1========================
2ftrace - Function Tracer
3========================
4
5Copyright 2008 Red Hat Inc.
6
7:Author:   Steven Rostedt <srostedt@redhat.com>
8:License:  The GNU Free Documentation License, Version 1.2
9          (dual licensed under the GPL v2)
10:Original Reviewers:  Elias Oltmanns, Randy Dunlap, Andrew Morton,
11		      John Kacur, and David Teigland.
12
13- Written for: 2.6.28-rc2
14- Updated for: 3.10
15- Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt
16- Converted to rst format - Changbin Du <changbin.du@intel.com>
17
18Introduction
19------------
20
21Ftrace is an internal tracer designed to help out developers and
22designers of systems to find what is going on inside the kernel.
23It can be used for debugging or analyzing latencies and
24performance issues that take place outside of user-space.
25
26Although ftrace is typically considered the function tracer, it
27is really a framework of several assorted tracing utilities.
28There's latency tracing to examine what occurs between interrupts
29disabled and enabled, as well as for preemption and from a time
30a task is woken to the task is actually scheduled in.
31
32One of the most common uses of ftrace is the event tracing.
33Throughout the kernel is hundreds of static event points that
34can be enabled via the tracefs file system to see what is
35going on in certain parts of the kernel.
36
37See events.rst for more information.
38
39
40Implementation Details
41----------------------
42
43See Documentation/trace/ftrace-design.rst for details for arch porters and such.
44
45
46The File System
47---------------
48
49Ftrace uses the tracefs file system to hold the control files as
50well as the files to display output.
51
52When tracefs is configured into the kernel (which selecting any ftrace
53option will do) the directory /sys/kernel/tracing will be created. To mount
54this directory, you can add to your /etc/fstab file::
55
56 tracefs       /sys/kernel/tracing       tracefs defaults        0       0
57
58Or you can mount it at run time with::
59
60 mount -t tracefs nodev /sys/kernel/tracing
61
62For quicker access to that directory you may want to make a soft link to
63it::
64
65 ln -s /sys/kernel/tracing /tracing
66
67.. attention::
68
69  Before 4.1, all ftrace tracing control files were within the debugfs
70  file system, which is typically located at /sys/kernel/debug/tracing.
71  For backward compatibility, when mounting the debugfs file system,
72  the tracefs file system will be automatically mounted at:
73
74  /sys/kernel/debug/tracing
75
76  All files located in the tracefs file system will be located in that
77  debugfs file system directory as well.
78
79.. attention::
80
81  Any selected ftrace option will also create the tracefs file system.
82  The rest of the document will assume that you are in the ftrace directory
83  (cd /sys/kernel/tracing) and will only concentrate on the files within that
84  directory and not distract from the content with the extended
85  "/sys/kernel/tracing" path name.
86
87That's it! (assuming that you have ftrace configured into your kernel)
88
89After mounting tracefs you will have access to the control and output files
90of ftrace. Here is a list of some of the key files:
91
92
93 Note: all time values are in microseconds.
94
95  current_tracer:
96
97	This is used to set or display the current tracer
98	that is configured. Changing the current tracer clears
99	the ring buffer content as well as the "snapshot" buffer.
100
101  available_tracers:
102
103	This holds the different types of tracers that
104	have been compiled into the kernel. The
105	tracers listed here can be configured by
106	echoing their name into current_tracer.
107
108  tracing_on:
109
110	This sets or displays whether writing to the trace
111	ring buffer is enabled. Echo 0 into this file to disable
112	the tracer or 1 to enable it. Note, this only disables
113	writing to the ring buffer, the tracing overhead may
114	still be occurring.
115
116	The kernel function tracing_off() can be used within the
117	kernel to disable writing to the ring buffer, which will
118	set this file to "0". User space can re-enable tracing by
119	echoing "1" into the file.
120
121	Note, the function and event trigger "traceoff" will also
122	set this file to zero and stop tracing. Which can also
123	be re-enabled by user space using this file.
124
125  trace:
126
127	This file holds the output of the trace in a human
128	readable format (described below). Opening this file for
129	writing with the O_TRUNC flag clears the ring buffer content.
130        Note, this file is not a consumer. If tracing is off
131        (no tracer running, or tracing_on is zero), it will produce
132        the same output each time it is read. When tracing is on,
133        it may produce inconsistent results as it tries to read
134        the entire buffer without consuming it.
135
136  trace_pipe:
137
138	The output is the same as the "trace" file but this
139	file is meant to be streamed with live tracing.
140	Reads from this file will block until new data is
141	retrieved.  Unlike the "trace" file, this file is a
142	consumer. This means reading from this file causes
143	sequential reads to display more current data. Once
144	data is read from this file, it is consumed, and
145	will not be read again with a sequential read. The
146	"trace" file is static, and if the tracer is not
147	adding more data, it will display the same
148	information every time it is read.
149
150  trace_options:
151
152	This file lets the user control the amount of data
153	that is displayed in one of the above output
154	files. Options also exist to modify how a tracer
155	or events work (stack traces, timestamps, etc).
156
157  options:
158
159	This is a directory that has a file for every available
160	trace option (also in trace_options). Options may also be set
161	or cleared by writing a "1" or "0" respectively into the
162	corresponding file with the option name.
163
164  tracing_max_latency:
165
166	Some of the tracers record the max latency.
167	For example, the maximum time that interrupts are disabled.
168	The maximum time is saved in this file. The max trace will also be
169	stored,	and displayed by "trace". A new max trace will only be
170	recorded if the latency is greater than the value in this file
171	(in microseconds).
172
173	By echoing in a time into this file, no latency will be recorded
174	unless it is greater than the time in this file.
175
176  tracing_thresh:
177
178	Some latency tracers will record a trace whenever the
179	latency is greater than the number in this file.
180	Only active when the file contains a number greater than 0.
181	(in microseconds)
182
183  buffer_size_kb:
184
185	This sets or displays the number of kilobytes each CPU
186	buffer holds. By default, the trace buffers are the same size
187	for each CPU. The displayed number is the size of the
188	CPU buffer and not total size of all buffers. The
189	trace buffers are allocated in pages (blocks of memory
190	that the kernel uses for allocation, usually 4 KB in size).
191	A few extra pages may be allocated to accommodate buffer management
192	meta-data. If the last page allocated has room for more bytes
193	than requested, the rest of the page will be used,
194	making the actual allocation bigger than requested or shown.
195	( Note, the size may not be a multiple of the page size
196	due to buffer management meta-data. )
197
198	Buffer sizes for individual CPUs may vary
199	(see "per_cpu/cpu0/buffer_size_kb" below), and if they do
200	this file will show "X".
201
202  buffer_total_size_kb:
203
204	This displays the total combined size of all the trace buffers.
205
206  free_buffer:
207
208	If a process is performing tracing, and the ring buffer	should be
209	shrunk "freed" when the process is finished, even if it were to be
210	killed by a signal, this file can be used for that purpose. On close
211	of this file, the ring buffer will be resized to its minimum size.
212	Having a process that is tracing also open this file, when the process
213	exits its file descriptor for this file will be closed, and in doing so,
214	the ring buffer will be "freed".
215
216	It may also stop tracing if disable_on_free option is set.
217
218  tracing_cpumask:
219
220	This is a mask that lets the user only trace on specified CPUs.
221	The format is a hex string representing the CPUs.
222
223  set_ftrace_filter:
224
225	When dynamic ftrace is configured in (see the
226	section below "dynamic ftrace"), the code is dynamically
227	modified (code text rewrite) to disable calling of the
228	function profiler (mcount). This lets tracing be configured
229	in with practically no overhead in performance.  This also
230	has a side effect of enabling or disabling specific functions
231	to be traced. Echoing names of functions into this file
232	will limit the trace to only those functions.
233	This influences the tracers "function" and "function_graph"
234	and thus also function profiling (see "function_profile_enabled").
235
236	The functions listed in "available_filter_functions" are what
237	can be written into this file.
238
239	This interface also allows for commands to be used. See the
240	"Filter commands" section for more details.
241
242	As a speed up, since processing strings can be quite expensive
243	and requires a check of all functions registered to tracing, instead
244	an index can be written into this file. A number (starting with "1")
245	written will instead select the same corresponding at the line position
246	of the "available_filter_functions" file.
247
248  set_ftrace_notrace:
249
250	This has an effect opposite to that of
251	set_ftrace_filter. Any function that is added here will not
252	be traced. If a function exists in both set_ftrace_filter
253	and set_ftrace_notrace,	the function will _not_ be traced.
254
255  set_ftrace_pid:
256
257	Have the function tracer only trace the threads whose PID are
258	listed in this file.
259
260	If the "function-fork" option is set, then when a task whose
261	PID is listed in this file forks, the child's PID will
262	automatically be added to this file, and the child will be
263	traced by the function tracer as well. This option will also
264	cause PIDs of tasks that exit to be removed from the file.
265
266  set_ftrace_notrace_pid:
267
268        Have the function tracer ignore threads whose PID are listed in
269        this file.
270
271        If the "function-fork" option is set, then when a task whose
272	PID is listed in this file forks, the child's PID will
273	automatically be added to this file, and the child will not be
274	traced by the function tracer as well. This option will also
275	cause PIDs of tasks that exit to be removed from the file.
276
277        If a PID is in both this file and "set_ftrace_pid", then this
278        file takes precedence, and the thread will not be traced.
279
280  set_event_pid:
281
282	Have the events only trace a task with a PID listed in this file.
283	Note, sched_switch and sched_wake_up will also trace events
284	listed in this file.
285
286	To have the PIDs of children of tasks with their PID in this file
287	added on fork, enable the "event-fork" option. That option will also
288	cause the PIDs of tasks to be removed from this file when the task
289	exits.
290
291  set_event_notrace_pid:
292
293	Have the events not trace a task with a PID listed in this file.
294	Note, sched_switch and sched_wakeup will trace threads not listed
295	in this file, even if a thread's PID is in the file if the
296        sched_switch or sched_wakeup events also trace a thread that should
297        be traced.
298
299	To have the PIDs of children of tasks with their PID in this file
300	added on fork, enable the "event-fork" option. That option will also
301	cause the PIDs of tasks to be removed from this file when the task
302	exits.
303
304  set_graph_function:
305
306	Functions listed in this file will cause the function graph
307	tracer to only trace these functions and the functions that
308	they call. (See the section "dynamic ftrace" for more details).
309	Note, set_ftrace_filter and set_ftrace_notrace still affects
310	what functions are being traced.
311
312  set_graph_notrace:
313
314	Similar to set_graph_function, but will disable function graph
315	tracing when the function is hit until it exits the function.
316	This makes it possible to ignore tracing functions that are called
317	by a specific function.
318
319  available_filter_functions:
320
321	This lists the functions that ftrace has processed and can trace.
322	These are the function names that you can pass to
323	"set_ftrace_filter", "set_ftrace_notrace",
324	"set_graph_function", or "set_graph_notrace".
325	(See the section "dynamic ftrace" below for more details.)
326
327  available_filter_functions_addrs:
328
329	Similar to available_filter_functions, but with address displayed
330	for each function. The displayed address is the patch-site address
331	and can differ from /proc/kallsyms address.
332
333  dyn_ftrace_total_info:
334
335	This file is for debugging purposes. The number of functions that
336	have been converted to nops and are available to be traced.
337
338  enabled_functions:
339
340	This file is more for debugging ftrace, but can also be useful
341	in seeing if any function has a callback attached to it.
342	Not only does the trace infrastructure use ftrace function
343	trace utility, but other subsystems might too. This file
344	displays all functions that have a callback attached to them
345	as well as the number of callbacks that have been attached.
346	Note, a callback may also call multiple functions which will
347	not be listed in this count.
348
349	If the callback registered to be traced by a function with
350	the "save regs" attribute (thus even more overhead), a 'R'
351	will be displayed on the same line as the function that
352	is returning registers.
353
354	If the callback registered to be traced by a function with
355	the "ip modify" attribute (thus the regs->ip can be changed),
356	an 'I' will be displayed on the same line as the function that
357	can be overridden.
358
359	If a non ftrace trampoline is attached (BPF) a 'D' will be displayed.
360	Note, normal ftrace trampolines can also be attached, but only one
361	"direct" trampoline can be attached to a given function at a time.
362
363	Some architectures can not call direct trampolines, but instead have
364	the ftrace ops function located above the function entry point. In
365	such cases an 'O' will be displayed.
366
367	If a function had either the "ip modify" or a "direct" call attached to
368	it in the past, a 'M' will be shown. This flag is never cleared. It is
369	used to know if a function was every modified by the ftrace infrastructure,
370	and can be used for debugging.
371
372	If the architecture supports it, it will also show what callback
373	is being directly called by the function. If the count is greater
374	than 1 it most likely will be ftrace_ops_list_func().
375
376	If the callback of a function jumps to a trampoline that is
377	specific to the callback and which is not the standard trampoline,
378	its address will be printed as well as the function that the
379	trampoline calls.
380
381  touched_functions:
382
383	This file contains all the functions that ever had a function callback
384	to it via the ftrace infrastructure. It has the same format as
385	enabled_functions but shows all functions that have every been
386	traced.
387
388	To see any function that has every been modified by "ip modify" or a
389	direct trampoline, one can perform the following command:
390
391	grep ' M ' /sys/kernel/tracing/touched_functions
392
393  function_profile_enabled:
394
395	When set it will enable all functions with either the function
396	tracer, or if configured, the function graph tracer. It will
397	keep a histogram of the number of functions that were called
398	and if the function graph tracer was configured, it will also keep
399	track of the time spent in those functions. The histogram
400	content can be displayed in the files:
401
402	trace_stat/function<cpu> ( function0, function1, etc).
403
404  trace_stat:
405
406	A directory that holds different tracing stats.
407
408  kprobe_events:
409
410	Enable dynamic trace points. See kprobetrace.rst.
411
412  kprobe_profile:
413
414	Dynamic trace points stats. See kprobetrace.rst.
415
416  max_graph_depth:
417
418	Used with the function graph tracer. This is the max depth
419	it will trace into a function. Setting this to a value of
420	one will show only the first kernel function that is called
421	from user space.
422
423  printk_formats:
424
425	This is for tools that read the raw format files. If an event in
426	the ring buffer references a string, only a pointer to the string
427	is recorded into the buffer and not the string itself. This prevents
428	tools from knowing what that string was. This file displays the string
429	and address for	the string allowing tools to map the pointers to what
430	the strings were.
431
432  saved_cmdlines:
433
434	Only the pid of the task is recorded in a trace event unless
435	the event specifically saves the task comm as well. Ftrace
436	makes a cache of pid mappings to comms to try to display
437	comms for events. If a pid for a comm is not listed, then
438	"<...>" is displayed in the output.
439
440	If the option "record-cmd" is set to "0", then comms of tasks
441	will not be saved during recording. By default, it is enabled.
442
443  saved_cmdlines_size:
444
445	By default, 128 comms are saved (see "saved_cmdlines" above). To
446	increase or decrease the amount of comms that are cached, echo
447	the number of comms to cache into this file.
448
449  saved_tgids:
450
451	If the option "record-tgid" is set, on each scheduling context switch
452	the Task Group ID of a task is saved in a table mapping the PID of
453	the thread to its TGID. By default, the "record-tgid" option is
454	disabled.
455
456  snapshot:
457
458	This displays the "snapshot" buffer and also lets the user
459	take a snapshot of the current running trace.
460	See the "Snapshot" section below for more details.
461
462  stack_max_size:
463
464	When the stack tracer is activated, this will display the
465	maximum stack size it has encountered.
466	See the "Stack Trace" section below.
467
468  stack_trace:
469
470	This displays the stack back trace of the largest stack
471	that was encountered when the stack tracer is activated.
472	See the "Stack Trace" section below.
473
474  stack_trace_filter:
475
476	This is similar to "set_ftrace_filter" but it limits what
477	functions the stack tracer will check.
478
479  trace_clock:
480
481	Whenever an event is recorded into the ring buffer, a
482	"timestamp" is added. This stamp comes from a specified
483	clock. By default, ftrace uses the "local" clock. This
484	clock is very fast and strictly per cpu, but on some
485	systems it may not be monotonic with respect to other
486	CPUs. In other words, the local clocks may not be in sync
487	with local clocks on other CPUs.
