unlock_irq <-finish_task_switch
	      ls-2269    3d..2    8us : do_IRQ <-ret_from_intr
	      ls-2269    3d..2    8us : irq_enter <-do_IRQ
	      ls-2269    3d..2    8us : rcu_irq_enter <-irq_enter
	      ls-2269    3d..2    9us : add_preempt_count <-irq_enter
	      ls-2269    3d.h2    9us : exit_idle <-do_IRQ
	[...]
	      ls-2269    3d.h3   20us : sub_preempt_count <-_raw_spin_unlock
	      ls-2269    3d.h2   20us : irq_exit <-do_IRQ
	      ls-2269    3d.h2   21us : sub_preempt_count <-irq_exit
	      ls-2269    3d..3   21us : do_softirq <-irq_exit
	      ls-2269    3d..3   21us : __do_softirq <-call_softirq
	      ls-2269    3d..3   21us+: __local_bh_disable <-__do_softirq
	      ls-2269    3d.s4   29us : sub_preempt_count <-_local_bh_enable_ip
	      ls-2269    3d.s5   29us : sub_preempt_count <-_local_bh_enable_ip
	      ls-2269    3d.s5   31us : do_IRQ <-ret_from_intr
	      ls-2269    3d.s5   31us : irq_enter <-do_IRQ
	      ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
	[...]
	      ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
	      ls-2269    3d.s5   32us : add_preempt_count <-irq_enter
	      ls-2269    3d.H5   32us : exit_idle <-do_IRQ
	      ls-2269    3d.H5   32us : handle_irq <-do_IRQ
	      ls-2269    3d.H5   32us : irq_to_desc <-handle_irq
	      ls-2269    3d.H5   33us : handle_fasteoi_irq <-handle_irq
	[...]
	      ls-2269    3d.s5  158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
	      ls-2269    3d.s3  158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
	      ls-2269    3d.s3  159us : __local_bh_enable <-__do_softirq
	      ls-2269    3d.s3  159us : sub_preempt_count <-__local_bh_enable
	      ls-2269    3d..3  159us : idle_cpu <-irq_exit
	      ls-2269    3d..3  159us : rcu_irq_exit <-irq_exit
	      ls-2269    3d..3  160us : sub_preempt_count <-irq_exit
	      ls-2269    3d...  161us : __mutex_unlock_slowpath <-mutex_unlock
	      ls-2269    3d...  162us+: trace_hardirqs_on <-mutex_unlock
	      ls-2269    3d...  186us : <stack trace>
	 => __mutex_unlock_slowpath
	 => mutex_unlock
	 => process_output
	 => n_tty_write
	 => tty_write
	 => vfs_write
	 => sys_write
	 => system_call_fastpath
	
	This is an interesting trace. It started with kworker running and
	scheduling out and ls taking over. But as soon as ls released the
	rq lock and enabled interrupts (but not preemption) an interrupt
	triggered. When the interrupt finished, it started running softirqs.
	But while the softirq was running, another interrupt triggered.
	When an interrupt is running inside a softirq, the annotation is 'H'.
	
	
	wakeup
	------
	
	One common case that people are interested in tracing is the
	time it takes for a task that is woken to actually wake up.
	Now for non Real-Time tasks, this can be arbitrary. But tracing
	it none the less can be interesting. 
	
	Without function tracing:
	
	 # echo 0 > options/function-trace
	 # echo wakeup > current_tracer
	 # echo 1 > tracing_on
	 # echo 0 > tracing_max_latency
	 # chrt -f 5 sleep 1
	 # echo 0 > tracing_on
	 # cat trace
	# tracer: wakeup
	#
	# wakeup latency trace v1.1.5 on 3.8.0-test+
	# --------------------------------------------------------------------
	# latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
	#    -----------------
	#    | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
	#    -----------------
	#
	#                  _------=> CPU#            
	#                 / _-----=> irqs-off        
	#                | / _----=> need-resched    
	#                || / _---=> hardirq/softirq 
	#                ||| / _--=> preempt-depth   
	#                |||| /     delay             
	#  cmd     pid   ||||| time  |   caller      
	#     \   /      |||||  \    |   /           
	  <idle>-0       3dNs7    0us :      0:120:R   + [003]   312:100:R kworker/3:1H
	  <idle>-0       3dNs7    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
	  <idle>-0       3d..3   15us : __schedule <-schedule
	  <idle>-0       3d..3   15us :      0:120:R ==> [003]   312:100:R kworker/3:1H
	
	The tracer only traces the highest priority task in the system
	to avoid tracing the normal circumstances. Here we see that
	the kworker with a nice priority of -20 (not very nice), took
	just 15 microseconds from the time it woke up, to the time it
	ran.
	
	Non Real-Time tasks are not that interesting. A more interesting
	trace is to concentrate only on Real-Time tasks.
	
	wakeup_rt
	---------
	
	In a Real-Time environment it is very important to know the
	wakeup time it takes for the highest priority task that is woken
	up to the time that it executes. This is also known as "schedule
	latency". I stress the point that this is about RT tasks. It is
	also important to know the scheduling latency of non-RT tasks,
	but the average schedule latency is better for non-RT tasks.
	Tools like LatencyTop are more appropriate for such
	measurements.
	
	Real-Time environments are interested in the worst case latency.
	That is the longest latency it takes for something to happen,
	and not the average. We can have a very fast scheduler that may
	only have a large latency once in a while, but that would not
	work well with Real-Time tasks.  The wakeup_rt tracer was designed
	to record the worst case wakeups of RT tasks. Non-RT tasks are
	not recorded because the tracer only records one worst case and
	tracing non-RT tasks that are unpredictable will overwrite the
	worst case latency of RT tasks (just run the normal wakeup
	tracer for a while to see that effect).
	
	Since this tracer only deals with RT tasks, we will run this
	slightly differently than we did with the previous tracers.
	Instead of performing an 'ls', we will run 'sleep 1' under
	'chrt' which changes the priority of the task.
	
	 # echo 0 > options/function-trace
	 # echo wakeup_rt > current_tracer
	 # echo 1 > tracing_on
	 # echo 0 > tracing_max_latency
	 # chrt -f 5 sleep 1
	 # echo 0 > tracing_on
	 # cat trace
	# tracer: wakeup
	#
	# tracer: wakeup_rt
	#
	# wakeup_rt latency trace v1.1.5 on 3.8.0-test+
	# --------------------------------------------------------------------
	# latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
	#    -----------------
	#    | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
	#    -----------------
	#
	#                  _------=> CPU#            
	#                 / _-----=> irqs-off        
	#                | / _----=> need-resched    
	#                || / _---=> hardirq/softirq 
	#                ||| / _--=> preempt-depth   
	#                |||| /     delay             
	#  cmd     pid   ||||| time  |   caller      
	#     \   /      |||||  \    |   /           
	  <idle>-0       3d.h4    0us :      0:120:R   + [003]  2389: 94:R sleep
	  <idle>-0       3d.h4    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
	  <idle>-0       3d..3    5us : __schedule <-schedule
	  <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
	
	
	Running this on an idle system, we see that it only took 5 microseconds
	to perform the task switch.  Note, since the trace point in the schedule
	is before the actual "switch", we stop the tracing when the recorded task
	is about to schedule in. This may change if we add a new marker at the
	end of the scheduler.
	
	Notice that the recorded task is 'sleep' with the PID of 2389
	and it has an rt_prio of 5. This priority is user-space priority
	and not the internal kernel priority. The policy is 1 for
	SCHED_FIFO and 2 for SCHED_RR.
	
	Note, that the trace data shows the internal priority (99 - rtprio).
	
	  <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
	
	The 0:120:R means idle was running with a nice priority of 0 (120 - 120)
	and in the running state 'R'. The sleep task was scheduled in with
	2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
	and it too is in the running state.
	