488
489	Usual clocks for tracing::
490
491	  # cat trace_clock
492	  [local] global counter x86-tsc
493
494	The clock with the square brackets around it is the one in effect.
495
496	local:
497		Default clock, but may not be in sync across CPUs
498
499	global:
500		This clock is in sync with all CPUs but may
501		be a bit slower than the local clock.
502
503	counter:
504		This is not a clock at all, but literally an atomic
505		counter. It counts up one by one, but is in sync
506		with all CPUs. This is useful when you need to
507		know exactly the order events occurred with respect to
508		each other on different CPUs.
509
510	uptime:
511		This uses the jiffies counter and the time stamp
512		is relative to the time since boot up.
513
514	perf:
515		This makes ftrace use the same clock that perf uses.
516		Eventually perf will be able to read ftrace buffers
517		and this will help out in interleaving the data.
518
519	x86-tsc:
520		Architectures may define their own clocks. For
521		example, x86 uses its own TSC cycle clock here.
522
523	ppc-tb:
524		This uses the powerpc timebase register value.
525		This is in sync across CPUs and can also be used
526		to correlate events across hypervisor/guest if
527		tb_offset is known.
528
529	mono:
530		This uses the fast monotonic clock (CLOCK_MONOTONIC)
531		which is monotonic and is subject to NTP rate adjustments.
532
533	mono_raw:
534		This is the raw monotonic clock (CLOCK_MONOTONIC_RAW)
535		which is monotonic but is not subject to any rate adjustments
536		and ticks at the same rate as the hardware clocksource.
537
538	boot:
539		This is the boot clock (CLOCK_BOOTTIME) and is based on the
540		fast monotonic clock, but also accounts for time spent in
541		suspend. Since the clock access is designed for use in
542		tracing in the suspend path, some side effects are possible
543		if clock is accessed after the suspend time is accounted before
544		the fast mono clock is updated. In this case, the clock update
545		appears to happen slightly sooner than it normally would have.
546		Also on 32-bit systems, it's possible that the 64-bit boot offset
547		sees a partial update. These effects are rare and post
548		processing should be able to handle them. See comments in the
549		ktime_get_boot_fast_ns() function for more information.
550
551	tai:
552		This is the tai clock (CLOCK_TAI) and is derived from the wall-
553		clock time. However, this clock does not experience
554		discontinuities and backwards jumps caused by NTP inserting leap
555		seconds. Since the clock access is designed for use in tracing,
556		side effects are possible. The clock access may yield wrong
557		readouts in case the internal TAI offset is updated e.g., caused
558		by setting the system time or using adjtimex() with an offset.
559		These effects are rare and post processing should be able to
560		handle them. See comments in the ktime_get_tai_fast_ns()
561		function for more information.
562
563	To set a clock, simply echo the clock name into this file::
564
565	  # echo global > trace_clock
566
567	Setting a clock clears the ring buffer content as well as the
568	"snapshot" buffer.
569
570  trace_marker:
571
572	This is a very useful file for synchronizing user space
573	with events happening in the kernel. Writing strings into
574	this file will be written into the ftrace buffer.
575
576	It is useful in applications to open this file at the start
577	of the application and just reference the file descriptor
578	for the file::
579
580		void trace_write(const char *fmt, ...)
581		{
582			va_list ap;
583			char buf[256];
584			int n;
585
586			if (trace_fd < 0)
587				return;
588
589			va_start(ap, fmt);
590			n = vsnprintf(buf, 256, fmt, ap);
591			va_end(ap);
592
593			write(trace_fd, buf, n);
594		}
595
596	start::
597
598		trace_fd = open("trace_marker", O_WRONLY);
599
600	Note: Writing into the trace_marker file can also initiate triggers
601	      that are written into /sys/kernel/tracing/events/ftrace/print/trigger
602	      See "Event triggers" in Documentation/trace/events.rst and an
603              example in Documentation/trace/histogram.rst (Section 3.)
604
605  trace_marker_raw:
606
607	This is similar to trace_marker above, but is meant for binary data
608	to be written to it, where a tool can be used to parse the data
609	from trace_pipe_raw.
610
611  uprobe_events:
612
613	Add dynamic tracepoints in programs.
614	See uprobetracer.rst
615
616  uprobe_profile:
617
618	Uprobe statistics. See uprobetrace.txt
619
620  instances:
621
622	This is a way to make multiple trace buffers where different
623	events can be recorded in different buffers.
624	See "Instances" section below.
625
626  events:
627
628	This is the trace event directory. It holds event tracepoints
629	(also known as static tracepoints) that have been compiled
630	into the kernel. It shows what event tracepoints exist
631	and how they are grouped by system. There are "enable"
632	files at various levels that can enable the tracepoints
633	when a "1" is written to them.
634
635	See events.rst for more information.
636
637  set_event:
638
639	By echoing in the event into this file, will enable that event.
640
641	See events.rst for more information.
642
643  available_events:
644
645	A list of events that can be enabled in tracing.
646
647	See events.rst for more information.
648
649  timestamp_mode:
650
651	Certain tracers may change the timestamp mode used when
652	logging trace events into the event buffer.  Events with
653	different modes can coexist within a buffer but the mode in
654	effect when an event is logged determines which timestamp mode
655	is used for that event.  The default timestamp mode is
656	'delta'.
657
658	Usual timestamp modes for tracing:
659
660	  # cat timestamp_mode
661	  [delta] absolute
662
663	  The timestamp mode with the square brackets around it is the
664	  one in effect.
665
666	  delta: Default timestamp mode - timestamp is a delta against
667	         a per-buffer timestamp.
668
669	  absolute: The timestamp is a full timestamp, not a delta
670                 against some other value.  As such it takes up more
671                 space and is less efficient.
672
673  hwlat_detector:
674
675	Directory for the Hardware Latency Detector.
676	See "Hardware Latency Detector" section below.
677
678  per_cpu:
679
680	This is a directory that contains the trace per_cpu information.
681
682  per_cpu/cpu0/buffer_size_kb:
683
684	The ftrace buffer is defined per_cpu. That is, there's a separate
685	buffer for each CPU to allow writes to be done atomically,
686	and free from cache bouncing. These buffers may have different
687	size buffers. This file is similar to the buffer_size_kb
688	file, but it only displays or sets the buffer size for the
689	specific CPU. (here cpu0).
690
691  per_cpu/cpu0/trace:
692
693	This is similar to the "trace" file, but it will only display
694	the data specific for the CPU. If written to, it only clears
695	the specific CPU buffer.
696
697  per_cpu/cpu0/trace_pipe
698
699	This is similar to the "trace_pipe" file, and is a consuming
700	read, but it will only display (and consume) the data specific
701	for the CPU.
702
703  per_cpu/cpu0/trace_pipe_raw
704
705	For tools that can parse the ftrace ring buffer binary format,
706	the trace_pipe_raw file can be used to extract the data
707	from the ring buffer directly. With the use of the splice()
708	system call, the buffer data can be quickly transferred to
709	a file or to the network where a server is collecting the
710	data.
711
712	Like trace_pipe, this is a consuming reader, where multiple
713	reads will always produce different data.
714
715  per_cpu/cpu0/snapshot:
716
717	This is similar to the main "snapshot" file, but will only
718	snapshot the current CPU (if supported). It only displays
719	the content of the snapshot for a given CPU, and if
720	written to, only clears this CPU buffer.
721
722  per_cpu/cpu0/snapshot_raw:
723
724	Similar to the trace_pipe_raw, but will read the binary format
725	from the snapshot buffer for the given CPU.
726
727  per_cpu/cpu0/stats:
728
729	This displays certain stats about the ring buffer:
730
731	entries:
732		The number of events that are still in the buffer.
733
734	overrun:
735		The number of lost events due to overwriting when
736		the buffer was full.
737
738	commit overrun:
739		Should always be zero.
740		This gets set if so many events happened within a nested
741		event (ring buffer is re-entrant), that it fills the
742		buffer and starts dropping events.
743
744	bytes:
745		Bytes actually read (not overwritten).
746
747	oldest event ts:
748		The oldest timestamp in the buffer
749
750	now ts:
751		The current timestamp
752
753	dropped events:
754		Events lost due to overwrite option being off.
755
756	read events:
757		The number of events read.
758
759The Tracers
760-----------
761
762Here is the list of current tracers that may be configured.
763
764  "function"
765
766	Function call tracer to trace all kernel functions.
767
768  "function_graph"
769
770	Similar to the function tracer except that the
771	function tracer probes the functions on their entry
772	whereas the function graph tracer traces on both entry
773	and exit of the functions. It then provides the ability
774	to draw a graph of function calls similar to C code
775	source.
776
777  "blk"
778
779	The block tracer. The tracer used by the blktrace user
780	application.
781
782  "hwlat"
783
784	The Hardware Latency tracer is used to detect if the hardware
785	produces any latency. See "Hardware Latency Detector" section
786	below.
787
788  "irqsoff"
789
790	Traces the areas that disable interrupts and saves
791	the trace with the longest max latency.
792	See tracing_max_latency. When a new max is recorded,
793	it replaces the old trace. It is best to view this
794	trace with the latency-format option enabled, which
795	happens automatically when the tracer is selected.
796
797  "preemptoff"
798
799	Similar to irqsoff but traces and records the amount of
800	time for which preemption is disabled.
801
802  "preemptirqsoff"
803
804	Similar to irqsoff and preemptoff, but traces and
805	records the largest time for which irqs and/or preemption
806	is disabled.
807
808  "wakeup"
809
810	Traces and records the max latency that it takes for
811	the highest priority task to get scheduled after
812	it has been woken up.
813        Traces all tasks as an average developer would expect.
814
815  "wakeup_rt"
816
817        Traces and records the max latency that it takes for just
818        RT tasks (as the current "wakeup" does). This is useful
819        for those interested in wake up timings of RT tasks.
820
821  "wakeup_dl"
822
823	Traces and records the max latency that it takes for
824	a SCHED_DEADLINE task to be woken (as the "wakeup" and
825	"wakeup_rt" does).
826
827  "mmiotrace"
828
829	A special tracer that is used to trace binary module.
830	It will trace all the calls that a module makes to the
831	hardware. Everything it writes and reads from the I/O
832	as well.
833
834  "branch"
835
836	This tracer can be configured when tracing likely/unlikely
837	calls within the kernel. It will trace when a likely and
838	unlikely branch is hit and if it was correct in its prediction
839	of being correct.
840
841  "nop"
842
843	This is the "trace nothing" tracer. To remove all
844	tracers from tracing simply echo "nop" into
845	current_tracer.
846
847Error conditions
848----------------
849
850  For most ftrace commands, failure modes are obvious and communicated
851  using standard return codes.
852
853  For other more involved commands, extended error information may be
854  available via the tracing/error_log file.  For the commands that
855  support it, reading the tracing/error_log file after an error will
856  display more detailed information about what went wrong, if
857  information is available.  The tracing/error_log file is a circular
858  error log displaying a small number (currently, 8) of ftrace errors
859  for the last (8) failed commands.
860
861  The extended error information and usage takes the form shown in
862  this example::
863
864    # echo xxx > /sys/kernel/tracing/events/sched/sched_wakeup/trigger
865    echo: write error: Invalid argument
866
867    # cat /sys/kernel/tracing/error_log
868    [ 5348.887237] location: error: Couldn't yyy: zzz
869      Command: xxx
870               ^
871    [ 7517.023364] location: error: Bad rrr: sss
872      Command: ppp qqq
873                   ^
874
875  To clear the error log, echo the empty string into it::
876
877    # echo > /sys/kernel/tracing/error_log
878
879Examples of using the tracer
880----------------------------
881
882Here are typical examples of using the tracers when controlling
883them only with the tracefs interface (without using any
884user-land utilities).
885
886Output format:
887--------------
888
889Here is an example of the output format of the file "trace"::
890
891  # tracer: function
892  #
893  # entries-in-buffer/entries-written: 140080/250280   #P:4
894  #
895  #                              _-----=> irqs-off
896  #                             / _----=> need-resched
897  #                            | / _---=> hardirq/softirq
898  #                            || / _--=> preempt-depth
899  #                            ||| /     delay
900  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
901  #              | |       |   ||||       |         |
902              bash-1977  [000] .... 17284.993652: sys_close <-system_call_fastpath
903              bash-1977  [000] .... 17284.993653: __close_fd <-sys_close
904              bash-1977  [000] .... 17284.993653: _raw_spin_lock <-__close_fd
905              sshd-1974  [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
906              bash-1977  [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
907              bash-1977  [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
908              bash-1977  [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
909              bash-1977  [000] .... 17284.993657: filp_close <-__close_fd
910              bash-1977  [000] .... 17284.993657: dnotify_flush <-filp_close
911              sshd-1974  [003] .... 17284.993658: sys_select <-system_call_fastpath
912              ....
913
914A header is printed with the tracer name that is represented by
915the trace. In this case the tracer is "function". Then it shows the
916number of events in the buffer as well as the total number of entries
917that were written. The difference is the number of entries that were
918lost due to the buffer filling up (250280 - 140080 = 110200 events
919lost).
920
921The header explains the content of the events. Task name "bash", the task
922PID "1977", the CPU that it was running on "000", the latency format
923(explained below), the timestamp in <secs>.<usecs> format, the
924function name that was traced "sys_close" and the parent function that
925called this function "system_call_fastpath". The timestamp is the time
926at which the function was entered.
927
928Latency trace format
929--------------------
930
931When the latency-format option is enabled or when one of the latency
932tracers is set, the trace file gives somewhat more information to see
933why a latency happened. Here is a typical trace::
934
935  # tracer: irqsoff
936  #
937  # irqsoff latency trace v1.1.5 on 3.8.0-test+
938  # --------------------------------------------------------------------
939  # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
940  #    -----------------
941  #    | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
942  #    -----------------
943  #  => started at: __lock_task_sighand
944  #  => ended at:   _raw_spin_unlock_irqrestore
945  #
946  #
947  #                  _------=> CPU#
948  #                 / _-----=> irqs-off
949  #                | / _----=> need-resched
950  #                || / _---=> hardirq/softirq
951  #                ||| / _--=> preempt-depth
952  #                |||| /     delay
953  #  cmd     pid   ||||| time  |   caller
954  #     \   /      |||||  \    |   /
955        ps-6143    2d...    0us!: trace_hardirqs_off <-__lock_task_sighand
956        ps-6143    2d..1  259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
957        ps-6143    2d..1  263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
958        ps-6143    2d..1  306us : <stack trace>
959   => trace_hardirqs_on_caller
960   => trace_hardirqs_on
961   => _raw_spin_unlock_irqrestore
962   => do_task_stat
963   => proc_tgid_stat
964   => proc_single_show
965   => seq_read
966   => vfs_read
967   => sys_read
968   => system_call_fastpath
969
970
971This shows that the current tracer is "irqsoff" tracing the time
972for which interrupts were disabled. It gives the trace version (which
973never changes) and the version of the kernel upon which this was executed on
974(3.8). Then it displays the max latency in microseconds (259 us). The number
975of trace entries displayed and the total number (both are four: #4/4).
976VP, KP, SP, and HP are always zero and are reserved for later use.
977#P is the number of online CPUs (#P:4).
978
979The task is the process that was running when the latency
980occurred. (ps pid: 6143).
981
982The start and stop (the functions in which the interrupts were
983disabled and enabled respectively) that caused the latencies:
984
985  - __lock_task_sighand is where the interrupts were disabled.
986  - _raw_spin_unlock_irqrestore is where they were enabled again.
987
988The next lines after the header are the trace itself. The header
989explains which is which.
990
991  cmd: The name of the process in the trace.
992
993  pid: The PID of that process.
994
995  CPU#: The CPU which the process was running on.
996
997  irqs-off: 'd' interrupts are disabled. '.' otherwise.
998	.. caution:: If the architecture does not support a way to
999		read the irq flags variable, an 'X' will always
1000		be printed here.
1001
1002  need-resched:
1003	- 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
1004	- 'n' only TIF_NEED_RESCHED is set,
1005	- 'p' only PREEMPT_NEED_RESCHED is set,
1006	- '.' otherwise.
1007
1008  hardirq/softirq:
1009	- 'Z' - NMI occurred inside a hardirq
1010	- 'z' - NMI is running
1011	- 'H' - hard irq occurred inside a softirq.
1012	- 'h' - hard irq is running
1013	- 's' - soft irq is running
1014	- '.' - normal context.
1015
1016  preempt-depth: The level of preempt_disabled
1017
1018The above is mostly meaningful for kernel developers.
1019
1020  time:
1021	When the latency-format option is enabled, the trace file
1022	output includes a timestamp relative to the start of the
1023	trace. This differs from the output when latency-format
1024	is disabled, which includes an absolute timestamp.
1025
1026  delay:
1027	This is just to help catch your eye a bit better. And
1028	needs to be fixed to be only relative to the same CPU.
1029	The marks are determined by the difference between this
1030	current trace and the next trace.
1031
1032	  - '$' - greater than 1 second
1033	  - '@' - greater than 100 millisecond
1034	  - '*' - greater than 10 millisecond
1035	  - '#' - greater than 1000 microsecond
1036	  - '!' - greater than 100 microsecond
1037	  - '+' - greater than 10 microsecond
1038	  - ' ' - less than or equal to 10 microsecond.
1039
1040  The rest is the same as the 'trace' file.
1041
1042  Note, the latency tracers will usually end with a back trace
1043  to easily find where the latency occurred.
1044
1045trace_options
1046-------------
1047
1048The trace_options file (or the options directory) is used to control
1049what gets printed in the trace output, or manipulate the tracers.
1050To see what is available, simply cat the file::
1051
1052  cat trace_options
1053	print-parent
1054	nosym-offset
1055	nosym-addr
1056	noverbose
1057	noraw
1058	nohex
1059	nobin
1060	noblock
1061	nofields
1062	trace_printk
1063	annotate
1064	nouserstacktrace
1065	nosym-userobj
1066	noprintk-msg-only
1067	context-info
1068	nolatency-format
1069	record-cmd
1070	norecord-tgid
1071	overwrite
1072	nodisable_on_free
1073	irq-info
1074	markers
1075	noevent-fork
1076	function-trace
1077	nofunction-fork
1078	nodisplay-graph
1079	nostacktrace
1080	nobranch
1081
1082To disable one of the options, echo in the option prepended with
1083"no"::
1084
1085  echo noprint-parent > trace_options
1086
1087To enable an option, leave off the "no"::
1088
1089  echo sym-offset > trace_options
1090
1091Here are the available options:
1092
1093  print-parent
1094	On function traces, display the calling (parent)
1095	function as well as the function being traced.
1096	::
1097
1098	  print-parent:
1099	   bash-4000  [01]  1477.606694: simple_strtoul <-kstrtoul
1100
1101	  noprint-parent:
1102	   bash-4000  [01]  1477.606694: simple_strtoul
1103
1104
1105  sym-offset
1106	Display not only the function name, but also the
1107	offset in the function. For example, instead of
1108	seeing just "ktime_get", you will see
1109	"ktime_get+0xb/0x20".
1110	::
1111
1112	  sym-offset:
1113	   bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0
1114
1115  sym-addr
1116	This will also display the function address as well
1117	as the function name.
1118	::
1119
1120	  sym-addr:
1121	   bash-4000  [01]  1477.606694: simple_strtoul <c0339346>
1122
1123  verbose
1124	This deals with the trace file when the
1125        latency-format option is enabled.
1126	::
1127
1128	    bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
1129	    (+0.000ms): simple_strtoul (kstrtoul)
1130
1131  raw
1132	This will display raw numbers. This option is best for
1133	use with user applications that can translate the raw
1134	numbers better than having it done in the kernel.
1135
1136  hex
1137	Similar to raw, but the numbers will be in a hexadecimal format.
1138
1139  bin
1140	This will print out the formats in raw binary.
1141
1142  block
1143	When set, reading trace_pipe will not block when polled.
1144
1145  fields
1146	Print the fields as described by their types. This is a better
1147	option than using hex, bin or raw, as it gives a better parsing
1148	of the content of the event.
1149
1150  trace_printk
1151	Can disable trace_printk() from writing into the buffer.
1152
1153  annotate
1154	It is sometimes confusing when the CPU buffers are full
1155	and one CPU buffer had a lot of events recently, thus
1156	a shorter time frame, were another CPU may have only had
1157	a few events, which lets it have older events. When
1158	the trace is reported, it shows the oldest events first,
1159	and it may look like only one CPU ran (the one with the
1160	oldest events). When the annotate option is set, it will
1161	display when a new CPU buffer started::
1162
1163			  <idle>-0     [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
1164			  <idle>-0     [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
1165			  <idle>-0     [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
1166		##### CPU 2 buffer started ####
1167			  <idle>-0     [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
1168			  <idle>-0     [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
1169			  <idle>-0     [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
1170
1171  userstacktrace
1172	This option changes the trace. It records a
1173	stacktrace of the current user space thread after
1174	each trace event.
1175
1176  sym-userobj
1177	when user stacktrace are enabled, look up which
1178	object the address belongs to, and print a
1179	relative address. This is especially useful when
1180	ASLR is on, otherwise you don't get a chance to
1181	resolve the address to object/file/line after
1182	the app is no longer running
1183
1184	The lookup is performed when you read
1185	trace,trace_pipe. Example::
1186
1187		  a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
1188		  x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
1189
1190
1191  printk-msg-only
1192	When set, trace_printk()s will only show the format
1193	and not their parameters (if trace_bprintk() or
1194	trace_bputs() was used to save the trace_printk()).
1195
1196  context-info
1197	Show only the event data. Hides the comm, PID,
1198	timestamp, CPU, and other useful data.
1199
1200  latency-format
1201	This option changes the trace output. When it is enabled,
1202	the trace displays additional information about the
1203	latency, as described in "Latency trace format".
1204
1205  pause-on-trace
1206	When set, opening the trace file for read, will pause
1207	writing to the ring buffer (as if tracing_on was set to zero).
1208	This simulates the original behavior of the trace file.
1209	When the file is closed, tracing will be enabled again.
1210
1211  hash-ptr
1212        When set, "%p" in the event printk format displays the
1213        hashed pointer value instead of real address.
1214        This will be useful if you want to find out which hashed
1215        value is corresponding to the real value in trace log.
1216
1217  record-cmd
1218	When any event or tracer is enabled, a hook is enabled
1219	in the sched_switch trace point to fill comm cache
1220	with mapped pids and comms. But this may cause some
1221	overhead, and if you only care about pids, and not the
1222	name of the task, disabling this option can lower the
1223	impact of tracing. See "saved_cmdlines".
1224
1225  record-tgid
1226	When any event or tracer is enabled, a hook is enabled
1227	in the sched_switch trace point to fill the cache of
1228	mapped Thread Group IDs (TGID) mapping to pids. See
1229	"saved_tgids".
1230
1231  overwrite
1232	This controls what happens when the trace buffer is
1233	full. If "1" (default), the oldest events are
1234	discarded and overwritten. If "0", then the newest
1235	events are discarded.
1236	(see per_cpu/cpu0/stats for overrun and dropped)
1237
1238  disable_on_free
1239	When the free_buffer is closed, tracing will
1240	stop (tracing_on set to 0).
1241
1242  irq-info
1243	Shows the interrupt, preempt count, need resched data.
1244	When disabled, the trace looks like::
1245
1246		# tracer: function
1247		#
1248		# entries-in-buffer/entries-written: 144405/9452052   #P:4
1249		#
1250		#           TASK-PID   CPU#      TIMESTAMP  FUNCTION
1251		#              | |       |          |         |
1252			  <idle>-0     [002]  23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
1253			  <idle>-0     [002]  23636.756054: activate_task <-ttwu_do_activate.constprop.89
1254			  <idle>-0     [002]  23636.756055: enqueue_task <-activate_task
1255
1256
1257  markers
1258	When set, the trace_marker is writable (only by root).
1259	When disabled, the trace_marker will error with EINVAL
1260	on write.
1261
1262  event-fork
1263	When set, tasks with PIDs listed in set_event_pid will have
1264	the PIDs of their children added to set_event_pid when those
1265	tasks fork. Also, when tasks with PIDs in set_event_pid exit,
1266	their PIDs will be removed from the file.
1267
1268        This affects PIDs listed in set_event_notrace_pid as well.
1269
1270  function-trace
1271	The latency tracers will enable function tracing
1272	if this option is enabled (default it is). When
1273	it is disabled, the latency tracers do not trace
1274	functions. This keeps the overhead of the tracer down
1275	when performing latency tests.
1276
1277  function-fork
1278	When set, tasks with PIDs listed in set_ftrace_pid will
1279	have the PIDs of their children added to set_ftrace_pid
1280	when those tasks fork. Also, when tasks with PIDs in
1281	set_ftrace_pid exit, their PIDs will be removed from the
1282	file.
1283
1284        This affects PIDs in set_ftrace_notrace_pid as well.
1285
1286  display-graph
1287	When set, the latency tracers (irqsoff, wakeup, etc) will
1288	use function graph tracing instead of function tracing.
1289
1290  stacktrace
1291	When set, a stack trace is recorded after any trace event
1292	is recorded.
1293
1294  branch
1295	Enable branch tracing with the tracer. This enables branch
1296	tracer along with the currently set tracer. Enabling this
1297	with the "nop" tracer is the same as just enabling the
1298	"branch" tracer.
1299
1300.. tip:: Some tracers have their own options. They only appear in this
1301       file when the tracer is active. They always appear in the
1302       options directory.
1303
1304
1305Here are the per tracer options:
1306
1307Options for function tracer:
1308
1309  func_stack_trace
1310	When set, a stack trace is recorded after every
1311	function that is recorded. NOTE! Limit the functions
1312	that are recorded before enabling this, with
1313	"set_ftrace_filter" otherwise the system performance
1314	will be critically degraded. Remember to disable
1315	this option before clearing the function filter.
1316
1317Options for function_graph tracer:
1318
1319 Since the function_graph tracer has a slightly different output
1320 it has its own options to control what is displayed.
1321
1322  funcgraph-overrun
1323	When set, the "overrun" of the graph stack is
1324	displayed after each function traced. The
1325	overrun, is when the stack depth of the calls
1326	is greater than what is reserved for each task.
1327	Each task has a fixed array of functions to
1328	trace in the call graph. If the depth of the
1329	calls exceeds that, the function is not traced.
1330	The overrun is the number of functions missed
1331	due to exceeding this array.
1332
1333  funcgraph-cpu
1334	When set, the CPU number of the CPU where the trace
1335	occurred is displayed.
1336
1337  funcgraph-overhead
1338	When set, if the function takes longer than
1339	A certain amount, then a delay marker is
1340	displayed. See "delay" above, under the
1341	header description.
1342
1343  funcgraph-proc
1344	Unlike other tracers, the process' command line
1345	is not displayed by default, but instead only
1346	when a task is traced in and out during a context
1347	switch. Enabling this options has the command
1348	of each process displayed at every line.
1349
1350  funcgraph-duration
1351	At the end of each function (the return)
1352	the duration of the amount of time in the
1353	function is displayed in microseconds.
1354
1355  funcgraph-abstime
1356	When set, the timestamp is displayed at each line.
1357
1358  funcgraph-irqs
1359	When disabled, functions that happen inside an
1360	interrupt will not be traced.
1361
1362  funcgraph-tail
1363	When set, the return event will include the function
1364	that it represents. By default this is off, and
1365	only a closing curly bracket "}" is displayed for
1366	the return of a function.
1367
1368  funcgraph-retval
1369	When set, the return value of each traced function
1370	will be printed after an equal sign "=". By default
1371	this is off.
1372
1373  funcgraph-retval-hex
1374	When set, the return value will always be printed
1375	in hexadecimal format. If the option is not set and
1376	the return value is an error code, it will be printed
1377	in signed decimal format; otherwise it will also be
1378	printed in hexadecimal format. By default, this option
1379	is off.
1380
1381  sleep-time
1382	When running function graph tracer, to include
1383	the time a task schedules out in its function.
1384	When enabled, it will account time the task has been
1385	scheduled out as part of the function call.
1386
1387  graph-time
1388	When running function profiler with function graph tracer,
1389	to include the time to call nested functions. When this is
1390	not set, the time reported for the function will only
1391	include the time the function itself executed for, not the
1392	time for functions that it called.
1393
1394Options for blk tracer:
1395
1396  blk_classic
1397	Shows a more minimalistic output.
1398
1399
1400irqsoff
1401-------
1402
1403When interrupts are disabled, the CPU can not react to any other
1404external event (besides NMIs and SMIs). This prevents the timer
1405interrupt from triggering or the mouse interrupt from letting
1406the kernel know of a new mouse event. The result is a latency
1407with the reaction time.
1408
1409The irqsoff tracer tracks the time for which interrupts are
1410disabled. When a new maximum latency is hit, the tracer saves
1411the trace leading up to that latency point so that every time a
1412new maximum is reached, the old saved trace is discarded and the
1413new trace is saved.
1414
1415To reset the maximum, echo 0 into tracing_max_latency. Here is
1416an example::
1417
1418  # echo 0 > options/function-trace
1419  # echo irqsoff > current_tracer
1420  # echo 1 > tracing_on
1421  # echo 0 > tracing_max_latency
1422  # ls -ltr
1423  [...]
1424  # echo 0 > tracing_on
1425  # cat trace
1426  # tracer: irqsoff
1427  #
1428  # irqsoff latency trace v1.1.5 on 3.8.0-test+
1429  # --------------------------------------------------------------------
1430  # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1431  #    -----------------
1432  #    | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
1433  #    -----------------
1434  #  => started at: run_timer_softirq
1435  #  => ended at:   run_timer_softirq
1436  #
1437  #
1438  #                  _------=> CPU#
1439  #                 / _-----=> irqs-off
1440  #                | / _----=> need-resched
1441  #                || / _---=> hardirq/softirq
1442  #                ||| / _--=> preempt-depth
1443  #                |||| /     delay
1444  #  cmd     pid   ||||| time  |   caller
1445  #     \   /      |||||  \    |   /
1446    <idle>-0       0d.s2    0us+: _raw_spin_lock_irq <-run_timer_softirq
1447    <idle>-0       0dNs3   17us : _raw_spin_unlock_irq <-run_timer_softirq
1448    <idle>-0       0dNs3   17us+: trace_hardirqs_on <-run_timer_softirq
1449    <idle>-0       0dNs3   25us : <stack trace>
1450   => _raw_spin_unlock_irq
1451   => run_timer_softirq
1452   => __do_softirq
1453   => call_softirq
1454   => do_softirq
1455   => irq_exit
1456   => smp_apic_timer_interrupt
1457   => apic_timer_interrupt
1458   => rcu_idle_exit
1459   => cpu_idle
1460   => rest_init
1461   => start_kernel
1462   => x86_64_start_reservations
1463   => x86_64_start_kernel
1464
1465Here we see that we had a latency of 16 microseconds (which is
1466very good). The _raw_spin_lock_irq in run_timer_softirq disabled
1467interrupts. The difference between the 16 and the displayed
1468timestamp 25us occurred because the clock was incremented
1469between the time of recording the max latency and the time of
1470recording the function that had that latency.
1471
1472Note the above example had function-trace not set. If we set
1473function-trace, we get a much larger output::
1474
1475 with echo 1 > options/function-trace
1476
1477  # tracer: irqsoff
1478  #
1479  # irqsoff latency trace v1.1.5 on 3.8.0-test+
1480  # --------------------------------------------------------------------
1481  # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1482  #    -----------------
1483  #    | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
1484  #    -----------------
1485  #  => started at: ata_scsi_queuecmd
1486  #  => ended at:   ata_scsi_queuecmd
1487  #
1488  #
1489  #                  _------=> CPU#
1490  #                 / _-----=> irqs-off
1491  #                | / _----=> need-resched
1492  #                || / _---=> hardirq/softirq
1493  #                ||| / _--=> preempt-depth
1494  #                |||| /     delay
1495  #  cmd     pid   ||||| time  |   caller
1496  #     \   /      |||||  \    |   /
1497      bash-2042    3d...    0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1498      bash-2042    3d...    0us : add_preempt_count <-_raw_spin_lock_irqsave
1499      bash-2042    3d..1    1us : ata_scsi_find_dev <-ata_scsi_queuecmd
1500      bash-2042    3d..1    1us : __ata_scsi_find_dev <-ata_scsi_find_dev
1501      bash-2042    3d..1    2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
1502      bash-2042    3d..1    2us : ata_qc_new_init <-__ata_scsi_queuecmd
1503      bash-2042    3d..1    3us : ata_sg_init <-__ata_scsi_queuecmd
1504      bash-2042    3d..1    4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
1505      bash-2042    3d..1    4us : ata_build_rw_tf <-ata_scsi_rw_xlat
1506  [...]