	Doing the same with chrt -r 5 and function-trace set.
	
	  echo 1 > options/function-trace
	
	# tracer: wakeup_rt
	#
	# wakeup_rt latency trace v1.1.5 on 3.8.0-test+
	# --------------------------------------------------------------------
	# latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
	#    -----------------
	#    | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
	#    -----------------
	#
	#                  _------=> CPU#            
	#                 / _-----=> irqs-off        
	#                | / _----=> need-resched    
	#                || / _---=> hardirq/softirq 
	#                ||| / _--=> preempt-depth   
	#                |||| /     delay             
	#  cmd     pid   ||||| time  |   caller      
	#     \   /      |||||  \    |   /           
	  <idle>-0       3d.h4    1us+:      0:120:R   + [003]  2448: 94:R sleep
	  <idle>-0       3d.h4    2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
	  <idle>-0       3d.h3    3us : check_preempt_curr <-ttwu_do_wakeup
	  <idle>-0       3d.h3    3us : resched_curr <-check_preempt_curr
	  <idle>-0       3dNh3    4us : task_woken_rt <-ttwu_do_wakeup
	  <idle>-0       3dNh3    4us : _raw_spin_unlock <-try_to_wake_up
	  <idle>-0       3dNh3    4us : sub_preempt_count <-_raw_spin_unlock
	  <idle>-0       3dNh2    5us : ttwu_stat <-try_to_wake_up
	  <idle>-0       3dNh2    5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
	  <idle>-0       3dNh2    6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
	  <idle>-0       3dNh1    6us : _raw_spin_lock <-__run_hrtimer
	  <idle>-0       3dNh1    6us : add_preempt_count <-_raw_spin_lock
	  <idle>-0       3dNh2    7us : _raw_spin_unlock <-hrtimer_interrupt
	  <idle>-0       3dNh2    7us : sub_preempt_count <-_raw_spin_unlock
	  <idle>-0       3dNh1    7us : tick_program_event <-hrtimer_interrupt
	  <idle>-0       3dNh1    7us : clockevents_program_event <-tick_program_event
	  <idle>-0       3dNh1    8us : ktime_get <-clockevents_program_event
	  <idle>-0       3dNh1    8us : lapic_next_event <-clockevents_program_event
	  <idle>-0       3dNh1    8us : irq_exit <-smp_apic_timer_interrupt
	  <idle>-0       3dNh1    9us : sub_preempt_count <-irq_exit
	  <idle>-0       3dN.2    9us : idle_cpu <-irq_exit
	  <idle>-0       3dN.2    9us : rcu_irq_exit <-irq_exit
	  <idle>-0       3dN.2   10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
	  <idle>-0       3dN.2   10us : sub_preempt_count <-irq_exit
	  <idle>-0       3.N.1   11us : rcu_idle_exit <-cpu_idle
	  <idle>-0       3dN.1   11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
	  <idle>-0       3.N.1   11us : tick_nohz_idle_exit <-cpu_idle
	  <idle>-0       3dN.1   12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
	  <idle>-0       3dN.1   12us : ktime_get <-tick_nohz_idle_exit
	  <idle>-0       3dN.1   12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
	  <idle>-0       3dN.1   13us : cpu_load_update_nohz <-tick_nohz_idle_exit
	  <idle>-0       3dN.1   13us : _raw_spin_lock <-cpu_load_update_nohz
	  <idle>-0       3dN.1   13us : add_preempt_count <-_raw_spin_lock
	  <idle>-0       3dN.2   13us : __cpu_load_update <-cpu_load_update_nohz
	  <idle>-0       3dN.2   14us : sched_avg_update <-__cpu_load_update
	  <idle>-0       3dN.2   14us : _raw_spin_unlock <-cpu_load_update_nohz
	  <idle>-0       3dN.2   14us : sub_preempt_count <-_raw_spin_unlock
	  <idle>-0       3dN.1   15us : calc_load_nohz_stop <-tick_nohz_idle_exit
	  <idle>-0       3dN.1   15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
	  <idle>-0       3dN.1   15us : hrtimer_cancel <-tick_nohz_idle_exit
	  <idle>-0       3dN.1   15us : hrtimer_try_to_cancel <-hrtimer_cancel
	  <idle>-0       3dN.1   16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
	  <idle>-0       3dN.1   16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
	  <idle>-0       3dN.1   16us : add_preempt_count <-_raw_spin_lock_irqsave
	  <idle>-0       3dN.2   17us : __remove_hrtimer <-remove_hrtimer.part.16
	  <idle>-0       3dN.2   17us : hrtimer_force_reprogram <-__remove_hrtimer
	  <idle>-0       3dN.2   17us : tick_program_event <-hrtimer_force_reprogram
	  <idle>-0       3dN.2   18us : clockevents_program_event <-tick_program_event
	  <idle>-0       3dN.2   18us : ktime_get <-clockevents_program_event
	  <idle>-0       3dN.2   18us : lapic_next_event <-clockevents_program_event
	  <idle>-0       3dN.2   19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
	  <idle>-0       3dN.2   19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
	  <idle>-0       3dN.1   19us : hrtimer_forward <-tick_nohz_idle_exit
	  <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
	  <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
	  <idle>-0       3dN.1   20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
	  <idle>-0       3dN.1   20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
	  <idle>-0       3dN.1   21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
	  <idle>-0       3dN.1   21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
	  <idle>-0       3dN.1   21us : add_preempt_count <-_raw_spin_lock_irqsave
	  <idle>-0       3dN.2   22us : ktime_add_safe <-__hrtimer_start_range_ns
	  <idle>-0       3dN.2   22us : enqueue_hrtimer <-__hrtimer_start_range_ns
	  <idle>-0       3dN.2   22us : tick_program_event <-__hrtimer_start_range_ns
	  <idle>-0       3dN.2   23us : clockevents_program_event <-tick_program_event
	  <idle>-0       3dN.2   23us : ktime_get <-clockevents_program_event
	  <idle>-0       3dN.2   23us : lapic_next_event <-clockevents_program_event
	  <idle>-0       3dN.2   24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
	  <idle>-0       3dN.2   24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
	  <idle>-0       3dN.1   24us : account_idle_ticks <-tick_nohz_idle_exit
	  <idle>-0       3dN.1   24us : account_idle_time <-account_idle_ticks
	  <idle>-0       3.N.1   25us : sub_preempt_count <-cpu_idle
	  <idle>-0       3.N..   25us : schedule <-cpu_idle
	  <idle>-0       3.N..   25us : __schedule <-preempt_schedule
	  <idle>-0       3.N..   26us : add_preempt_count <-__schedule
	  <idle>-0       3.N.1   26us : rcu_note_context_switch <-__schedule
	  <idle>-0       3.N.1   26us : rcu_sched_qs <-rcu_note_context_switch
	  <idle>-0       3dN.1   27us : rcu_preempt_qs <-rcu_note_context_switch
	  <idle>-0       3.N.1   27us : _raw_spin_lock_irq <-__schedule
	  <idle>-0       3dN.1   27us : add_preempt_count <-_raw_spin_lock_irq
	  <idle>-0       3dN.2   28us : put_prev_task_idle <-__schedule
	  <idle>-0       3dN.2   28us : pick_next_task_stop <-pick_next_task
	  <idle>-0       3dN.2   28us : pick_next_task_rt <-pick_next_task
	  <idle>-0       3dN.2   29us : dequeue_pushable_task <-pick_next_task_rt
	  <idle>-0       3d..3   29us : __schedule <-preempt_schedule
	  <idle>-0       3d..3   30us :      0:120:R ==> [003]  2448: 94:R sleep
	
	This isn't that big of a trace, even with function tracing enabled,
	so I included the entire trace.
	
	The interrupt went off while when the system was idle. Somewhere
	before task_woken_rt() was called, the NEED_RESCHED flag was set,
	this is indicated by the first occurrence of the 'N' flag.
	
	Latency tracing and events
	--------------------------
	As function tracing can induce a much larger latency, but without
	seeing what happens within the latency it is hard to know what
	caused it. There is a middle ground, and that is with enabling
	events.
	
	 # echo 0 > options/function-trace
	 # echo wakeup_rt > current_tracer
	 # echo 1 > events/enable
	 # echo 1 > tracing_on
	 # echo 0 > tracing_max_latency
	 # chrt -f 5 sleep 1
	 # echo 0 > tracing_on
	 # cat trace
	# tracer: wakeup_rt
	#
	# wakeup_rt latency trace v1.1.5 on 3.8.0-test+
	# --------------------------------------------------------------------
	# latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
	#    -----------------
	#    | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
	#    -----------------
	#
	#                  _------=> CPU#            
	#                 / _-----=> irqs-off        
	#                | / _----=> need-resched    
	#                || / _---=> hardirq/softirq 
	#                ||| / _--=> preempt-depth   
	#                |||| /     delay             
	#  cmd     pid   ||||| time  |   caller      
	#     \   /      |||||  \    |   /           
	  <idle>-0       2d.h4    0us :      0:120:R   + [002]  5882: 94:R sleep
	  <idle>-0       2d.h4    0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
	  <idle>-0       2d.h4    1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
	  <idle>-0       2dNh2    1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
	  <idle>-0       2.N.2    2us : power_end: cpu_id=2
	  <idle>-0       2.N.2    3us : cpu_idle: state=4294967295 cpu_id=2
	  <idle>-0       2dN.3    4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
	  <idle>-0       2dN.3    4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
	  <idle>-0       2.N.2    5us : rcu_utilization: Start context switch
	  <idle>-0       2.N.2    5us : rcu_utilization: End context switch
	  <idle>-0       2d..3    6us : __schedule <-schedule
	  <idle>-0       2d..3    6us :      0:120:R ==> [002]  5882: 94:R sleep
	
	
	Hardware Latency Detector
	-------------------------
	
	The hardware latency detector is executed by enabling the "hwlat" tracer.
	
	NOTE, this tracer will affect the performance of the system as it will
	periodically make a CPU constantly busy with interrupts disabled.
	
	 # echo hwlat > current_tracer
	 # sleep 100
	 # cat trace
	# tracer: hwlat
	#
	#                              _-----=> irqs-off
	#                             / _----=> need-resched
	#                            | / _---=> hardirq/softirq
	#                            || / _--=> preempt-depth
	#                            ||| /     delay
	#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
	#              | |       |   ||||       |         |
	           <...>-3638  [001] d... 19452.055471: #1     inner/outer(us):   12/14    ts:1499801089.066141940
	           <...>-3638  [003] d... 19454.071354: #2     inner/outer(us):   11/9     ts:1499801091.082164365
	           <...>-3638  [002] dn.. 19461.126852: #3     inner/outer(us):   12/9     ts:1499801098.138150062
	           <...>-3638  [001] d... 19488.340960: #4     inner/outer(us):    8/12    ts:1499801125.354139633
	           <...>-3638  [003] d... 19494.388553: #5     inner/outer(us):    8/12    ts:1499801131.402150961
	           <...>-3638  [003] d... 19501.283419: #6     inner/outer(us):    0/12    ts:1499801138.297435289 nmi-total:4 nmi-count:1
	
	
	The above output is somewhat the same in the header. All events will have
	interrupts disabled 'd'. Under the FUNCTION title there is:
	
	 #1 - This is the count of events recorded that were greater than the
	      tracing_threshold (See below).
	
	 inner/outer(us):   12/14
	
	      This shows two numbers as "inner latency" and "outer latency". The test
	      runs in a loop checking a timestamp twice. The latency detected within
	      the two timestamps is the "inner latency" and the latency detected
	      after the previous timestamp and the next timestamp in the loop is
	      the "outer latency".
	
	 ts:1499801089.066141940
	
	      The absolute timestamp that the event happened.
	
	 nmi-total:4 nmi-count:1
	
	      On architectures that support it, if an NMI comes in during the
	      test, the time spent in NMI is reported in "nmi-total" (in
	      microseconds).
	
	      All architectures that have NMIs will show the "nmi-count" if an
	      NMI comes in during the test.
	
	hwlat files:
	
	  tracing_threshold - This gets automatically set to "10" to represent 10
			      microseconds. This is the threshold of latency that
			      needs to be detected before the trace will be recorded.
	
			      Note, when hwlat tracer is finished (another tracer is
			      written into "current_tracer"), the original value for
			      tracing_threshold is placed back into this file.
	
	  hwlat_detector/width - The length of time the test runs with interrupts
				 disabled.
	
	  hwlat_detector/window - The length of time of the window which the test
				  runs. That is, the test will run for "width"
				  microseconds per "window" microseconds
	
	  tracing_cpumask - When the test is started. A kernel thread is created that
			    runs the test. This thread will alternate between CPUs
			    listed in the tracing_cpumask between each period
			    (one "window"). To limit the test to specific CPUs
			    set the mask in this file to only the CPUs that the test
			    should run on.
	
	function
	--------
	
	This tracer is the function tracer. Enabling the function tracer
	can be done from the debug file system. Make sure the
	ftrace_enabled is set; otherwise this tracer is a nop.
	See the "ftrace_enabled" section below.
	
	 # sysctl kernel.ftrace_enabled=1
	 # echo function > current_tracer
	 # echo 1 > tracing_on
	 # usleep 1
	 # echo 0 > tracing_on
	 # cat trace
	# tracer: function
	#
	# entries-in-buffer/entries-written: 24799/24799   #P:4
	#
	#                              _-----=> irqs-off
	#                             / _----=> need-resched
	#                            | / _---=> hardirq/softirq
	#                            || / _--=> preempt-depth
	#                            ||| /     delay
	#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
	#              | |       |   ||||       |         |
	            bash-1994  [002] ....  3082.063030: mutex_unlock <-rb_simple_write
	            bash-1994  [002] ....  3082.063031: __mutex_unlock_slowpath <-mutex_unlock
	            bash-1994  [002] ....  3082.063031: __fsnotify_parent <-fsnotify_modify
	            bash-1994  [002] ....  3082.063032: fsnotify <-fsnotify_modify
	            bash-1994  [002] ....  3082.063032: __srcu_read_lock <-fsnotify
	            bash-1994  [002] ....  3082.063032: add_preempt_count <-__srcu_read_lock
	            bash-1994  [002] ...1  3082.063032: sub_preempt_count <-__srcu_read_lock
	            bash-1994  [002] ....  3082.063033: __srcu_read_unlock <-fsnotify
	[...]
	
	
	Note: function tracer uses ring buffers to store the above
	entries. The newest data may overwrite the oldest data.
	Sometimes using echo to stop the trace is not sufficient because
	the tracing could have overwritten the data that you wanted to
	record. For this reason, it is sometimes better to disable
	tracing directly from a program. This allows you to stop the
	tracing at the point that you hit the part that you are
	interested in. To disable the tracing directly from a C program,
	something like following code snippet can be used:
	
	int trace_fd;
	[...]
	int main(int argc, char *argv[]) {
		[...]
		trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
		[...]
		if (condition_hit()) {
			write(trace_fd, "0", 1);
		}
		[...]
	}
	
	
	Single thread tracing
	---------------------
	
	By writing into set_ftrace_pid you can trace a
	single thread. For example:
	
	# cat set_ftrace_pid
	no pid
	# echo 3111 > set_ftrace_pid
	# cat set_ftrace_pid
	3111
	# echo function > current_tracer
	# cat trace | head
	 # tracer: function
	 #
	 #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
	 #              | |       |          |         |
	     yum-updatesd-3111  [003]  1637.254676: finish_task_switch <-thread_return
	     yum-updatesd-3111  [003]  1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
	     yum-updatesd-3111  [003]  1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
	     yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
	     yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
	     yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll
	# echo > set_ftrace_pid
	# cat trace |head
	 # tracer: function
	 #
	 #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
	 #              | |       |          |         |
	 ##### CPU 3 buffer started ####
	     yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
	     yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
	     yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
	     yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
	     yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit
	