1507      bash-2042    3d..1   67us : delay_tsc <-__delay
1508      bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
1509      bash-2042    3d..2   67us : sub_preempt_count <-delay_tsc
1510      bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
1511      bash-2042    3d..2   68us : sub_preempt_count <-delay_tsc
1512      bash-2042    3d..1   68us+: ata_bmdma_start <-ata_bmdma_qc_issue
1513      bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1514      bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1515      bash-2042    3d..1   72us+: trace_hardirqs_on <-ata_scsi_queuecmd
1516      bash-2042    3d..1  120us : <stack trace>
1517   => _raw_spin_unlock_irqrestore
1518   => ata_scsi_queuecmd
1519   => scsi_dispatch_cmd
1520   => scsi_request_fn
1521   => __blk_run_queue_uncond
1522   => __blk_run_queue
1523   => blk_queue_bio
1524   => submit_bio_noacct
1525   => submit_bio
1526   => submit_bh
1527   => __ext3_get_inode_loc
1528   => ext3_iget
1529   => ext3_lookup
1530   => lookup_real
1531   => __lookup_hash
1532   => walk_component
1533   => lookup_last
1534   => path_lookupat
1535   => filename_lookup
1536   => user_path_at_empty
1537   => user_path_at
1538   => vfs_fstatat
1539   => vfs_stat
1540   => sys_newstat
1541   => system_call_fastpath
1542
1543
1544Here we traced a 71 microsecond latency. But we also see all the
1545functions that were called during that time. Note that by
1546enabling function tracing, we incur an added overhead. This
1547overhead may extend the latency times. But nevertheless, this
1548trace has provided some very helpful debugging information.
1549
1550If we prefer function graph output instead of function, we can set
1551display-graph option::
1552
1553 with echo 1 > options/display-graph
1554
1555  # tracer: irqsoff
1556  #
1557  # irqsoff latency trace v1.1.5 on 4.20.0-rc6+
1558  # --------------------------------------------------------------------
1559  # latency: 3751 us, #274/274, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4)
1560  #    -----------------
1561  #    | task: bash-1507 (uid:0 nice:0 policy:0 rt_prio:0)
1562  #    -----------------
1563  #  => started at: free_debug_processing
1564  #  => ended at:   return_to_handler
1565  #
1566  #
1567  #                                       _-----=> irqs-off
1568  #                                      / _----=> need-resched
1569  #                                     | / _---=> hardirq/softirq
1570  #                                     || / _--=> preempt-depth
1571  #                                     ||| /
1572  #   REL TIME      CPU  TASK/PID       ||||     DURATION                  FUNCTION CALLS
1573  #      |          |     |    |        ||||      |   |                     |   |   |   |
1574          0 us |   0)   bash-1507    |  d... |   0.000 us    |  _raw_spin_lock_irqsave();
1575          0 us |   0)   bash-1507    |  d..1 |   0.378 us    |    do_raw_spin_trylock();
1576          1 us |   0)   bash-1507    |  d..2 |               |    set_track() {
1577          2 us |   0)   bash-1507    |  d..2 |               |      save_stack_trace() {
1578          2 us |   0)   bash-1507    |  d..2 |               |        __save_stack_trace() {
1579          3 us |   0)   bash-1507    |  d..2 |               |          __unwind_start() {
1580          3 us |   0)   bash-1507    |  d..2 |               |            get_stack_info() {
1581          3 us |   0)   bash-1507    |  d..2 |   0.351 us    |              in_task_stack();
1582          4 us |   0)   bash-1507    |  d..2 |   1.107 us    |            }
1583  [...]
1584       3750 us |   0)   bash-1507    |  d..1 |   0.516 us    |      do_raw_spin_unlock();
1585       3750 us |   0)   bash-1507    |  d..1 |   0.000 us    |  _raw_spin_unlock_irqrestore();
1586       3764 us |   0)   bash-1507    |  d..1 |   0.000 us    |  tracer_hardirqs_on();
1587      bash-1507    0d..1 3792us : <stack trace>
1588   => free_debug_processing
1589   => __slab_free
1590   => kmem_cache_free
1591   => vm_area_free
1592   => remove_vma
1593   => exit_mmap
1594   => mmput
1595   => begin_new_exec
1596   => load_elf_binary
1597   => search_binary_handler
1598   => __do_execve_file.isra.32
1599   => __x64_sys_execve
1600   => do_syscall_64
1601   => entry_SYSCALL_64_after_hwframe
1602
1603preemptoff
1604----------
1605
1606When preemption is disabled, we may be able to receive
1607interrupts but the task cannot be preempted and a higher
1608priority task must wait for preemption to be enabled again
1609before it can preempt a lower priority task.
1610
1611The preemptoff tracer traces the places that disable preemption.
1612Like the irqsoff tracer, it records the maximum latency for
1613which preemption was disabled. The control of preemptoff tracer
1614is much like the irqsoff tracer.
1615::
1616
1617  # echo 0 > options/function-trace
1618  # echo preemptoff > current_tracer
1619  # echo 1 > tracing_on
1620  # echo 0 > tracing_max_latency
1621  # ls -ltr
1622  [...]
1623  # echo 0 > tracing_on
1624  # cat trace
1625  # tracer: preemptoff
1626  #
1627  # preemptoff latency trace v1.1.5 on 3.8.0-test+
1628  # --------------------------------------------------------------------
1629  # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1630  #    -----------------
1631  #    | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
1632  #    -----------------
1633  #  => started at: do_IRQ
1634  #  => ended at:   do_IRQ
1635  #
1636  #
1637  #                  _------=> CPU#
1638  #                 / _-----=> irqs-off
1639  #                | / _----=> need-resched
1640  #                || / _---=> hardirq/softirq
1641  #                ||| / _--=> preempt-depth
1642  #                |||| /     delay
1643  #  cmd     pid   ||||| time  |   caller
1644  #     \   /      |||||  \    |   /
1645      sshd-1991    1d.h.    0us+: irq_enter <-do_IRQ
1646      sshd-1991    1d..1   46us : irq_exit <-do_IRQ
1647      sshd-1991    1d..1   47us+: trace_preempt_on <-do_IRQ
1648      sshd-1991    1d..1   52us : <stack trace>
1649   => sub_preempt_count
1650   => irq_exit
1651   => do_IRQ
1652   => ret_from_intr
1653
1654
1655This has some more changes. Preemption was disabled when an
1656interrupt came in (notice the 'h'), and was enabled on exit.
1657But we also see that interrupts have been disabled when entering
1658the preempt off section and leaving it (the 'd'). We do not know if
1659interrupts were enabled in the mean time or shortly after this
1660was over.
1661::
1662
1663  # tracer: preemptoff
1664  #
1665  # preemptoff latency trace v1.1.5 on 3.8.0-test+
1666  # --------------------------------------------------------------------
1667  # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1668  #    -----------------
1669  #    | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
1670  #    -----------------
1671  #  => started at: wake_up_new_task
1672  #  => ended at:   task_rq_unlock
1673  #
1674  #
1675  #                  _------=> CPU#
1676  #                 / _-----=> irqs-off
1677  #                | / _----=> need-resched
1678  #                || / _---=> hardirq/softirq
1679  #                ||| / _--=> preempt-depth
1680  #                |||| /     delay
1681  #  cmd     pid   ||||| time  |   caller
1682  #     \   /      |||||  \    |   /
1683      bash-1994    1d..1    0us : _raw_spin_lock_irqsave <-wake_up_new_task
1684      bash-1994    1d..1    0us : select_task_rq_fair <-select_task_rq
1685      bash-1994    1d..1    1us : __rcu_read_lock <-select_task_rq_fair
1686      bash-1994    1d..1    1us : source_load <-select_task_rq_fair
1687      bash-1994    1d..1    1us : source_load <-select_task_rq_fair
1688  [...]
1689      bash-1994    1d..1   12us : irq_enter <-smp_apic_timer_interrupt
1690      bash-1994    1d..1   12us : rcu_irq_enter <-irq_enter
1691      bash-1994    1d..1   13us : add_preempt_count <-irq_enter
1692      bash-1994    1d.h1   13us : exit_idle <-smp_apic_timer_interrupt
1693      bash-1994    1d.h1   13us : hrtimer_interrupt <-smp_apic_timer_interrupt
1694      bash-1994    1d.h1   13us : _raw_spin_lock <-hrtimer_interrupt
1695      bash-1994    1d.h1   14us : add_preempt_count <-_raw_spin_lock
1696      bash-1994    1d.h2   14us : ktime_get_update_offsets <-hrtimer_interrupt
1697  [...]
1698      bash-1994    1d.h1   35us : lapic_next_event <-clockevents_program_event
1699      bash-1994    1d.h1   35us : irq_exit <-smp_apic_timer_interrupt
1700      bash-1994    1d.h1   36us : sub_preempt_count <-irq_exit
1701      bash-1994    1d..2   36us : do_softirq <-irq_exit
1702      bash-1994    1d..2   36us : __do_softirq <-call_softirq
1703      bash-1994    1d..2   36us : __local_bh_disable <-__do_softirq
1704      bash-1994    1d.s2   37us : add_preempt_count <-_raw_spin_lock_irq
1705      bash-1994    1d.s3   38us : _raw_spin_unlock <-run_timer_softirq
1706      bash-1994    1d.s3   39us : sub_preempt_count <-_raw_spin_unlock
1707      bash-1994    1d.s2   39us : call_timer_fn <-run_timer_softirq
1708  [...]
1709      bash-1994    1dNs2   81us : cpu_needs_another_gp <-rcu_process_callbacks
1710      bash-1994    1dNs2   82us : __local_bh_enable <-__do_softirq
1711      bash-1994    1dNs2   82us : sub_preempt_count <-__local_bh_enable
1712      bash-1994    1dN.2   82us : idle_cpu <-irq_exit
1713      bash-1994    1dN.2   83us : rcu_irq_exit <-irq_exit
1714      bash-1994    1dN.2   83us : sub_preempt_count <-irq_exit
1715      bash-1994    1.N.1   84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
1716      bash-1994    1.N.1   84us+: trace_preempt_on <-task_rq_unlock
1717      bash-1994    1.N.1  104us : <stack trace>
1718   => sub_preempt_count
1719   => _raw_spin_unlock_irqrestore
1720   => task_rq_unlock
1721   => wake_up_new_task
1722   => do_fork
1723   => sys_clone
1724   => stub_clone
1725
1726
1727The above is an example of the preemptoff trace with
1728function-trace set. Here we see that interrupts were not disabled
1729the entire time. The irq_enter code lets us know that we entered
1730an interrupt 'h'. Before that, the functions being traced still
1731show that it is not in an interrupt, but we can see from the
1732functions themselves that this is not the case.
1733
1734preemptirqsoff
1735--------------
1736
1737Knowing the locations that have interrupts disabled or
1738preemption disabled for the longest times is helpful. But
1739sometimes we would like to know when either preemption and/or
1740interrupts are disabled.
1741
1742Consider the following code::
1743
1744    local_irq_disable();
1745    call_function_with_irqs_off();
1746    preempt_disable();
1747    call_function_with_irqs_and_preemption_off();
1748    local_irq_enable();
1749    call_function_with_preemption_off();
1750    preempt_enable();
1751
1752The irqsoff tracer will record the total length of
1753call_function_with_irqs_off() and
1754call_function_with_irqs_and_preemption_off().
1755
1756The preemptoff tracer will record the total length of
1757call_function_with_irqs_and_preemption_off() and
1758call_function_with_preemption_off().
1759
1760But neither will trace the time that interrupts and/or
1761preemption is disabled. This total time is the time that we can
1762not schedule. To record this time, use the preemptirqsoff
1763tracer.
1764
1765Again, using this trace is much like the irqsoff and preemptoff
1766tracers.
1767::
1768
1769  # echo 0 > options/function-trace
1770  # echo preemptirqsoff > current_tracer
1771  # echo 1 > tracing_on
1772  # echo 0 > tracing_max_latency
1773  # ls -ltr
1774  [...]
1775  # echo 0 > tracing_on
1776  # cat trace
1777  # tracer: preemptirqsoff
1778  #
1779  # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1780  # --------------------------------------------------------------------
1781  # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1782  #    -----------------
1783  #    | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
1784  #    -----------------
1785  #  => started at: ata_scsi_queuecmd
1786  #  => ended at:   ata_scsi_queuecmd
1787  #
1788  #
1789  #                  _------=> CPU#
1790  #                 / _-----=> irqs-off
1791  #                | / _----=> need-resched
1792  #                || / _---=> hardirq/softirq
1793  #                ||| / _--=> preempt-depth
1794  #                |||| /     delay
1795  #  cmd     pid   ||||| time  |   caller
1796  #     \   /      |||||  \    |   /
1797        ls-2230    3d...    0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1798        ls-2230    3...1  100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1799        ls-2230    3...1  101us+: trace_preempt_on <-ata_scsi_queuecmd
1800        ls-2230    3...1  111us : <stack trace>
1801   => sub_preempt_count
1802   => _raw_spin_unlock_irqrestore
1803   => ata_scsi_queuecmd
1804   => scsi_dispatch_cmd
1805   => scsi_request_fn
1806   => __blk_run_queue_uncond
1807   => __blk_run_queue
1808   => blk_queue_bio
1809   => submit_bio_noacct
1810   => submit_bio
1811   => submit_bh
1812   => ext3_bread
1813   => ext3_dir_bread
1814   => htree_dirblock_to_tree
1815   => ext3_htree_fill_tree
1816   => ext3_readdir
1817   => vfs_readdir
1818   => sys_getdents
1819   => system_call_fastpath
1820
1821
1822The trace_hardirqs_off_thunk is called from assembly on x86 when
1823interrupts are disabled in the assembly code. Without the
1824function tracing, we do not know if interrupts were enabled
1825within the preemption points. We do see that it started with
1826preemption enabled.
1827
1828Here is a trace with function-trace set::
1829
1830  # tracer: preemptirqsoff
1831  #
1832  # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1833  # --------------------------------------------------------------------
1834  # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1835  #    -----------------
1836  #    | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
1837  #    -----------------
1838  #  => started at: schedule
1839  #  => ended at:   mutex_unlock
1840  #
1841  #
1842  #                  _------=> CPU#
1843  #                 / _-----=> irqs-off
1844  #                | / _----=> need-resched
1845  #                || / _---=> hardirq/softirq
1846  #                ||| / _--=> preempt-depth
1847  #                |||| /     delay
1848  #  cmd     pid   ||||| time  |   caller
1849  #     \   /      |||||  \    |   /
1850  kworker/-59      3...1    0us : __schedule <-schedule
1851  kworker/-59      3d..1    0us : rcu_preempt_qs <-rcu_note_context_switch
1852  kworker/-59      3d..1    1us : add_preempt_count <-_raw_spin_lock_irq
1853  kworker/-59      3d..2    1us : deactivate_task <-__schedule
1854  kworker/-59      3d..2    1us : dequeue_task <-deactivate_task
1855  kworker/-59      3d..2    2us : update_rq_clock <-dequeue_task
1856  kworker/-59      3d..2    2us : dequeue_task_fair <-dequeue_task
1857  kworker/-59      3d..2    2us : update_curr <-dequeue_task_fair
1858  kworker/-59      3d..2    2us : update_min_vruntime <-update_curr
1859  kworker/-59      3d..2    3us : cpuacct_charge <-update_curr
1860  kworker/-59      3d..2    3us : __rcu_read_lock <-cpuacct_charge
1861  kworker/-59      3d..2    3us : __rcu_read_unlock <-cpuacct_charge
1862  kworker/-59      3d..2    3us : update_cfs_rq_blocked_load <-dequeue_task_fair
1863  kworker/-59      3d..2    4us : clear_buddies <-dequeue_task_fair
1864  kworker/-59      3d..2    4us : account_entity_dequeue <-dequeue_task_fair
1865  kworker/-59      3d..2    4us : update_min_vruntime <-dequeue_task_fair
1866  kworker/-59      3d..2    4us : update_cfs_shares <-dequeue_task_fair
1867  kworker/-59      3d..2    5us : hrtick_update <-dequeue_task_fair
1868  kworker/-59      3d..2    5us : wq_worker_sleeping <-__schedule
1869  kworker/-59      3d..2    5us : kthread_data <-wq_worker_sleeping
1870  kworker/-59      3d..2    5us : put_prev_task_fair <-__schedule
1871  kworker/-59      3d..2    6us : pick_next_task_fair <-pick_next_task
1872  kworker/-59      3d..2    6us : clear_buddies <-pick_next_task_fair
1873  kworker/-59      3d..2    6us : set_next_entity <-pick_next_task_fair
1874  kworker/-59      3d..2    6us : update_stats_wait_end <-set_next_entity
1875        ls-2269    3d..2    7us : finish_task_switch <-__schedule
1876        ls-2269    3d..2    7us : _raw_spin_unlock_irq <-finish_task_switch
1877        ls-2269    3d..2    8us : do_IRQ <-ret_from_intr
1878        ls-2269    3d..2    8us : irq_enter <-do_IRQ
1879        ls-2269    3d..2    8us : rcu_irq_enter <-irq_enter
1880        ls-2269    3d..2    9us : add_preempt_count <-irq_enter
1881        ls-2269    3d.h2    9us : exit_idle <-do_IRQ
1882  [...]
1883        ls-2269    3d.h3   20us : sub_preempt_count <-_raw_spin_unlock
1884        ls-2269    3d.h2   20us : irq_exit <-do_IRQ
1885        ls-2269    3d.h2   21us : sub_preempt_count <-irq_exit
1886        ls-2269    3d..3   21us : do_softirq <-irq_exit
1887        ls-2269    3d..3   21us : __do_softirq <-call_softirq
1888        ls-2269    3d..3   21us+: __local_bh_disable <-__do_softirq
1889        ls-2269    3d.s4   29us : sub_preempt_count <-_local_bh_enable_ip
1890        ls-2269    3d.s5   29us : sub_preempt_count <-_local_bh_enable_ip
1891        ls-2269    3d.s5   31us : do_IRQ <-ret_from_intr
1892        ls-2269    3d.s5   31us : irq_enter <-do_IRQ
1893        ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
1894  [...]
1895        ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
1896        ls-2269    3d.s5   32us : add_preempt_count <-irq_enter
1897        ls-2269    3d.H5   32us : exit_idle <-do_IRQ
1898        ls-2269    3d.H5   32us : handle_irq <-do_IRQ
1899        ls-2269    3d.H5   32us : irq_to_desc <-handle_irq
1900        ls-2269    3d.H5   33us : handle_fasteoi_irq <-handle_irq
1901  [...]
1902        ls-2269    3d.s5  158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
1903        ls-2269    3d.s3  158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
1904        ls-2269    3d.s3  159us : __local_bh_enable <-__do_softirq
1905        ls-2269    3d.s3  159us : sub_preempt_count <-__local_bh_enable
1906        ls-2269    3d..3  159us : idle_cpu <-irq_exit
1907        ls-2269    3d..3  159us : rcu_irq_exit <-irq_exit
1908        ls-2269    3d..3  160us : sub_preempt_count <-irq_exit
1909        ls-2269    3d...  161us : __mutex_unlock_slowpath <-mutex_unlock
1910        ls-2269    3d...  162us+: trace_hardirqs_on <-mutex_unlock
1911        ls-2269    3d...  186us : <stack trace>
1912   => __mutex_unlock_slowpath
1913   => mutex_unlock
1914   => process_output
1915   => n_tty_write
1916   => tty_write
1917   => vfs_write
1918   => sys_write
1919   => system_call_fastpath
1920
1921This is an interesting trace. It started with kworker running and
1922scheduling out and ls taking over. But as soon as ls released the
1923rq lock and enabled interrupts (but not preemption) an interrupt
1924triggered. When the interrupt finished, it started running softirqs.
1925But while the softirq was running, another interrupt triggered.
1926When an interrupt is running inside a softirq, the annotation is 'H'.
1927
1928
1929wakeup
1930------
1931
1932One common case that people are interested in tracing is the
1933time it takes for a task that is woken to actually wake up.
1934Now for non Real-Time tasks, this can be arbitrary. But tracing
1935it none the less can be interesting.
1936
1937Without function tracing::
1938
1939  # echo 0 > options/function-trace
1940  # echo wakeup > current_tracer
1941  # echo 1 > tracing_on
1942  # echo 0 > tracing_max_latency
1943  # chrt -f 5 sleep 1
1944  # echo 0 > tracing_on
1945  # cat trace
1946  # tracer: wakeup
1947  #
1948  # wakeup latency trace v1.1.5 on 3.8.0-test+
1949  # --------------------------------------------------------------------
1950  # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1951  #    -----------------
1952  #    | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
1953  #    -----------------
1954  #
1955  #                  _------=> CPU#
1956  #                 / _-----=> irqs-off
1957  #                | / _----=> need-resched
1958  #                || / _---=> hardirq/softirq
1959  #                ||| / _--=> preempt-depth
1960  #                |||| /     delay
1961  #  cmd     pid   ||||| time  |   caller
1962  #     \   /      |||||  \    |   /
1963    <idle>-0       3dNs7    0us :      0:120:R   + [003]   312:100:R kworker/3:1H
1964    <idle>-0       3dNs7    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1965    <idle>-0       3d..3   15us : __schedule <-schedule
1966    <idle>-0       3d..3   15us :      0:120:R ==> [003]   312:100:R kworker/3:1H
1967
1968The tracer only traces the highest priority task in the system
1969to avoid tracing the normal circumstances. Here we see that
1970the kworker with a nice priority of -20 (not very nice), took
1971just 15 microseconds from the time it woke up, to the time it
1972ran.
1973
1974Non Real-Time tasks are not that interesting. A more interesting
1975trace is to concentrate only on Real-Time tasks.
1976
1977wakeup_rt
1978---------
1979
1980In a Real-Time environment it is very important to know the
1981wakeup time it takes for the highest priority task that is woken
1982up to the time that it executes. This is also known as "schedule
1983latency". I stress the point that this is about RT tasks. It is
1984also important to know the scheduling latency of non-RT tasks,
1985but the average schedule latency is better for non-RT tasks.
1986Tools like LatencyTop are more appropriate for such
1987measurements.
1988
1989Real-Time environments are interested in the worst case latency.
1990That is the longest latency it takes for something to happen,
1991and not the average. We can have a very fast scheduler that may
1992only have a large latency once in a while, but that would not
1993work well with Real-Time tasks.  The wakeup_rt tracer was designed
1994to record the worst case wakeups of RT tasks. Non-RT tasks are
1995not recorded because the tracer only records one worst case and
1996tracing non-RT tasks that are unpredictable will overwrite the
1997worst case latency of RT tasks (just run the normal wakeup
1998tracer for a while to see that effect).
1999
2000Since this tracer only deals with RT tasks, we will run this
2001slightly differently than we did with the previous tracers.
2002Instead of performing an 'ls', we will run 'sleep 1' under
2003'chrt' which changes the priority of the task.
2004::
2005
2006  # echo 0 > options/function-trace
2007  # echo wakeup_rt > current_tracer
2008  # echo 1 > tracing_on
2009  # echo 0 > tracing_max_latency
2010  # chrt -f 5 sleep 1
2011  # echo 0 > tracing_on
2012  # cat trace
2013  # tracer: wakeup
2014  #
2015  # tracer: wakeup_rt
2016  #
2017  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
2018  # --------------------------------------------------------------------
2019  # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
2020  #    -----------------
2021  #    | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
2022  #    -----------------
2023  #
2024  #                  _------=> CPU#
2025  #                 / _-----=> irqs-off
2026  #                | / _----=> need-resched
2027  #                || / _---=> hardirq/softirq
2028  #                ||| / _--=> preempt-depth
2029  #                |||| /     delay
2030  #  cmd     pid   ||||| time  |   caller
2031  #     \   /      |||||  \    |   /
2032    <idle>-0       3d.h4    0us :      0:120:R   + [003]  2389: 94:R sleep
2033    <idle>-0       3d.h4    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
2034    <idle>-0       3d..3    5us : __schedule <-schedule
2035    <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
2036
2037
2038Running this on an idle system, we see that it only took 5 microseconds
2039to perform the task switch.  Note, since the trace point in the schedule
2040is before the actual "switch", we stop the tracing when the recorded task
2041is about to schedule in. This may change if we add a new marker at the
2042end of the scheduler.
2043
2044Notice that the recorded task is 'sleep' with the PID of 2389
2045and it has an rt_prio of 5. This priority is user-space priority
2046and not the internal kernel priority. The policy is 1 for
2047SCHED_FIFO and 2 for SCHED_RR.
2048
2049Note, that the trace data shows the internal priority (99 - rtprio).
2050::
2051
2052  <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
2053
2054The 0:120:R means idle was running with a nice priority of 0 (120 - 120)
2055and in the running state 'R'. The sleep task was scheduled in with
20562389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
2057and it too is in the running state.
2058
2059Doing the same with chrt -r 5 and function-trace set.
2060::
2061
2062  echo 1 > options/function-trace
2063
2064  # tracer: wakeup_rt
2065  #
2066  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
2067  # --------------------------------------------------------------------
2068  # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
2069  #    -----------------
2070  #    | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
2071  #    -----------------
2072  #
2073  #                  _------=> CPU#
2074  #                 / _-----=> irqs-off
2075  #                | / _----=> need-resched
2076  #                || / _---=> hardirq/softirq
2077  #                ||| / _--=> preempt-depth
2078  #                |||| /     delay
2079  #  cmd     pid   ||||| time  |   caller
2080  #     \   /      |||||  \    |   /
2081    <idle>-0       3d.h4    1us+:      0:120:R   + [003]  2448: 94:R sleep
2082    <idle>-0       3d.h4    2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
2083    <idle>-0       3d.h3    3us : check_preempt_curr <-ttwu_do_wakeup
2084    <idle>-0       3d.h3    3us : resched_curr <-check_preempt_curr
2085    <idle>-0       3dNh3    4us : task_woken_rt <-ttwu_do_wakeup
2086    <idle>-0       3dNh3    4us : _raw_spin_unlock <-try_to_wake_up
2087    <idle>-0       3dNh3    4us : sub_preempt_count <-_raw_spin_unlock
2088    <idle>-0       3dNh2    5us : ttwu_stat <-try_to_wake_up
2089    <idle>-0       3dNh2    5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
2090    <idle>-0       3dNh2    6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
2091    <idle>-0       3dNh1    6us : _raw_spin_lock <-__run_hrtimer
2092    <idle>-0       3dNh1    6us : add_preempt_count <-_raw_spin_lock
2093    <idle>-0       3dNh2    7us : _raw_spin_unlock <-hrtimer_interrupt
2094    <idle>-0       3dNh2    7us : sub_preempt_count <-_raw_spin_unlock
2095    <idle>-0       3dNh1    7us : tick_program_event <-hrtimer_interrupt
2096    <idle>-0       3dNh1    7us : clockevents_program_event <-tick_program_event
2097    <idle>-0       3dNh1    8us : ktime_get <-clockevents_program_event
2098    <idle>-0       3dNh1    8us : lapic_next_event <-clockevents_program_event
2099    <idle>-0       3dNh1    8us : irq_exit <-smp_apic_timer_interrupt
2100    <idle>-0       3dNh1    9us : sub_preempt_count <-irq_exit
2101    <idle>-0       3dN.2    9us : idle_cpu <-irq_exit
2102    <idle>-0       3dN.2    9us : rcu_irq_exit <-irq_exit
2103    <idle>-0       3dN.2   10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
2104    <idle>-0       3dN.2   10us : sub_preempt_count <-irq_exit
2105    <idle>-0       3.N.1   11us : rcu_idle_exit <-cpu_idle
2106    <idle>-0       3dN.1   11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
2107    <idle>-0       3.N.1   11us : tick_nohz_idle_exit <-cpu_idle
2108    <idle>-0       3dN.1   12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
2109    <idle>-0       3dN.1   12us : ktime_get <-tick_nohz_idle_exit
2110    <idle>-0       3dN.1   12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
2111    <idle>-0       3dN.1   13us : cpu_load_update_nohz <-tick_nohz_idle_exit
2112    <idle>-0       3dN.1   13us : _raw_spin_lock <-cpu_load_update_nohz
2113    <idle>-0       3dN.1   13us : add_preempt_count <-_raw_spin_lock
2114    <idle>-0       3dN.2   13us : __cpu_load_update <-cpu_load_update_nohz
2115    <idle>-0       3dN.2   14us : sched_avg_update <-__cpu_load_update
2116    <idle>-0       3dN.2   14us : _raw_spin_unlock <-cpu_load_update_nohz
2117    <idle>-0       3dN.2   14us : sub_preempt_count <-_raw_spin_unlock
2118    <idle>-0       3dN.1   15us : calc_load_nohz_stop <-tick_nohz_idle_exit
2119    <idle>-0       3dN.1   15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
2120    <idle>-0       3dN.1   15us : hrtimer_cancel <-tick_nohz_idle_exit
2121    <idle>-0       3dN.1   15us : hrtimer_try_to_cancel <-hrtimer_cancel
2122    <idle>-0       3dN.1   16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
2123    <idle>-0       3dN.1   16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
2124    <idle>-0       3dN.1   16us : add_preempt_count <-_raw_spin_lock_irqsave
2125    <idle>-0       3dN.2   17us : __remove_hrtimer <-remove_hrtimer.part.16
2126    <idle>-0       3dN.2   17us : hrtimer_force_reprogram <-__remove_hrtimer
2127    <idle>-0       3dN.2   17us : tick_program_event <-hrtimer_force_reprogram
2128    <idle>-0       3dN.2   18us : clockevents_program_event <-tick_program_event
2129    <idle>-0       3dN.2   18us : ktime_get <-clockevents_program_event
2130    <idle>-0       3dN.2   18us : lapic_next_event <-clockevents_program_event
2131    <idle>-0       3dN.2   19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
2132    <idle>-0       3dN.2   19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
2133    <idle>-0       3dN.1   19us : hrtimer_forward <-tick_nohz_idle_exit
2134    <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
2135    <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
2136    <idle>-0       3dN.1   20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
2137    <idle>-0       3dN.1   20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
2138    <idle>-0       3dN.1   21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
2139    <idle>-0       3dN.1   21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
2140    <idle>-0       3dN.1   21us : add_preempt_count <-_raw_spin_lock_irqsave
2141    <idle>-0       3dN.2   22us : ktime_add_safe <-__hrtimer_start_range_ns
2142    <idle>-0       3dN.2   22us : enqueue_hrtimer <-__hrtimer_start_range_ns
2143    <idle>-0       3dN.2   22us : tick_program_event <-__hrtimer_start_range_ns
2144    <idle>-0       3dN.2   23us : clockevents_program_event <-tick_program_event
2145    <idle>-0       3dN.2   23us : ktime_get <-clockevents_program_event
2146    <idle>-0       3dN.2   23us : lapic_next_event <-clockevents_program_event
2147    <idle>-0       3dN.2   24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
2148    <idle>-0       3dN.2   24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
2149    <idle>-0       3dN.1   24us : account_idle_ticks <-tick_nohz_idle_exit
2150    <idle>-0       3dN.1   24us : account_idle_time <-account_idle_ticks
2151    <idle>-0       3.N.1   25us : sub_preempt_count <-cpu_idle
2152    <idle>-0       3.N..   25us : schedule <-cpu_idle
2153    <idle>-0       3.N..   25us : __schedule <-preempt_schedule
2154    <idle>-0       3.N..   26us : add_preempt_count <-__schedule
2155    <idle>-0       3.N.1   26us : rcu_note_context_switch <-__schedule
2156    <idle>-0       3.N.1   26us : rcu_sched_qs <-rcu_note_context_switch
2157    <idle>-0       3dN.1   27us : rcu_preempt_qs <-rcu_note_context_switch
2158    <idle>-0       3.N.1   27us : _raw_spin_lock_irq <-__schedule
2159    <idle>-0       3dN.1   27us : add_preempt_count <-_raw_spin_lock_irq
2160    <idle>-0       3dN.2   28us : put_prev_task_idle <-__schedule
2161    <idle>-0       3dN.2   28us : pick_next_task_stop <-pick_next_task
2162    <idle>-0       3dN.2   28us : pick_next_task_rt <-pick_next_task
2163    <idle>-0       3dN.2   29us : dequeue_pushable_task <-pick_next_task_rt
2164    <idle>-0       3d..3   29us : __schedule <-preempt_schedule
2165    <idle>-0       3d..3   30us :      0:120:R ==> [003]  2448: 94:R sleep
2166
2167This isn't that big of a trace, even with function tracing enabled,
2168so I included the entire trace.