	If you want to trace a function when executing, you could use
	something like this simple program:
	
	#include <stdio.h>
	#include <stdlib.h>
	#include <sys/types.h>
	#include <sys/stat.h>
	#include <fcntl.h>
	#include <unistd.h>
	#include <string.h>
	
	#define _STR(x) #x
	#define STR(x) _STR(x)
	#define MAX_PATH 256
	
	const char *find_tracefs(void)
	{
	       static char tracefs[MAX_PATH+1];
	       static int tracefs_found;
	       char type[100];
	       FILE *fp;
	
	       if (tracefs_found)
	               return tracefs;
	
	       if ((fp = fopen("/proc/mounts","r")) == NULL) {
	               perror("/proc/mounts");
	               return NULL;
	       }
	
	       while (fscanf(fp, "%*s %"
	                     STR(MAX_PATH)
	                     "s %99s %*s %*d %*d\n",
	                     tracefs, type) == 2) {
	               if (strcmp(type, "tracefs") == 0)
	                       break;
	       }
	       fclose(fp);
	
	       if (strcmp(type, "tracefs") != 0) {
	               fprintf(stderr, "tracefs not mounted");
	               return NULL;
	       }
	
	       strcat(tracefs, "/tracing/");
	       tracefs_found = 1;
	
	       return tracefs;
	}
	
	const char *tracing_file(const char *file_name)
	{
	       static char trace_file[MAX_PATH+1];
	       snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name);
	       return trace_file;
	}
	
	int main (int argc, char **argv)
	{
	        if (argc < 1)
	                exit(-1);
	
	        if (fork() > 0) {
	                int fd, ffd;
	                char line[64];
	                int s;
	
	                ffd = open(tracing_file("current_tracer"), O_WRONLY);
	                if (ffd < 0)
	                        exit(-1);
	                write(ffd, "nop", 3);
	
	                fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
	                s = sprintf(line, "%d\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
	
	tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts`
	echo nop > $tracefs/tracing/current_tracer
	echo 0 > $tracefs/tracing/tracing_on
	echo $$ > $tracefs/tracing/set_ftrace_pid
	echo function > $tracefs/tracing/current_tracer
	echo 1 > $tracefs/tracing/tracing_on
	exec "$@"
	------
	
	
	function graph tracer
	---------------------------
	
	This tracer is similar to the function tracer except that it
	probes a function on its entry and its exit. This is done by
	using a dynamically allocated stack of return addresses in each
	task_struct. On function entry the tracer overwrites the return
	address of each function traced to set a custom probe. Thus the
	original return address is stored on the stack of return address
	in the task_struct.
	
	Probing on both ends of a function leads to special features
	such as:
	
	- measure of a function's time execution
	- having a reliable call stack to draw function calls graph
	
	This tracer is useful in several situations:
	
	- you want to find the reason of a strange kernel behavior and
	  need to see what happens in detail on any areas (or specific
	  ones).
	
	- you are experiencing weird latencies but it's difficult to
	  find its origin.
	
	- you want to find quickly which path is taken by a specific
	  function
	
	- you just want to peek inside a working kernel and want to see
	  what happens there.
	
	# tracer: function_graph
	#
	# CPU  DURATION                  FUNCTION CALLS
	# |     |   |                     |   |   |   |
	
	 0)               |  sys_open() {
	 0)               |    do_sys_open() {
	 0)               |      getname() {
	 0)               |        kmem_cache_alloc() {
	 0)   1.382 us    |          __might_sleep();
	 0)   2.478 us    |        }
	 0)               |        strncpy_from_user() {
	 0)               |          might_fault() {
	 0)   1.389 us    |            __might_sleep();
	 0)   2.553 us    |          }
	 0)   3.807 us    |        }
	 0)   7.876 us    |      }
	 0)               |      alloc_fd() {
	 0)   0.668 us    |        _spin_lock();
	 0)   0.570 us    |        expand_files();
	 0)   0.586 us    |        _spin_unlock();
	
	
	There are several columns that can be dynamically
	enabled/disabled. You can use every combination of options you
	want, depending on your needs.
	
	- The cpu number on which the function executed is default
	  enabled.  It is sometimes better to only trace one cpu (see
	  tracing_cpu_mask file) or you might sometimes see unordered
	  function calls while cpu tracing switch.
	
		hide: echo nofuncgraph-cpu > trace_options
		show: echo funcgraph-cpu > trace_options
	
	- The duration (function's time of execution) is displayed on
	  the closing bracket line of a function or on the same line
	  than the current function in case of a leaf one. It is default
	  enabled.
	
		hide: echo nofuncgraph-duration > trace_options
		show: echo funcgraph-duration > trace_options
	
	- The overhead field precedes the duration field in case of
	  reached duration thresholds.
	
		hide: echo nofuncgraph-overhead > trace_options
		show: echo funcgraph-overhead > trace_options
		depends on: funcgraph-duration
	
	  ie:
	
	  3) # 1837.709 us |          } /* __switch_to */
	  3)               |          finish_task_switch() {
	  3)   0.313 us    |            _raw_spin_unlock_irq();
	  3)   3.177 us    |          }
	  3) # 1889.063 us |        } /* __schedule */
	  3) ! 140.417 us  |      } /* __schedule */
	  3) # 2034.948 us |    } /* schedule */
	  3) * 33998.59 us |  } /* schedule_preempt_disabled */
	
	  [...]
	
	  1)   0.260 us    |              msecs_to_jiffies();
	  1)   0.313 us    |              __rcu_read_unlock();
	  1) + 61.770 us   |            }
	  1) + 64.479 us   |          }
	  1)   0.313 us    |          rcu_bh_qs();
	  1)   0.313 us    |          __local_bh_enable();
	  1) ! 217.240 us  |        }
	  1)   0.365 us    |        idle_cpu();
	  1)               |        rcu_irq_exit() {
	  1)   0.417 us    |          rcu_eqs_enter_common.isra.47();
	  1)   3.125 us    |        }
	  1) ! 227.812 us  |      }
	  1) ! 457.395 us  |    }
	  1) @ 119760.2 us |  }
	
	  [...]
	
	  2)               |    handle_IPI() {
	  1)   6.979 us    |                  }
	  2)   0.417 us    |      scheduler_ipi();
	  1)   9.791 us    |                }
	  1) + 12.917 us   |              }
	  2)   3.490 us    |    }
	  1) + 15.729 us   |            }
	  1) + 18.542 us   |          }
	  2) $ 3594274 us  |  }
	
	  + means that the function exceeded 10 usecs.
	  ! means that the function exceeded 100 usecs.
	  # means that the function exceeded 1000 usecs.
	  * means that the function exceeded 10 msecs.
	  @ means that the function exceeded 100 msecs.
	  $ means that the function exceeded 1 sec.
	
	
	- The task/pid field displays the thread cmdline and pid which
	  executed the function. It is default disabled.
	
		hide: echo nofuncgraph-proc > trace_options
		show: echo funcgraph-proc > trace_options
	
	  ie:
	
	  # tracer: function_graph
	  #
	  # CPU  TASK/PID        DURATION                  FUNCTION CALLS
	  # |    |    |           |   |                     |   |   |   |
	  0)    sh-4802     |               |                  d_free() {
	  0)    sh-4802     |               |                    call_rcu() {
	  0)    sh-4802     |               |                      __call_rcu() {
	  0)    sh-4802     |   0.616 us    |                        rcu_process_gp_end();
	  0)    sh-4802     |   0.586 us    |                        check_for_new_grace_period();
	  0)    sh-4802     |   2.899 us    |                      }
	  0)    sh-4802     |   4.040 us    |                    }
	  0)    sh-4802     |   5.151 us    |                  }
	  0)    sh-4802     | + 49.370 us   |                }
	
	
	- The absolute time field is an absolute timestamp given by the
	  system clock since it started. A snapshot of this time is
	  given on each entry/exit of functions
	
		hide: echo nofuncgraph-abstime > trace_options
		show: echo funcgraph-abstime > trace_options
	
	  ie:
	
	  #
	  #      TIME       CPU  DURATION                  FUNCTION CALLS
	  #       |         |     |   |                     |   |   |   |
	  360.774522 |   1)   0.541 us    |                                          }
	  360.774522 |   1)   4.663 us    |                                        }
	  360.774523 |   1)   0.541 us    |                                        __wake_up_bit();
	  360.774524 |   1)   6.796 us    |                                      }
	  360.774524 |   1)   7.952 us    |                                    }
	  360.774525 |   1)   9.063 us    |                                  }
	  360.774525 |   1)   0.615 us    |                                  journal_mark_dirty();
	  360.774527 |   1)   0.578 us    |                                  __brelse();
	  360.774528 |   1)               |                                  reiserfs_prepare_for_journal() {
	  360.774528 |   1)               |                                    unlock_buffer() {
	  360.774529 |   1)               |                                      wake_up_bit() {
	  360.774529 |   1)               |                                        bit_waitqueue() {
	  360.774530 |   1)   0.594 us    |                                          __phys_addr();
	
	
	The function name is always displayed after the closing bracket
	for a function if the start of that function is not in the
	trace buffer.
	
	Display of the function name after the closing bracket may be
	enabled for functions whose start is in the trace buffer,
	allowing easier searching with grep for function durations.
	It is default disabled.
	
		hide: echo nofuncgraph-tail > trace_options
		show: echo funcgraph-tail > trace_options
	
	  Example with nofuncgraph-tail (default):
	  0)               |      putname() {
	  0)               |        kmem_cache_free() {
	  0)   0.518 us    |          __phys_addr();
	  0)   1.757 us    |        }
	  0)   2.861 us    |      }
	
	  Example with funcgraph-tail:
	  0)               |      putname() {
	  0)               |        kmem_cache_free() {
	  0)   0.518 us    |          __phys_addr();
	  0)   1.757 us    |        } /* kmem_cache_free() */
	  0)   2.861 us    |      } /* putname() */
	
	You can put some comments on specific functions by using
	trace_printk() For example, if you want to put a comment inside
	the __might_sleep() function, you just have to include
	<linux/ftrace.h> and call trace_printk() inside __might_sleep()
	
	trace_printk("I'm a comment!\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. Starting
	with gcc verson 4.6, the -mfentry has been added for x86, which
	calls "__fentry__" instead of "mcount". Which is called before
	the creation of the stack frame.
	
	Note, not all sections are traced. They may be prevented by either
	a notrace, or blocked another way and all inline functions are not
	traced. Check the "available_filter_functions" file to see what functions
	can be traced.
	
	A section called "__mcount_loc" is created that holds
	references to all the mcount/fentry call sites in the .text section.
	The recordmcount program re-links this section back into the
	original object. The final linking stage of the kernel will add all these
	references into a single table.
	
	On boot up, before SMP is initialized, the dynamic ftrace code
	scans this table and updates all the locations into nops. It
	also records the locations, which are added to the
	available_filter_functions list.  Modules are processed as they
	are loaded and before they are executed.  When a module is
	unloaded, it also removes its functions from the ftrace function
	list. This is automatic in the module unload code, and the
	module author does not need to worry about it.
	
	When tracing is enabled, the process of modifying the function
	tracepoints is dependent on architecture. The old method is to use
	kstop_machine to prevent races with the CPUs executing code being
	modified (which can cause the CPU to do undesirable things, especially
	if the modified code crosses cache (or page) boundaries), and the nops are
	patched back to calls. But this time, they do not call mcount
	(which is just a function stub). They now call into the ftrace
	infrastructure.
	
	The new method of modifying the function tracepoints is to place
	a breakpoint at the location to be modified, sync all CPUs, modify
	the rest of the instruction not covered by the breakpoint. Sync
	all CPUs again, and then remove the breakpoint with the finished
	version to the ftrace call site.
	
	Some archs do not even need to monkey around with the synchronization,
	and can just slap the new code on top of the old without any
	problems with other CPUs executing it at the same time.
	
	One special side-effect to the recording of the functions being
	traced is that we can now selectively choose which functions we
	wish to trace and which ones we want the mcount calls to remain
	as nops.
	
	Two files are used, one for enabling and one for disabling the
	tracing of specified functions. They are:
	
	  set_ftrace_filter
	
	and
	
	  set_ftrace_notrace
	
	A list of available functions that you can add to these files is
	listed in:
	
	   available_filter_functions
	
	 # cat available_filter_functions
	put_prev_task_idle
	kmem_cache_create
	pick_next_task_rt
	get_online_cpus
	pick_next_task_fair
	mutex_lock
	[...]
	
	If I am only interested in sys_nanosleep and hrtimer_interrupt:
	
	 # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
	 # echo function > current_tracer
	 # echo 1 > tracing_on
	 # usleep 1
	 # echo 0 > tracing_on
	 # cat trace
	# tracer: function
	#
	# entries-in-buffer/entries-written: 5/5   #P:4
	#
	#                              _-----=> irqs-off
	#                             / _----=> need-resched
	#                            | / _---=> hardirq/softirq
	#                            || / _--=> preempt-depth
	#                            ||| /     delay
	#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
	#              | |       |   ||||       |         |
	          usleep-2665  [001] ....  4186.475355: sys_nanosleep <-system_call_fastpath
	          <idle>-0     [001] d.h1  4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
	          usleep-2665  [001] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
	          <idle>-0     [003] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
	          <idle>-0     [002] d.h1  4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
	
	To see which functions are being traced, you can cat the file:
	
	 # cat set_ftrace_filter
	hrtimer_interrupt
	sys_nanosleep
	
	
	Perhaps this is not enough. The filters also allow glob(7) matching.
	
	  <match>*  - will match functions that begin with <match>
	  *<match>  - will match functions that end with <match>
	  *<match>* - will match functions that have <match> in it
	  <match1>*<match2> - will match functions that begin with
	                      <match1> and end with <match2>
	
	Note: It is better to use quotes to enclose the wild cards,
	      otherwise the shell may expand the parameters into names
	      of files in the local directory.
	
	 # echo 'hrtimer_*' > set_ftrace_filter
	
	Produces:
	
	# tracer: function
	#
	# entries-in-buffer/entries-written: 897/897   #P:4
	#
	#                              _-----=> irqs-off
	#                             / _----=> need-resched
	#                            | / _---=> hardirq/softirq
	#                            || / _--=> preempt-depth
	#                            ||| /     delay
	#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
	#              | |       |   ||||       |         |
	          <idle>-0     [003] dN.1  4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
	          <idle>-0     [003] dN.1  4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
	          <idle>-0     [003] dN.2  4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
	          <idle>-0     [003] dN.1  4228.547805: hrtimer_forward <-tick_nohz_idle_exit
	          <idle>-0     [003] dN.1  4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
	          <idle>-0     [003] d..1  4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
	          <idle>-0     [003] d..1  4228.547859: hrtimer_start <-__tick_nohz_idle_enter
	          <idle>-0     [003] d..2  4228.547860: hrtimer_force_reprogram <-__rem
	
	Notice that we lost the sys_nanosleep.
	
	 # cat set_ftrace_filter
	hrtimer_run_queues
	hrtimer_run_pending
	hrtimer_init
	hrtimer_cancel
	hrtimer_try_to_cancel
	hrtimer_forward
	hrtimer_start
	hrtimer_reprogram
	hrtimer_force_reprogram
	hrtimer_get_next_event
	hrtimer_interrupt
	hrtimer_nanosleep
	hrtimer_wakeup
	hrtimer_get_remaining
	hrtimer_get_res
	hrtimer_init_sleeper
	
	
	This is because the '>' and '>>' act just like they do in bash.
	To rewrite the filters, use '>'
	To append to the filters, use '>>'
	
	To clear out a filter so that all functions will be recorded
	again:
	
	 # echo > set_ftrace_filter
	 # cat set_ftrace_filter
	 #
	
	Again, now we want to append.
	