2169
2170The interrupt went off while when the system was idle. Somewhere
2171before task_woken_rt() was called, the NEED_RESCHED flag was set,
2172this is indicated by the first occurrence of the 'N' flag.
2173
2174Latency tracing and events
2175--------------------------
2176As function tracing can induce a much larger latency, but without
2177seeing what happens within the latency it is hard to know what
2178caused it. There is a middle ground, and that is with enabling
2179events.
2180::
2181
2182  # echo 0 > options/function-trace
2183  # echo wakeup_rt > current_tracer
2184  # echo 1 > events/enable
2185  # echo 1 > tracing_on
2186  # echo 0 > tracing_max_latency
2187  # chrt -f 5 sleep 1
2188  # echo 0 > tracing_on
2189  # cat trace
2190  # tracer: wakeup_rt
2191  #
2192  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
2193  # --------------------------------------------------------------------
2194  # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
2195  #    -----------------
2196  #    | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
2197  #    -----------------
2198  #
2199  #                  _------=> CPU#
2200  #                 / _-----=> irqs-off
2201  #                | / _----=> need-resched
2202  #                || / _---=> hardirq/softirq
2203  #                ||| / _--=> preempt-depth
2204  #                |||| /     delay
2205  #  cmd     pid   ||||| time  |   caller
2206  #     \   /      |||||  \    |   /
2207    <idle>-0       2d.h4    0us :      0:120:R   + [002]  5882: 94:R sleep
2208    <idle>-0       2d.h4    0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
2209    <idle>-0       2d.h4    1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
2210    <idle>-0       2dNh2    1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
2211    <idle>-0       2.N.2    2us : power_end: cpu_id=2
2212    <idle>-0       2.N.2    3us : cpu_idle: state=4294967295 cpu_id=2
2213    <idle>-0       2dN.3    4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
2214    <idle>-0       2dN.3    4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
2215    <idle>-0       2.N.2    5us : rcu_utilization: Start context switch
2216    <idle>-0       2.N.2    5us : rcu_utilization: End context switch
2217    <idle>-0       2d..3    6us : __schedule <-schedule
2218    <idle>-0       2d..3    6us :      0:120:R ==> [002]  5882: 94:R sleep
2219
2220
2221Hardware Latency Detector
2222-------------------------
2223
2224The hardware latency detector is executed by enabling the "hwlat" tracer.
2225
2226NOTE, this tracer will affect the performance of the system as it will
2227periodically make a CPU constantly busy with interrupts disabled.
2228::
2229
2230  # echo hwlat > current_tracer
2231  # sleep 100
2232  # cat trace
2233  # tracer: hwlat
2234  #
2235  # entries-in-buffer/entries-written: 13/13   #P:8
2236  #
2237  #                              _-----=> irqs-off
2238  #                             / _----=> need-resched
2239  #                            | / _---=> hardirq/softirq
2240  #                            || / _--=> preempt-depth
2241  #                            ||| /     delay
2242  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2243  #              | |       |   ||||       |         |
2244             <...>-1729  [001] d...   678.473449: #1     inner/outer(us):   11/12    ts:1581527483.343962693 count:6
2245             <...>-1729  [004] d...   689.556542: #2     inner/outer(us):   16/9     ts:1581527494.889008092 count:1
2246             <...>-1729  [005] d...   714.756290: #3     inner/outer(us):   16/16    ts:1581527519.678961629 count:5
2247             <...>-1729  [001] d...   718.788247: #4     inner/outer(us):    9/17    ts:1581527523.889012713 count:1
2248             <...>-1729  [002] d...   719.796341: #5     inner/outer(us):   13/9     ts:1581527524.912872606 count:1
2249             <...>-1729  [006] d...   844.787091: #6     inner/outer(us):    9/12    ts:1581527649.889048502 count:2
2250             <...>-1729  [003] d...   849.827033: #7     inner/outer(us):   18/9     ts:1581527654.889013793 count:1
2251             <...>-1729  [007] d...   853.859002: #8     inner/outer(us):    9/12    ts:1581527658.889065736 count:1
2252             <...>-1729  [001] d...   855.874978: #9     inner/outer(us):    9/11    ts:1581527660.861991877 count:1
2253             <...>-1729  [001] d...   863.938932: #10    inner/outer(us):    9/11    ts:1581527668.970010500 count:1 nmi-total:7 nmi-count:1
2254             <...>-1729  [007] d...   878.050780: #11    inner/outer(us):    9/12    ts:1581527683.385002600 count:1 nmi-total:5 nmi-count:1
2255             <...>-1729  [007] d...   886.114702: #12    inner/outer(us):    9/12    ts:1581527691.385001600 count:1
2256
2257
2258The above output is somewhat the same in the header. All events will have
2259interrupts disabled 'd'. Under the FUNCTION title there is:
2260
2261 #1
2262	This is the count of events recorded that were greater than the
2263	tracing_threshold (See below).
2264
2265 inner/outer(us):   11/11
2266
2267      This shows two numbers as "inner latency" and "outer latency". The test
2268      runs in a loop checking a timestamp twice. The latency detected within
2269      the two timestamps is the "inner latency" and the latency detected
2270      after the previous timestamp and the next timestamp in the loop is
2271      the "outer latency".
2272
2273 ts:1581527483.343962693
2274
2275      The absolute timestamp that the first latency was recorded in the window.
2276
2277 count:6
2278
2279      The number of times a latency was detected during the window.
2280
2281 nmi-total:7 nmi-count:1
2282
2283      On architectures that support it, if an NMI comes in during the
2284      test, the time spent in NMI is reported in "nmi-total" (in
2285      microseconds).
2286
2287      All architectures that have NMIs will show the "nmi-count" if an
2288      NMI comes in during the test.
2289
2290hwlat files:
2291
2292  tracing_threshold
2293	This gets automatically set to "10" to represent 10
2294	microseconds. This is the threshold of latency that
2295	needs to be detected before the trace will be recorded.
2296
2297	Note, when hwlat tracer is finished (another tracer is
2298	written into "current_tracer"), the original value for
2299	tracing_threshold is placed back into this file.
2300
2301  hwlat_detector/width
2302	The length of time the test runs with interrupts disabled.
2303
2304  hwlat_detector/window
2305	The length of time of the window which the test
2306	runs. That is, the test will run for "width"
2307	microseconds per "window" microseconds
2308
2309  tracing_cpumask
2310	When the test is started. A kernel thread is created that
2311	runs the test. This thread will alternate between CPUs
2312	listed in the tracing_cpumask between each period
2313	(one "window"). To limit the test to specific CPUs
2314	set the mask in this file to only the CPUs that the test
2315	should run on.
2316
2317function
2318--------
2319
2320This tracer is the function tracer. Enabling the function tracer
2321can be done from the debug file system. Make sure the
2322ftrace_enabled is set; otherwise this tracer is a nop.
2323See the "ftrace_enabled" section below.
2324::
2325
2326  # sysctl kernel.ftrace_enabled=1
2327  # echo function > current_tracer
2328  # echo 1 > tracing_on
2329  # usleep 1
2330  # echo 0 > tracing_on
2331  # cat trace
2332  # tracer: function
2333  #
2334  # entries-in-buffer/entries-written: 24799/24799   #P:4
2335  #
2336  #                              _-----=> irqs-off
2337  #                             / _----=> need-resched
2338  #                            | / _---=> hardirq/softirq
2339  #                            || / _--=> preempt-depth
2340  #                            ||| /     delay
2341  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2342  #              | |       |   ||||       |         |
2343              bash-1994  [002] ....  3082.063030: mutex_unlock <-rb_simple_write
2344              bash-1994  [002] ....  3082.063031: __mutex_unlock_slowpath <-mutex_unlock
2345              bash-1994  [002] ....  3082.063031: __fsnotify_parent <-fsnotify_modify
2346              bash-1994  [002] ....  3082.063032: fsnotify <-fsnotify_modify
2347              bash-1994  [002] ....  3082.063032: __srcu_read_lock <-fsnotify
2348              bash-1994  [002] ....  3082.063032: add_preempt_count <-__srcu_read_lock
2349              bash-1994  [002] ...1  3082.063032: sub_preempt_count <-__srcu_read_lock
2350              bash-1994  [002] ....  3082.063033: __srcu_read_unlock <-fsnotify
2351  [...]
2352
2353
2354Note: function tracer uses ring buffers to store the above
2355entries. The newest data may overwrite the oldest data.
2356Sometimes using echo to stop the trace is not sufficient because
2357the tracing could have overwritten the data that you wanted to
2358record. For this reason, it is sometimes better to disable
2359tracing directly from a program. This allows you to stop the
2360tracing at the point that you hit the part that you are
2361interested in. To disable the tracing directly from a C program,
2362something like following code snippet can be used::
2363
2364	int trace_fd;
2365	[...]
2366	int main(int argc, char *argv[]) {
2367		[...]
2368		trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
2369		[...]
2370		if (condition_hit()) {
2371			write(trace_fd, "0", 1);
2372		}
2373		[...]
2374	}
2375
2376
2377Single thread tracing
2378---------------------
2379
2380By writing into set_ftrace_pid you can trace a
2381single thread. For example::
2382
2383  # cat set_ftrace_pid
2384  no pid
2385  # echo 3111 > set_ftrace_pid
2386  # cat set_ftrace_pid
2387  3111
2388  # echo function > current_tracer
2389  # cat trace | head
2390  # tracer: function
2391  #
2392  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
2393  #              | |       |          |         |
2394      yum-updatesd-3111  [003]  1637.254676: finish_task_switch <-thread_return
2395      yum-updatesd-3111  [003]  1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
2396      yum-updatesd-3111  [003]  1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
2397      yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
2398      yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
2399      yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll
2400  # echo > set_ftrace_pid
2401  # cat trace |head
2402  # tracer: function
2403  #
2404  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
2405  #              | |       |          |         |
2406  ##### CPU 3 buffer started ####
2407      yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
2408      yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
2409      yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
2410      yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
2411      yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit
2412
2413If you want to trace a function when executing, you could use
2414something like this simple program.
2415::
2416
2417	#include <stdio.h>
2418	#include <stdlib.h>
2419	#include <sys/types.h>
2420	#include <sys/stat.h>
2421	#include <fcntl.h>
2422	#include <unistd.h>
2423	#include <string.h>
2424
2425	#define _STR(x) #x
2426	#define STR(x) _STR(x)
2427	#define MAX_PATH 256
2428
2429	const char *find_tracefs(void)
2430	{
2431	       static char tracefs[MAX_PATH+1];
2432	       static int tracefs_found;
2433	       char type[100];
2434	       FILE *fp;
2435
2436	       if (tracefs_found)
2437		       return tracefs;
2438
2439	       if ((fp = fopen("/proc/mounts","r")) == NULL) {
2440		       perror("/proc/mounts");
2441		       return NULL;
2442	       }
2443
2444	       while (fscanf(fp, "%*s %"
2445		             STR(MAX_PATH)
2446		             "s %99s %*s %*d %*d\n",
2447		             tracefs, type) == 2) {
2448		       if (strcmp(type, "tracefs") == 0)
2449		               break;
2450	       }
2451	       fclose(fp);
2452
2453	       if (strcmp(type, "tracefs") != 0) {
2454		       fprintf(stderr, "tracefs not mounted");
2455		       return NULL;
2456	       }
2457
2458	       strcat(tracefs, "/tracing/");
2459	       tracefs_found = 1;
2460
2461	       return tracefs;
2462	}
2463
2464	const char *tracing_file(const char *file_name)
2465	{
2466	       static char trace_file[MAX_PATH+1];
2467	       snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name);
2468	       return trace_file;
2469	}
2470
2471	int main (int argc, char **argv)
2472	{
2473		if (argc < 1)
2474		        exit(-1);
2475
2476		if (fork() > 0) {
2477		        int fd, ffd;
2478		        char line[64];
2479		        int s;
2480
2481		        ffd = open(tracing_file("current_tracer"), O_WRONLY);
2482		        if (ffd < 0)
2483		                exit(-1);
2484		        write(ffd, "nop", 3);
2485
2486		        fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
2487		        s = sprintf(line, "%d\n", getpid());
2488		        write(fd, line, s);
2489
2490		        write(ffd, "function", 8);
2491
2492		        close(fd);
2493		        close(ffd);
2494
2495		        execvp(argv[1], argv+1);
2496		}
2497
2498		return 0;
2499	}
2500
2501Or this simple script!
2502::
2503
2504  #!/bin/bash
2505
2506  tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts`
2507  echo 0 > $tracefs/tracing_on
2508  echo $$ > $tracefs/set_ftrace_pid
2509  echo function > $tracefs/current_tracer
2510  echo 1 > $tracefs/tracing_on
2511  exec "$@"
2512
2513
2514function graph tracer
2515---------------------------
2516
2517This tracer is similar to the function tracer except that it
2518probes a function on its entry and its exit. This is done by
2519using a dynamically allocated stack of return addresses in each
2520task_struct. On function entry the tracer overwrites the return
2521address of each function traced to set a custom probe. Thus the
2522original return address is stored on the stack of return address
2523in the task_struct.
2524
2525Probing on both ends of a function leads to special features
2526such as:
2527
2528- measure of a function's time execution
2529- having a reliable call stack to draw function calls graph
2530
2531This tracer is useful in several situations:
2532
2533- you want to find the reason of a strange kernel behavior and
2534  need to see what happens in detail on any areas (or specific
2535  ones).
2536
2537- you are experiencing weird latencies but it's difficult to
2538  find its origin.
2539
2540- you want to find quickly which path is taken by a specific
2541  function
2542
2543- you just want to peek inside a working kernel and want to see
2544  what happens there.
2545
2546::
2547
2548  # tracer: function_graph
2549  #
2550  # CPU  DURATION                  FUNCTION CALLS
2551  # |     |   |                     |   |   |   |
2552
2553   0)               |  sys_open() {
2554   0)               |    do_sys_open() {
2555   0)               |      getname() {
2556   0)               |        kmem_cache_alloc() {
2557   0)   1.382 us    |          __might_sleep();
2558   0)   2.478 us    |        }
2559   0)               |        strncpy_from_user() {
2560   0)               |          might_fault() {
2561   0)   1.389 us    |            __might_sleep();
2562   0)   2.553 us    |          }
2563   0)   3.807 us    |        }
2564   0)   7.876 us    |      }
2565   0)               |      alloc_fd() {
2566   0)   0.668 us    |        _spin_lock();
2567   0)   0.570 us    |        expand_files();
2568   0)   0.586 us    |        _spin_unlock();
2569
2570
2571There are several columns that can be dynamically
2572enabled/disabled. You can use every combination of options you
2573want, depending on your needs.
2574
2575- The cpu number on which the function executed is default
2576  enabled.  It is sometimes better to only trace one cpu (see
2577  tracing_cpu_mask file) or you might sometimes see unordered
2578  function calls while cpu tracing switch.
2579
2580	- hide: echo nofuncgraph-cpu > trace_options
2581	- show: echo funcgraph-cpu > trace_options
2582
2583- The duration (function's time of execution) is displayed on
2584  the closing bracket line of a function or on the same line
2585  than the current function in case of a leaf one. It is default
2586  enabled.
2587
2588	- hide: echo nofuncgraph-duration > trace_options
2589	- show: echo funcgraph-duration > trace_options
2590
2591- The overhead field precedes the duration field in case of
2592  reached duration thresholds.