	 # echo sys_nanosleep > set_ftrace_filter
	 # cat set_ftrace_filter
	sys_nanosleep
	 # echo 'hrtimer_*' >> set_ftrace_filter
	 # cat set_ftrace_filter
	hrtimer_run_queues
	hrtimer_run_pending
	hrtimer_init
	hrtimer_cancel
	hrtimer_try_to_cancel
	hrtimer_forward
	hrtimer_start
	hrtimer_reprogram
	hrtimer_force_reprogram
	hrtimer_get_next_event
	hrtimer_interrupt
	sys_nanosleep
	hrtimer_nanosleep
	hrtimer_wakeup
	hrtimer_get_remaining
	hrtimer_get_res
	hrtimer_init_sleeper
	
	
	The set_ftrace_notrace prevents those functions from being
	traced.
	
	 # echo '*preempt*' '*lock*' > set_ftrace_notrace
	
	Produces:
	
	# tracer: function
	#
	# entries-in-buffer/entries-written: 39608/39608   #P:4
	#
	#                              _-----=> irqs-off
	#                             / _----=> need-resched
	#                            | / _---=> hardirq/softirq
	#                            || / _--=> preempt-depth
	#                            ||| /     delay
	#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
	#              | |       |   ||||       |         |
	            bash-1994  [000] ....  4342.324896: file_ra_state_init <-do_dentry_open
	            bash-1994  [000] ....  4342.324897: open_check_o_direct <-do_last
	            bash-1994  [000] ....  4342.324897: ima_file_check <-do_last
	            bash-1994  [000] ....  4342.324898: process_measurement <-ima_file_check
	            bash-1994  [000] ....  4342.324898: ima_get_action <-process_measurement
	            bash-1994  [000] ....  4342.324898: ima_match_policy <-ima_get_action
	            bash-1994  [000] ....  4342.324899: do_truncate <-do_last
	            bash-1994  [000] ....  4342.324899: should_remove_suid <-do_truncate
	            bash-1994  [000] ....  4342.324899: notify_change <-do_truncate
	            bash-1994  [000] ....  4342.324900: current_fs_time <-notify_change
	            bash-1994  [000] ....  4342.324900: current_kernel_time <-current_fs_time
	            bash-1994  [000] ....  4342.324900: timespec_trunc <-current_fs_time
	
	We can see that there's no more lock or preempt tracing.
	
	
	Dynamic ftrace with the function graph tracer
	---------------------------------------------
	
	Although what has been explained above concerns both the
	function tracer and the function-graph-tracer, there are some
	special features only available in the function-graph tracer.
	
	If you want to trace only one function and all of its children,
	you just have to echo its name into set_graph_function:
	
	 echo __do_fault > set_graph_function
	
	will produce the following "expanded" trace of the __do_fault()
	function:
	
	 0)               |  __do_fault() {
	 0)               |    filemap_fault() {
	 0)               |      find_lock_page() {
	 0)   0.804 us    |        find_get_page();
	 0)               |        __might_sleep() {
	 0)   1.329 us    |        }
	 0)   3.904 us    |      }
	 0)   4.979 us    |    }
	 0)   0.653 us    |    _spin_lock();
	 0)   0.578 us    |    page_add_file_rmap();
	 0)   0.525 us    |    native_set_pte_at();
	 0)   0.585 us    |    _spin_unlock();
	 0)               |    unlock_page() {
	 0)   0.541 us    |      page_waitqueue();
	 0)   0.639 us    |      __wake_up_bit();
	 0)   2.786 us    |    }
	 0) + 14.237 us   |  }
	 0)               |  __do_fault() {
	 0)               |    filemap_fault() {
	 0)               |      find_lock_page() {
	 0)   0.698 us    |        find_get_page();
	 0)               |        __might_sleep() {
	 0)   1.412 us    |        }
	 0)   3.950 us    |      }
	 0)   5.098 us    |    }
	 0)   0.631 us    |    _spin_lock();
	 0)   0.571 us    |    page_add_file_rmap();
	 0)   0.526 us    |    native_set_pte_at();
	 0)   0.586 us    |    _spin_unlock();
	 0)               |    unlock_page() {
	 0)   0.533 us    |      page_waitqueue();
	 0)   0.638 us    |      __wake_up_bit();
	 0)   2.793 us    |    }
	 0) + 14.012 us   |  }
	
	You can also expand several functions at once:
	
	 echo sys_open > set_graph_function
	 echo sys_close >> set_graph_function
	
	Now if you want to go back to trace all functions you can clear
	this special filter via:
	
	 echo > set_graph_function
	
	
	ftrace_enabled
	--------------
	
	Note, the proc sysctl ftrace_enable is a big on/off switch for the
	function tracer. By default it is enabled (when function tracing is
	enabled in the kernel). If it is disabled, all function tracing is
	disabled. This includes not only the function tracers for ftrace, but
	also for any other uses (perf, kprobes, stack tracing, profiling, etc).
	
	Please disable this with care.
	
	This can be disable (and enabled) with:
	
	  sysctl kernel.ftrace_enabled=0
	  sysctl kernel.ftrace_enabled=1
	
	 or
	
	  echo 0 > /proc/sys/kernel/ftrace_enabled
	  echo 1 > /proc/sys/kernel/ftrace_enabled
	
	
	Filter commands
	---------------
	
	A few commands are supported by the set_ftrace_filter interface.
	Trace commands have the following format:
	
	<function>:<command>:<parameter>
	
	The following commands are supported:
	
	- mod
	  This command enables function filtering per module. The
	  parameter defines the module. For example, if only the write*
	  functions in the ext3 module are desired, run:
	
	   echo 'write*:mod:ext3' > set_ftrace_filter
	
	  This command interacts with the filter in the same way as
	  filtering based on function names. Thus, adding more functions
	  in a different module is accomplished by appending (>>) to the
	  filter file. Remove specific module functions by prepending
	  '!':
	
	   echo '!writeback*:mod:ext3' >> set_ftrace_filter
	
	  Mod command supports module globbing. Disable tracing for all
	  functions except a specific module:
	
	   echo '!*:mod:!ext3' >> set_ftrace_filter
	
	  Disable tracing for all modules, but still trace kernel:
	
	   echo '!*:mod:*' >> set_ftrace_filter
	
	  Enable filter only for kernel:
	
	   echo '*write*:mod:!*' >> set_ftrace_filter
	
	  Enable filter for module globbing:
	
	   echo '*write*:mod:*snd*' >> set_ftrace_filter
	
	- traceon/traceoff
	  These commands turn tracing on and off when the specified
	  functions are hit. The parameter determines how many times the
	  tracing system is turned on and off. If unspecified, there is
	  no limit. For example, to disable tracing when a schedule bug
	  is hit the first 5 times, run:
	
	   echo '__schedule_bug:traceoff:5' > set_ftrace_filter
	
	  To always disable tracing when __schedule_bug is hit:
	
	   echo '__schedule_bug:traceoff' > set_ftrace_filter
	
	  These commands are cumulative whether or not they are appended
	  to set_ftrace_filter. To remove a command, prepend it by '!'
	  and drop the parameter:
	
	   echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
	
	    The above removes the traceoff command for __schedule_bug
	    that have a counter. To remove commands without counters:
	
	   echo '!__schedule_bug:traceoff' > set_ftrace_filter
	
	- snapshot
	  Will cause a snapshot to be triggered when the function is hit.
	
	   echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
	
	  To only snapshot once:
	
	   echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
	
	  To remove the above commands:
	
	   echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
	   echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
	
	- enable_event/disable_event
	  These commands can enable or disable a trace event. Note, because
	  function tracing callbacks are very sensitive, when these commands
	  are registered, the trace point is activated, but disabled in
	  a "soft" mode. That is, the tracepoint will be called, but
	  just will not be traced. The event tracepoint stays in this mode
	  as long as there's a command that triggers it.
	
	   echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
	   	 set_ftrace_filter
	
	  The format is:
	
	    <function>:enable_event:<system>:<event>[:count]
	    <function>:disable_event:<system>:<event>[:count]
	
	  To remove the events commands:
	
	
	   echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
	   	 set_ftrace_filter
	   echo '!schedule:disable_event:sched:sched_switch' > \
	   	 set_ftrace_filter
	
	- dump
	  When the function is hit, it will dump the contents of the ftrace
	  ring buffer to the console. This is useful if you need to debug
	  something, and want to dump the trace when a certain function
	  is hit. Perhaps its a function that is called before a tripple
	  fault happens and does not allow you to get a regular dump.
	