2593
2594	- hide: echo nofuncgraph-overhead > trace_options
2595	- show: echo funcgraph-overhead > trace_options
2596	- depends on: funcgraph-duration
2597
2598  ie::
2599
2600    3) # 1837.709 us |          } /* __switch_to */
2601    3)               |          finish_task_switch() {
2602    3)   0.313 us    |            _raw_spin_unlock_irq();
2603    3)   3.177 us    |          }
2604    3) # 1889.063 us |        } /* __schedule */
2605    3) ! 140.417 us  |      } /* __schedule */
2606    3) # 2034.948 us |    } /* schedule */
2607    3) * 33998.59 us |  } /* schedule_preempt_disabled */
2608
2609    [...]
2610
2611    1)   0.260 us    |              msecs_to_jiffies();
2612    1)   0.313 us    |              __rcu_read_unlock();
2613    1) + 61.770 us   |            }
2614    1) + 64.479 us   |          }
2615    1)   0.313 us    |          rcu_bh_qs();
2616    1)   0.313 us    |          __local_bh_enable();
2617    1) ! 217.240 us  |        }
2618    1)   0.365 us    |        idle_cpu();
2619    1)               |        rcu_irq_exit() {
2620    1)   0.417 us    |          rcu_eqs_enter_common.isra.47();
2621    1)   3.125 us    |        }
2622    1) ! 227.812 us  |      }
2623    1) ! 457.395 us  |    }
2624    1) @ 119760.2 us |  }
2625
2626    [...]
2627
2628    2)               |    handle_IPI() {
2629    1)   6.979 us    |                  }
2630    2)   0.417 us    |      scheduler_ipi();
2631    1)   9.791 us    |                }
2632    1) + 12.917 us   |              }
2633    2)   3.490 us    |    }
2634    1) + 15.729 us   |            }
2635    1) + 18.542 us   |          }
2636    2) $ 3594274 us  |  }
2637
2638Flags::
2639
2640  + means that the function exceeded 10 usecs.
2641  ! means that the function exceeded 100 usecs.
2642  # means that the function exceeded 1000 usecs.
2643  * means that the function exceeded 10 msecs.
2644  @ means that the function exceeded 100 msecs.
2645  $ means that the function exceeded 1 sec.
2646
2647
2648- The task/pid field displays the thread cmdline and pid which
2649  executed the function. It is default disabled.
2650
2651	- hide: echo nofuncgraph-proc > trace_options
2652	- show: echo funcgraph-proc > trace_options
2653
2654  ie::
2655
2656    # tracer: function_graph
2657    #
2658    # CPU  TASK/PID        DURATION                  FUNCTION CALLS
2659    # |    |    |           |   |                     |   |   |   |
2660    0)    sh-4802     |               |                  d_free() {
2661    0)    sh-4802     |               |                    call_rcu() {
2662    0)    sh-4802     |               |                      __call_rcu() {
2663    0)    sh-4802     |   0.616 us    |                        rcu_process_gp_end();
2664    0)    sh-4802     |   0.586 us    |                        check_for_new_grace_period();
2665    0)    sh-4802     |   2.899 us    |                      }
2666    0)    sh-4802     |   4.040 us    |                    }
2667    0)    sh-4802     |   5.151 us    |                  }
2668    0)    sh-4802     | + 49.370 us   |                }
2669
2670
2671- The absolute time field is an absolute timestamp given by the
2672  system clock since it started. A snapshot of this time is
2673  given on each entry/exit of functions
2674
2675	- hide: echo nofuncgraph-abstime > trace_options
2676	- show: echo funcgraph-abstime > trace_options
2677
2678  ie::
2679
2680    #
2681    #      TIME       CPU  DURATION                  FUNCTION CALLS
2682    #       |         |     |   |                     |   |   |   |
2683    360.774522 |   1)   0.541 us    |                                          }
2684    360.774522 |   1)   4.663 us    |                                        }
2685    360.774523 |   1)   0.541 us    |                                        __wake_up_bit();
2686    360.774524 |   1)   6.796 us    |                                      }
2687    360.774524 |   1)   7.952 us    |                                    }
2688    360.774525 |   1)   9.063 us    |                                  }
2689    360.774525 |   1)   0.615 us    |                                  journal_mark_dirty();
2690    360.774527 |   1)   0.578 us    |                                  __brelse();
2691    360.774528 |   1)               |                                  reiserfs_prepare_for_journal() {
2692    360.774528 |   1)               |                                    unlock_buffer() {
2693    360.774529 |   1)               |                                      wake_up_bit() {
2694    360.774529 |   1)               |                                        bit_waitqueue() {
2695    360.774530 |   1)   0.594 us    |                                          __phys_addr();
2696
2697
2698The function name is always displayed after the closing bracket
2699for a function if the start of that function is not in the
2700trace buffer.
2701
2702Display of the function name after the closing bracket may be
2703enabled for functions whose start is in the trace buffer,
2704allowing easier searching with grep for function durations.
2705It is default disabled.
2706
2707	- hide: echo nofuncgraph-tail > trace_options
2708	- show: echo funcgraph-tail > trace_options
2709
2710  Example with nofuncgraph-tail (default)::
2711
2712    0)               |      putname() {
2713    0)               |        kmem_cache_free() {
2714    0)   0.518 us    |          __phys_addr();
2715    0)   1.757 us    |        }
2716    0)   2.861 us    |      }
2717
2718  Example with funcgraph-tail::
2719
2720    0)               |      putname() {
2721    0)               |        kmem_cache_free() {
2722    0)   0.518 us    |          __phys_addr();
2723    0)   1.757 us    |        } /* kmem_cache_free() */
2724    0)   2.861 us    |      } /* putname() */
2725
2726The return value of each traced function can be displayed after
2727an equal sign "=". When encountering system call failures, it
2728can be very helpful to quickly locate the function that first
2729returns an error code.
2730
2731	- hide: echo nofuncgraph-retval > trace_options
2732	- show: echo funcgraph-retval > trace_options
2733
2734  Example with funcgraph-retval::
2735
2736    1)               |    cgroup_migrate() {
2737    1)   0.651 us    |      cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
2738    1)               |      cgroup_migrate_execute() {
2739    1)               |        cpu_cgroup_can_attach() {
2740    1)               |          cgroup_taskset_first() {
2741    1)   0.732 us    |            cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
2742    1)   1.232 us    |          } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
2743    1)   0.380 us    |          sched_rt_can_attach(); /* = 0x0 */
2744    1)   2.335 us    |        } /* cpu_cgroup_can_attach = -22 */
2745    1)   4.369 us    |      } /* cgroup_migrate_execute = -22 */
2746    1)   7.143 us    |    } /* cgroup_migrate = -22 */
2747
2748The above example shows that the function cpu_cgroup_can_attach
2749returned the error code -22 firstly, then we can read the code
2750of this function to get the root cause.
2751
2752When the option funcgraph-retval-hex is not set, the return value can
2753be displayed in a smart way. Specifically, if it is an error code,
2754it will be printed in signed decimal format, otherwise it will
2755printed in hexadecimal format.
2756
2757	- smart: echo nofuncgraph-retval-hex > trace_options
2758	- hexadecimal: echo funcgraph-retval-hex > trace_options
2759
2760  Example with funcgraph-retval-hex::
2761
2762    1)               |      cgroup_migrate() {
2763    1)   0.651 us    |        cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
2764    1)               |        cgroup_migrate_execute() {
2765    1)               |          cpu_cgroup_can_attach() {
2766    1)               |            cgroup_taskset_first() {
2767    1)   0.732 us    |              cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
2768    1)   1.232 us    |            } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
2769    1)   0.380 us    |            sched_rt_can_attach(); /* = 0x0 */
2770    1)   2.335 us    |          } /* cpu_cgroup_can_attach = 0xffffffea */
2771    1)   4.369 us    |        } /* cgroup_migrate_execute = 0xffffffea */
2772    1)   7.143 us    |      } /* cgroup_migrate = 0xffffffea */
2773
2774At present, there are some limitations when using the funcgraph-retval
2775option, and these limitations will be eliminated in the future:
2776
2777- Even if the function return type is void, a return value will still
2778  be printed, and you can just ignore it.
2779
2780- Even if return values are stored in multiple registers, only the
2781  value contained in the first register will be recorded and printed.
2782  To illustrate, in the x86 architecture, eax and edx are used to store
2783  a 64-bit return value, with the lower 32 bits saved in eax and the
2784  upper 32 bits saved in edx. However, only the value stored in eax
2785  will be recorded and printed.
2786
2787- In certain procedure call standards, such as arm64's AAPCS64, when a
2788  type is smaller than a GPR, it is the responsibility of the consumer
2789  to perform the narrowing, and the upper bits may contain UNKNOWN values.
2790  Therefore, it is advisable to check the code for such cases. For instance,
2791  when using a u8 in a 64-bit GPR, bits [63:8] may contain arbitrary values,
2792  especially when larger types are truncated, whether explicitly or implicitly.
2793  Here are some specific cases to illustrate this point:
2794
2795  **Case One**:
2796
2797  The function narrow_to_u8 is defined as follows::
2798
2799	u8 narrow_to_u8(u64 val)
2800	{
2801		// implicitly truncated
2802		return val;
2803	}
2804
2805  It may be compiled to::
2806
2807	narrow_to_u8:
2808		< ... ftrace instrumentation ... >
2809		RET
2810
2811  If you pass 0x123456789abcdef to this function and want to narrow it,
2812  it may be recorded as 0x123456789abcdef instead of 0xef.
2813
2814  **Case Two**:
2815
2816  The function error_if_not_4g_aligned is defined as follows::
2817
2818	int error_if_not_4g_aligned(u64 val)
2819	{
2820		if (val & GENMASK(31, 0))
2821			return -EINVAL;
2822
2823		return 0;
2824	}
2825
2826  It could be compiled to::
2827
2828	error_if_not_4g_aligned:
2829		CBNZ    w0, .Lnot_aligned
2830		RET			// bits [31:0] are zero, bits
2831					// [63:32] are UNKNOWN
2832	.Lnot_aligned:
2833		MOV    x0, #-EINVAL
2834		RET
2835
2836  When passing 0x2_0000_0000 to it, the return value may be recorded as
2837  0x2_0000_0000 instead of 0.
2838
2839You can put some comments on specific functions by using
2840trace_printk() For example, if you want to put a comment inside
2841the __might_sleep() function, you just have to include
2842<linux/ftrace.h> and call trace_printk() inside __might_sleep()::
2843
2844	trace_printk("I'm a comment!\n")
2845
2846will produce::
2847
2848   1)               |             __might_sleep() {
2849   1)               |                /* I'm a comment! */
2850   1)   1.449 us    |             }
2851
2852
2853You might find other useful features for this tracer in the
2854following "dynamic ftrace" section such as tracing only specific
2855functions or tasks.
2856
2857dynamic ftrace
2858--------------
2859
2860If CONFIG_DYNAMIC_FTRACE is set, the system will run with
2861virtually no overhead when function tracing is disabled. The way
2862this works is the mcount function call (placed at the start of
2863every kernel function, produced by the -pg switch in gcc),
2864starts of pointing to a simple return. (Enabling FTRACE will
2865include the -pg switch in the compiling of the kernel.)
2866
2867At compile time every C file object is run through the
2868recordmcount program (located in the scripts directory). This
2869program will parse the ELF headers in the C object to find all
2870the locations in the .text section that call mcount. Starting
2871with gcc version 4.6, the -mfentry has been added for x86, which
2872calls "__fentry__" instead of "mcount". Which is called before
2873the creation of the stack frame.
2874
2875Note, not all sections are traced. They may be prevented by either
2876a notrace, or blocked another way and all inline functions are not
2877traced. Check the "available_filter_functions" file to see what functions
2878can be traced.
2879
2880A section called "__mcount_loc" is created that holds
2881references to all the mcount/fentry call sites in the .text section.
2882The recordmcount program re-links this section back into the
2883original object. The final linking stage of the kernel will add all these
2884references into a single table.
2885
2886On boot up, before SMP is initialized, the dynamic ftrace code
2887scans this table and updates all the locations into nops. It
2888also records the locations, which are added to the
2889available_filter_functions list.  Modules are processed as they
2890are loaded and before they are executed.  When a module is
2891unloaded, it also removes its functions from the ftrace function
2892list. This is automatic in the module unload code, and the
2893module author does not need to worry about it.
2894
2895When tracing is enabled, the process of modifying the function
2896tracepoints is dependent on architecture. The old method is to use
2897kstop_machine to prevent races with the CPUs executing code being
2898modified (which can cause the CPU to do undesirable things, especially
2899if the modified code crosses cache (or page) boundaries), and the nops are
2900patched back to calls. But this time, they do not call mcount
2901(which is just a function stub). They now call into the ftrace
2902infrastructure.
2903
2904The new method of modifying the function tracepoints is to place
2905a breakpoint at the location to be modified, sync all CPUs, modify
2906the rest of the instruction not covered by the breakpoint. Sync
2907all CPUs again, and then remove the breakpoint with the finished
2908version to the ftrace call site.
2909
2910Some archs do not even need to monkey around with the synchronization,
2911and can just slap the new code on top of the old without any
2912problems with other CPUs executing it at the same time.
2913
2914One special side-effect to the recording of the functions being
2915traced is that we can now selectively choose which functions we
2916wish to trace and which ones we want the mcount calls to remain
2917as nops.
2918
2919Two files are used, one for enabling and one for disabling the
2920tracing of specified functions. They are:
2921
2922  set_ftrace_filter
2923
2924and
2925
2926  set_ftrace_notrace
2927
2928A list of available functions that you can add to these files is
2929listed in:
2930
2931   available_filter_functions
2932
2933::
2934
2935  # cat available_filter_functions
2936  put_prev_task_idle
2937  kmem_cache_create
2938  pick_next_task_rt
2939  cpus_read_lock
2940  pick_next_task_fair
2941  mutex_lock
2942  [...]
2943
2944If I am only interested in sys_nanosleep and hrtimer_interrupt::
2945
2946  # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
2947  # echo function > current_tracer
2948  # echo 1 > tracing_on
2949  # usleep 1
2950  # echo 0 > tracing_on
2951  # cat trace
2952  # tracer: function
2953  #
2954  # entries-in-buffer/entries-written: 5/5   #P:4
2955  #
2956  #                              _-----=> irqs-off
2957  #                             / _----=> need-resched
2958  #                            | / _---=> hardirq/softirq
2959  #                            || / _--=> preempt-depth
2960  #                            ||| /     delay
2961  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2962  #              | |       |   ||||       |         |
2963            usleep-2665  [001] ....  4186.475355: sys_nanosleep <-system_call_fastpath
2964            <idle>-0     [001] d.h1  4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
2965            usleep-2665  [001] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2966            <idle>-0     [003] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2967            <idle>-0     [002] d.h1  4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
2968
2969To see which functions are being traced, you can cat the file:
2970::
2971
2972  # cat set_ftrace_filter
2973  hrtimer_interrupt
2974  sys_nanosleep
2975
2976
2977Perhaps this is not enough. The filters also allow glob(7) matching.
2978
2979  ``<match>*``
2980	will match functions that begin with <match>
2981  ``*<match>``
2982	will match functions that end with <match>
2983  ``*<match>*``
2984	will match functions that have <match> in it
2985  ``<match1>*<match2>``
2986	will match functions that begin with <match1> and end with <match2>
2987
2988.. note::
2989      It is better to use quotes to enclose the wild cards,
2990      otherwise the shell may expand the parameters into names
2991      of files in the local directory.
2992
2993::
2994
2995  # echo 'hrtimer_*' > set_ftrace_filter
2996
2997Produces::
2998
2999  # tracer: function
3000  #
3001  # entries-in-buffer/entries-written: 897/897   #P:4
3002  #
3003  #                              _-----=> irqs-off
3004  #                             / _----=> need-resched
3005  #                            | / _---=> hardirq/softirq
3006  #                            || / _--=> preempt-depth
3007  #                            ||| /     delay
3008  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
3009  #              | |       |   ||||       |         |
3010            <idle>-0     [003] dN.1  4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
3011            <idle>-0     [003] dN.1  4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
3012            <idle>-0     [003] dN.2  4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
3013            <idle>-0     [003] dN.1  4228.547805: hrtimer_forward <-tick_nohz_idle_exit
3014            <idle>-0     [003] dN.1  4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
3015            <idle>-0     [003] d..1  4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
3016            <idle>-0     [003] d..1  4228.547859: hrtimer_start <-__tick_nohz_idle_enter
3017            <idle>-0     [003] d..2  4228.547860: hrtimer_force_reprogram <-__rem
3018
3019Notice that we lost the sys_nanosleep.