	- cpudump
	  When the function is hit, it will dump the contents of the ftrace
	  ring buffer for the current CPU to the console. Unlike the "dump"
	  command, it only prints out the contents of the ring buffer for the
	  CPU that executed the function that triggered the dump.
	
	trace_pipe
	----------
	
	The trace_pipe outputs the same content as the trace file, but
	the effect on the tracing is different. Every read from
	trace_pipe is consumed. This means that subsequent reads will be
	different. The trace is live.
	
	 # echo function > current_tracer
	 # cat trace_pipe > /tmp/trace.out &
	[1] 4153
	 # echo 1 > tracing_on
	 # usleep 1
	 # echo 0 > tracing_on
	 # cat trace
	# tracer: function
	#
	# entries-in-buffer/entries-written: 0/0   #P:4
	#
	#                              _-----=> irqs-off
	#                             / _----=> need-resched
	#                            | / _---=> hardirq/softirq
	#                            || / _--=> preempt-depth
	#                            ||| /     delay
	#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
	#              | |       |   ||||       |         |
	
	 #
	 # cat /tmp/trace.out
	            bash-1994  [000] ....  5281.568961: mutex_unlock <-rb_simple_write
	            bash-1994  [000] ....  5281.568963: __mutex_unlock_slowpath <-mutex_unlock
	            bash-1994  [000] ....  5281.568963: __fsnotify_parent <-fsnotify_modify
	            bash-1994  [000] ....  5281.568964: fsnotify <-fsnotify_modify
	            bash-1994  [000] ....  5281.568964: __srcu_read_lock <-fsnotify
	            bash-1994  [000] ....  5281.568964: add_preempt_count <-__srcu_read_lock
	            bash-1994  [000] ...1  5281.568965: sub_preempt_count <-__srcu_read_lock
	            bash-1994  [000] ....  5281.568965: __srcu_read_unlock <-fsnotify
	            bash-1994  [000] ....  5281.568967: sys_dup2 <-system_call_fastpath
	
	
	Note, reading the trace_pipe file will block until more input is
	added.
	
	trace entries
	-------------
	
	Having too much or not enough data can be troublesome in
	diagnosing an issue in the kernel. The file buffer_size_kb is
	used to modify the size of the internal trace buffers. The
	number listed is the number of entries that can be recorded per
	CPU. To know the full size, multiply the number of possible CPUs
	with the number of entries.
	
	 # cat buffer_size_kb
	1408 (units kilobytes)
	
	Or simply read buffer_total_size_kb
	
	 # cat buffer_total_size_kb 
	5632
	
	To modify the buffer, simple echo in a number (in 1024 byte segments).
	
	 # echo 10000 > buffer_size_kb
	 # cat buffer_size_kb
	10000 (units kilobytes)
	
	It will try to allocate as much as possible. If you allocate too
	much, it can cause Out-Of-Memory to trigger.
	
	 # echo 1000000000000 > buffer_size_kb
	-bash: echo: write error: Cannot allocate memory
	 # cat buffer_size_kb
	85
	
	The per_cpu buffers can be changed individually as well:
	
	 # echo 10000 > per_cpu/cpu0/buffer_size_kb
	 # echo 100 > per_cpu/cpu1/buffer_size_kb
	
	When the per_cpu buffers are not the same, the buffer_size_kb
	at the top level will just show an X
	
	 # cat buffer_size_kb
	X
	
	This is where the buffer_total_size_kb is useful:
	
	 # cat buffer_total_size_kb 
	12916
	
	Writing to the top level buffer_size_kb will reset all the buffers
	to be the same again.
	
	Snapshot
	--------
	CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
	available to all non latency tracers. (Latency tracers which
	record max latency, such as "irqsoff" or "wakeup", can't use
	this feature, since those are already using the snapshot
	mechanism internally.)
	
	Snapshot preserves a current trace buffer at a particular point
	in time without stopping tracing. Ftrace swaps the current
	buffer with a spare buffer, and tracing continues in the new
	current (=previous spare) buffer.
	
	The following tracefs files in "tracing" are related to this
	feature:
	
	  snapshot:
	
		This is used to take a snapshot and to read the output
		of the snapshot. Echo 1 into this file to allocate a
		spare buffer and to take a snapshot (swap), then read
		the snapshot from this file in the same format as
		"trace" (described above in the section "The File
		System"). Both reads snapshot and tracing are executable
		in parallel. When the spare buffer is allocated, echoing
		0 frees it, and echoing else (positive) values clear the
		snapshot contents.
		More details are shown in the table below.
	
		status\input  |     0      |     1      |    else    |
		--------------+------------+------------+------------+
		not allocated |(do nothing)| alloc+swap |(do nothing)|
		--------------+------------+------------+------------+
		allocated     |    free    |    swap    |   clear    |
		--------------+------------+------------+------------+
	
	Here is an example of using the snapshot feature.
	
	 # echo 1 > events/sched/enable
	 # echo 1 > snapshot
	 # cat snapshot
	# tracer: nop
	#
	# entries-in-buffer/entries-written: 71/71   #P:8
	#
	#                              _-----=> irqs-off
	#                             / _----=> need-resched
	#                            | / _---=> hardirq/softirq
	#                            || / _--=> preempt-depth
	#                            ||| /     delay
	#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
	#              | |       |   ||||       |         |
	          <idle>-0     [005] d...  2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120
	           sleep-2242  [005] d...  2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120
	[...]
	          <idle>-0     [002] d...  2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120
	
	 # cat trace
	# tracer: nop
	#
	# entries-in-buffer/entries-written: 77/77   #P:8
	#
	#                              _-----=> irqs-off
	#                             / _----=> need-resched
	#                            | / _---=> hardirq/softirq
	#                            || / _--=> preempt-depth
	#                            ||| /     delay
	#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
	#              | |       |   ||||       |         |
	          <idle>-0     [007] d...  2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120
	 snapshot-test-2-2229  [002] d...  2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120
	[...]
	
	
	If you try to use this snapshot feature when current tracer is
	one of the latency tracers, you will get the following results.
	
	 # echo wakeup > current_tracer
	 # echo 1 > snapshot
	bash: echo: write error: Device or resource busy
	 # cat snapshot
	cat: snapshot: Device or resource busy
	
	
	Instances
	---------
	In the tracefs tracing directory is a directory called "instances".
	This directory can have new directories created inside of it using
	mkdir, and removing directories with rmdir. The directory created
	with mkdir in this directory will already contain files and other
	directories after it is created.
	
	 # mkdir instances/foo
	 # ls instances/foo
	buffer_size_kb  buffer_total_size_kb  events  free_buffer  per_cpu
	set_event  snapshot  trace  trace_clock  trace_marker  trace_options
	trace_pipe  tracing_on
	
	As you can see, the new directory looks similar to the tracing directory
	itself. In fact, it is very similar, except that the buffer and
	events are agnostic from the main director, or from any other
	instances that are created.
	
	The files in the new directory work just like the files with the
	same name in the tracing directory except the buffer that is used
	is a separate and new buffer. The files affect that buffer but do not
	affect the main buffer with the exception of trace_options. Currently,
	the trace_options affect all instances and the top level buffer
	the same, but this may change in future releases. That is, options
	may become specific to the instance they reside in.
	
	Notice that none of the function tracer files are there, nor is
	current_tracer and available_tracers. This is because the buffers
	can currently only have events enabled for them.
	
	 # mkdir instances/foo
	 # mkdir instances/bar
	 # mkdir instances/zoot
	 # echo 100000 > buffer_size_kb
	 # echo 1000 > instances/foo/buffer_size_kb
	 # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
	 # echo function > current_trace
	 # echo 1 > instances/foo/events/sched/sched_wakeup/enable
	 # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
	 # echo 1 > instances/foo/events/sched/sched_switch/enable
	 # echo 1 > instances/bar/events/irq/enable
	 # echo 1 > instances/zoot/events/syscalls/enable
	 # cat trace_pipe
	CPU:2 [LOST 11745 EVENTS]
	            bash-2044  [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
	            bash-2044  [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
	            bash-2044  [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
	            bash-2044  [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
	            bash-2044  [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
	            bash-2044  [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
	            bash-2044  [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
	            bash-2044  [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
	            bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
	            bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
	            bash-2044  [002] .... 10594.481035: arch_dup_task_struct <-copy_process
	[...]
	
	 # cat instances/foo/trace_pipe
	            bash-1998  [000] d..4   136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
	            bash-1998  [000] dN.4   136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
	          <idle>-0     [003] d.h3   136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
	          <idle>-0     [003] d..3   136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120
	     rcu_preempt-9     [003] d..3   136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120
	            bash-1998  [000] d..4   136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
	            bash-1998  [000] dN.4   136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
	            bash-1998  [000] d..3   136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120
	     kworker/0:1-59    [000] d..4   136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
	     kworker/0:1-59    [000] d..3   136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120
	[...]
	
	 # cat instances/bar/trace_pipe
	     migration/1-14    [001] d.h3   138.732674: softirq_raise: vec=3 [action=NET_RX]
	          <idle>-0     [001] dNh3   138.732725: softirq_raise: vec=3 [action=NET_RX]
	            bash-1998  [000] d.h1   138.733101: softirq_raise: vec=1 [action=TIMER]
	            bash-1998  [000] d.h1   138.733102: softirq_raise: vec=9 [action=RCU]
	            bash-1998  [000] ..s2   138.733105: softirq_entry: vec=1 [action=TIMER]
	            bash-1998  [000] ..s2   138.733106: softirq_exit: vec=1 [action=TIMER]
	            bash-1998  [000] ..s2   138.733106: softirq_entry: vec=9 [action=RCU]
	            bash-1998  [000] ..s2   138.733109: softirq_exit: vec=9 [action=RCU]
	            sshd-1995  [001] d.h1   138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
	            sshd-1995  [001] d.h1   138.733280: irq_handler_exit: irq=21 ret=unhandled
	            sshd-1995  [001] d.h1   138.733281: irq_handler_entry: irq=21 name=eth0
	            sshd-1995  [001] d.h1   138.733283: irq_handler_exit: irq=21 ret=handled
	[...]
	
	 # cat instances/zoot/trace
	# tracer: nop
	#
	# entries-in-buffer/entries-written: 18996/18996   #P:4
	#
	#                              _-----=> irqs-off
	#                             / _----=> need-resched
	#                            | / _---=> hardirq/softirq
	#                            || / _--=> preempt-depth
	#                            ||| /     delay
	#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
	#              | |       |   ||||       |         |
	            bash-1998  [000] d...   140.733501: sys_write -> 0x2
	            bash-1998  [000] d...   140.733504: sys_dup2(oldfd: a, newfd: 1)
	            bash-1998  [000] d...   140.733506: sys_dup2 -> 0x1
	            bash-1998  [000] d...   140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
	            bash-1998  [000] d...   140.733509: sys_fcntl -> 0x1
	            bash-1998  [000] d...   140.733510: sys_close(fd: a)
	            bash-1998  [000] d...   140.733510: sys_close -> 0x0
	            bash-1998  [000] d...   140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
	            bash-1998  [000] d...   140.733515: sys_rt_sigprocmask -> 0x0
	            bash-1998  [000] d...   140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
	            bash-1998  [000] d...   140.733516: sys_rt_sigaction -> 0x0
	
	You can see that the trace of the top most trace buffer shows only
	the function tracing. The foo instance displays wakeups and task
	switches.
	
	To remove the instances, simply delete their directories:
	
	 # rmdir instances/foo
	 # rmdir instances/bar
	 # rmdir instances/zoot
	
	Note, if a process has a trace file open in one of the instance
	directories, the rmdir will fail with EBUSY.
	
	
	Stack trace
	-----------
	Since the kernel has a fixed sized stack, it is important not to
	waste it in functions. A kernel developer must be conscience of
	what they allocate on the stack. If they add too much, the system
	can be in danger of a stack overflow, and corruption will occur,
	usually leading to a system panic.
	
	There are some tools that check this, usually with interrupts
	periodically checking usage. But if you can perform a check
	at every function call that will become very useful. As ftrace provides
	a function tracer, it makes it convenient to check the stack size
	at every function call. This is enabled via the stack tracer.
	
	CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
	To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
	
	 # echo 1 > /proc/sys/kernel/stack_tracer_enabled
	
	You can also enable it from the kernel command line to trace
	the stack size of the kernel during boot up, by adding "stacktrace"
	to the kernel command line parameter.
	
	After running it for a few minutes, the output looks like:
	
	 # cat stack_max_size
	2928
	
	 # cat stack_trace
	        Depth    Size   Location    (18 entries)
	        -----    ----   --------
	  0)     2928     224   update_sd_lb_stats+0xbc/0x4ac
	  1)     2704     160   find_busiest_group+0x31/0x1f1
	  2)     2544     256   load_balance+0xd9/0x662
	  3)     2288      80   idle_balance+0xbb/0x130
	  4)     2208     128   __schedule+0x26e/0x5b9
	  5)     2080      16   schedule+0x64/0x66
	  6)     2064     128   schedule_timeout+0x34/0xe0
	  7)     1936     112   wait_for_common+0x97/0xf1
	  8)     1824      16   wait_for_completion+0x1d/0x1f
	  9)     1808     128   flush_work+0xfe/0x119
	 10)     1680      16   tty_flush_to_ldisc+0x1e/0x20
	 11)     1664      48   input_available_p+0x1d/0x5c
	 12)     1616      48   n_tty_poll+0x6d/0x134
	 13)     1568      64   tty_poll+0x64/0x7f
	 14)     1504     880   do_select+0x31e/0x511
	 15)      624     400   core_sys_select+0x177/0x216
	 16)      224      96   sys_select+0x91/0xb9
	 17)      128     128   system_call_fastpath+0x16/0x1b
	
	Note, if -mfentry is being used by gcc, functions get traced before
	they set up the stack frame. This means that leaf level functions
	are not tested by the stack tracer when -mfentry is used.
	
	Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
	
	---------
	
	More details can be found in the source code, in the
	kernel/trace/*.c files.

• [ trace ]
• coresight-cpu-debug.txt
• coresight.txt
• events-kmem.txt
• events-msr.txt
• events-nmi.txt
• events-power.txt
• events.txt
• ftrace-design.txt
• ftrace-uses.rst
• ftrace.txt
• function-graph-fold.vim
• hwlat_detector.txt
• intel_th.txt
• kprobetrace.txt
• mmiotrace.txt
• [ postprocess ]
• ring-buffer-design.txt
• stm.txt
• tracepoint-analysis.txt
• tracepoints.txt
• uprobetracer.txt
•  

---