3020::
3021
3022  # cat set_ftrace_filter
3023  hrtimer_run_queues
3024  hrtimer_run_pending
3025  hrtimer_init
3026  hrtimer_cancel
3027  hrtimer_try_to_cancel
3028  hrtimer_forward
3029  hrtimer_start
3030  hrtimer_reprogram
3031  hrtimer_force_reprogram
3032  hrtimer_get_next_event
3033  hrtimer_interrupt
3034  hrtimer_nanosleep
3035  hrtimer_wakeup
3036  hrtimer_get_remaining
3037  hrtimer_get_res
3038  hrtimer_init_sleeper
3039
3040
3041This is because the '>' and '>>' act just like they do in bash.
3042To rewrite the filters, use '>'
3043To append to the filters, use '>>'
3044
3045To clear out a filter so that all functions will be recorded
3046again::
3047
3048 # echo > set_ftrace_filter
3049 # cat set_ftrace_filter
3050 #
3051
3052Again, now we want to append.
3053
3054::
3055
3056  # echo sys_nanosleep > set_ftrace_filter
3057  # cat set_ftrace_filter
3058  sys_nanosleep
3059  # echo 'hrtimer_*' >> set_ftrace_filter
3060  # cat set_ftrace_filter
3061  hrtimer_run_queues
3062  hrtimer_run_pending
3063  hrtimer_init
3064  hrtimer_cancel
3065  hrtimer_try_to_cancel
3066  hrtimer_forward
3067  hrtimer_start
3068  hrtimer_reprogram
3069  hrtimer_force_reprogram
3070  hrtimer_get_next_event
3071  hrtimer_interrupt
3072  sys_nanosleep
3073  hrtimer_nanosleep
3074  hrtimer_wakeup
3075  hrtimer_get_remaining
3076  hrtimer_get_res
3077  hrtimer_init_sleeper
3078
3079
3080The set_ftrace_notrace prevents those functions from being
3081traced.
3082::
3083
3084  # echo '*preempt*' '*lock*' > set_ftrace_notrace
3085
3086Produces::
3087
3088  # tracer: function
3089  #
3090  # entries-in-buffer/entries-written: 39608/39608   #P:4
3091  #
3092  #                              _-----=> irqs-off
3093  #                             / _----=> need-resched
3094  #                            | / _---=> hardirq/softirq
3095  #                            || / _--=> preempt-depth
3096  #                            ||| /     delay
3097  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
3098  #              | |       |   ||||       |         |
3099              bash-1994  [000] ....  4342.324896: file_ra_state_init <-do_dentry_open
3100              bash-1994  [000] ....  4342.324897: open_check_o_direct <-do_last
3101              bash-1994  [000] ....  4342.324897: ima_file_check <-do_last
3102              bash-1994  [000] ....  4342.324898: process_measurement <-ima_file_check
3103              bash-1994  [000] ....  4342.324898: ima_get_action <-process_measurement
3104              bash-1994  [000] ....  4342.324898: ima_match_policy <-ima_get_action
3105              bash-1994  [000] ....  4342.324899: do_truncate <-do_last
3106              bash-1994  [000] ....  4342.324899: setattr_should_drop_suidgid <-do_truncate
3107              bash-1994  [000] ....  4342.324899: notify_change <-do_truncate
3108              bash-1994  [000] ....  4342.324900: current_fs_time <-notify_change
3109              bash-1994  [000] ....  4342.324900: current_kernel_time <-current_fs_time
3110              bash-1994  [000] ....  4342.324900: timespec_trunc <-current_fs_time
3111
3112We can see that there's no more lock or preempt tracing.
3113
3114Selecting function filters via index
3115------------------------------------
3116
3117Because processing of strings is expensive (the address of the function
3118needs to be looked up before comparing to the string being passed in),
3119an index can be used as well to enable functions. This is useful in the
3120case of setting thousands of specific functions at a time. By passing
3121in a list of numbers, no string processing will occur. Instead, the function
3122at the specific location in the internal array (which corresponds to the
3123functions in the "available_filter_functions" file), is selected.
3124
3125::
3126
3127  # echo 1 > set_ftrace_filter
3128
3129Will select the first function listed in "available_filter_functions"
3130
3131::
3132
3133  # head -1 available_filter_functions
3134  trace_initcall_finish_cb
3135
3136  # cat set_ftrace_filter
3137  trace_initcall_finish_cb
3138
3139  # head -50 available_filter_functions | tail -1
3140  x86_pmu_commit_txn
3141
3142  # echo 1 50 > set_ftrace_filter
3143  # cat set_ftrace_filter
3144  trace_initcall_finish_cb
3145  x86_pmu_commit_txn
3146
3147Dynamic ftrace with the function graph tracer
3148---------------------------------------------
3149
3150Although what has been explained above concerns both the
3151function tracer and the function-graph-tracer, there are some
3152special features only available in the function-graph tracer.
3153
3154If you want to trace only one function and all of its children,
3155you just have to echo its name into set_graph_function::
3156
3157 echo __do_fault > set_graph_function
3158
3159will produce the following "expanded" trace of the __do_fault()
3160function::
3161
3162   0)               |  __do_fault() {
3163   0)               |    filemap_fault() {
3164   0)               |      find_lock_page() {
3165   0)   0.804 us    |        find_get_page();
3166   0)               |        __might_sleep() {
3167   0)   1.329 us    |        }
3168   0)   3.904 us    |      }
3169   0)   4.979 us    |    }
3170   0)   0.653 us    |    _spin_lock();
3171   0)   0.578 us    |    page_add_file_rmap();
3172   0)   0.525 us    |    native_set_pte_at();
3173   0)   0.585 us    |    _spin_unlock();
3174   0)               |    unlock_page() {
3175   0)   0.541 us    |      page_waitqueue();
3176   0)   0.639 us    |      __wake_up_bit();
3177   0)   2.786 us    |    }
3178   0) + 14.237 us   |  }
3179   0)               |  __do_fault() {
3180   0)               |    filemap_fault() {
3181   0)               |      find_lock_page() {
3182   0)   0.698 us    |        find_get_page();
3183   0)               |        __might_sleep() {
3184   0)   1.412 us    |        }
3185   0)   3.950 us    |      }
3186   0)   5.098 us    |    }
3187   0)   0.631 us    |    _spin_lock();
3188   0)   0.571 us    |    page_add_file_rmap();
3189   0)   0.526 us    |    native_set_pte_at();
3190   0)   0.586 us    |    _spin_unlock();
3191   0)               |    unlock_page() {
3192   0)   0.533 us    |      page_waitqueue();
3193   0)   0.638 us    |      __wake_up_bit();
3194   0)   2.793 us    |    }
3195   0) + 14.012 us   |  }
3196
3197You can also expand several functions at once::
3198
3199 echo sys_open > set_graph_function
3200 echo sys_close >> set_graph_function
3201
3202Now if you want to go back to trace all functions you can clear
3203this special filter via::
3204
3205 echo > set_graph_function
3206
3207
3208ftrace_enabled
3209--------------
3210
3211Note, the proc sysctl ftrace_enable is a big on/off switch for the
3212function tracer. By default it is enabled (when function tracing is
3213enabled in the kernel). If it is disabled, all function tracing is
3214disabled. This includes not only the function tracers for ftrace, but
3215also for any other uses (perf, kprobes, stack tracing, profiling, etc). It
3216cannot be disabled if there is a callback with FTRACE_OPS_FL_PERMANENT set
3217registered.
3218
3219Please disable this with care.
3220
3221This can be disable (and enabled) with::
3222
3223  sysctl kernel.ftrace_enabled=0
3224  sysctl kernel.ftrace_enabled=1
3225
3226 or
3227
3228  echo 0 > /proc/sys/kernel/ftrace_enabled
3229  echo 1 > /proc/sys/kernel/ftrace_enabled
3230
3231
3232Filter commands
3233---------------
3234
3235A few commands are supported by the set_ftrace_filter interface.
3236Trace commands have the following format::
3237
3238  <function>:<command>:<parameter>
3239
3240The following commands are supported:
3241
3242- mod:
3243  This command enables function filtering per module. The
3244  parameter defines the module. For example, if only the write*
3245  functions in the ext3 module are desired, run:
3246
3247   echo 'write*:mod:ext3' > set_ftrace_filter
3248
3249  This command interacts with the filter in the same way as
3250  filtering based on function names. Thus, adding more functions
3251  in a different module is accomplished by appending (>>) to the
3252  filter file. Remove specific module functions by prepending
3253  '!'::
3254
3255   echo '!writeback*:mod:ext3' >> set_ftrace_filter
3256
3257  Mod command supports module globbing. Disable tracing for all
3258  functions except a specific module::
3259
3260   echo '!*:mod:!ext3' >> set_ftrace_filter
3261
3262  Disable tracing for all modules, but still trace kernel::
3263
3264   echo '!*:mod:*' >> set_ftrace_filter
3265
3266  Enable filter only for kernel::
3267
3268   echo '*write*:mod:!*' >> set_ftrace_filter
3269
3270  Enable filter for module globbing::
3271
3272   echo '*write*:mod:*snd*' >> set_ftrace_filter
3273
3274- traceon/traceoff:
3275  These commands turn tracing on and off when the specified
3276  functions are hit. The parameter determines how many times the
3277  tracing system is turned on and off. If unspecified, there is
3278  no limit. For example, to disable tracing when a schedule bug
3279  is hit the first 5 times, run::
3280
3281   echo '__schedule_bug:traceoff:5' > set_ftrace_filter
3282
3283  To always disable tracing when __schedule_bug is hit::
3284
3285   echo '__schedule_bug:traceoff' > set_ftrace_filter
3286
3287  These commands are cumulative whether or not they are appended
3288  to set_ftrace_filter. To remove a command, prepend it by '!'
3289  and drop the parameter::
3290
3291   echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
3292
3293  The above removes the traceoff command for __schedule_bug
3294  that have a counter. To remove commands without counters::
3295
3296   echo '!__schedule_bug:traceoff' > set_ftrace_filter
3297
3298- snapshot:
3299  Will cause a snapshot to be triggered when the function is hit.
3300  ::
3301
3302   echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
3303
3304  To only snapshot once:
3305  ::
3306
3307   echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
3308
3309  To remove the above commands::
3310
3311   echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
3312   echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
3313
3314- enable_event/disable_event:
3315  These commands can enable or disable a trace event. Note, because
3316  function tracing callbacks are very sensitive, when these commands
3317  are registered, the trace point is activated, but disabled in
3318  a "soft" mode. That is, the tracepoint will be called, but
3319  just will not be traced. The event tracepoint stays in this mode
3320  as long as there's a command that triggers it.
3321  ::
3322
3323   echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
3324   	 set_ftrace_filter
3325
3326  The format is::
3327
3328    <function>:enable_event:<system>:<event>[:count]
3329    <function>:disable_event:<system>:<event>[:count]
3330
3331  To remove the events commands::
3332
3333   echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
3334   	 set_ftrace_filter
3335   echo '!schedule:disable_event:sched:sched_switch' > \
3336   	 set_ftrace_filter
3337
3338- dump:
3339  When the function is hit, it will dump the contents of the ftrace
3340  ring buffer to the console. This is useful if you need to debug
3341  something, and want to dump the trace when a certain function
3342  is hit. Perhaps it's a function that is called before a triple
3343  fault happens and does not allow you to get a regular dump.
3344
3345- cpudump:
3346  When the function is hit, it will dump the contents of the ftrace
3347  ring buffer for the current CPU to the console. Unlike the "dump"
3348  command, it only prints out the contents of the ring buffer for the
3349  CPU that executed the function that triggered the dump.
3350
3351- stacktrace:
3352  When the function is hit, a stack trace is recorded.
3353
3354trace_pipe
3355----------
3356
3357The trace_pipe outputs the same content as the trace file, but
3358the effect on the tracing is different. Every read from
3359trace_pipe is consumed. This means that subsequent reads will be
3360different. The trace is live.
3361::
3362
3363  # echo function > current_tracer
3364  # cat trace_pipe > /tmp/trace.out &
3365  [1] 4153
3366  # echo 1 > tracing_on
3367  # usleep 1
3368  # echo 0 > tracing_on
3369  # cat trace
3370  # tracer: function
3371  #
3372  # entries-in-buffer/entries-written: 0/0   #P:4
3373  #
3374  #                              _-----=> irqs-off
3375  #                             / _----=> need-resched
3376  #                            | / _---=> hardirq/softirq
3377  #                            || / _--=> preempt-depth
3378  #                            ||| /     delay
3379  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
3380  #              | |       |   ||||       |         |
3381
3382  #
3383  # cat /tmp/trace.out
3384             bash-1994  [000] ....  5281.568961: mutex_unlock <-rb_simple_write
3385             bash-1994  [000] ....  5281.568963: __mutex_unlock_slowpath <-mutex_unlock
3386             bash-1994  [000] ....  5281.568963: __fsnotify_parent <-fsnotify_modify
3387             bash-1994  [000] ....  5281.568964: fsnotify <-fsnotify_modify
3388             bash-1994  [000] ....  5281.568964: __srcu_read_lock <-fsnotify
3389             bash-1994  [000] ....  5281.568964: add_preempt_count <-__srcu_read_lock
3390             bash-1994  [000] ...1  5281.568965: sub_preempt_count <-__srcu_read_lock
3391             bash-1994  [000] ....  5281.568965: __srcu_read_unlock <-fsnotify
3392             bash-1994  [000] ....  5281.568967: sys_dup2 <-system_call_fastpath
3393
3394
3395Note, reading the trace_pipe file will block until more input is
3396added. This is contrary to the trace file. If any process opened
3397the trace file for reading, it will actually disable tracing and
3398prevent new entries from being added. The trace_pipe file does
3399not have this limitation.
3400
3401trace entries
3402-------------
3403
3404Having too much or not enough data can be troublesome in
3405diagnosing an issue in the kernel. The file buffer_size_kb is
3406used to modify the size of the internal trace buffers. The
3407number listed is the number of entries that can be recorded per
3408CPU. To know the full size, multiply the number of possible CPUs
3409with the number of entries.
3410::
3411
3412  # cat buffer_size_kb
3413  1408 (units kilobytes)
3414
3415Or simply read buffer_total_size_kb
3416::
3417
3418  # cat buffer_total_size_kb
3419  5632
3420
3421To modify the buffer, simple echo in a number (in 1024 byte segments).
3422::
3423
3424  # echo 10000 > buffer_size_kb
3425  # cat buffer_size_kb
3426  10000 (units kilobytes)
3427
3428It will try to allocate as much as possible. If you allocate too
3429much, it can cause Out-Of-Memory to trigger.
3430::
3431
3432  # echo 1000000000000 > buffer_size_kb
3433  -bash: echo: write error: Cannot allocate memory
3434  # cat buffer_size_kb
3435  85
3436
3437The per_cpu buffers can be changed individually as well:
3438::
3439
3440  # echo 10000 > per_cpu/cpu0/buffer_size_kb
3441  # echo 100 > per_cpu/cpu1/buffer_size_kb
3442
3443When the per_cpu buffers are not the same, the buffer_size_kb
3444at the top level will just show an X
3445::
3446
3447  # cat buffer_size_kb
3448  X
3449
3450This is where the buffer_total_size_kb is useful:
3451::
3452
3453  # cat buffer_total_size_kb
3454  12916
3455
3456Writing to the top level buffer_size_kb will reset all the buffers
3457to be the same again.
3458
3459Snapshot
3460--------
3461CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
3462available to all non latency tracers. (Latency tracers which
3463record max latency, such as "irqsoff" or "wakeup", can't use
3464this feature, since those are already using the snapshot
3465mechanism internally.)
3466
3467Snapshot preserves a current trace buffer at a particular point
3468in time without stopping tracing. Ftrace swaps the current
3469buffer with a spare buffer, and tracing continues in the new
3470current (=previous spare) buffer.
3471
3472The following tracefs files in "tracing" are related to this
3473feature:
3474
3475  snapshot:
3476
3477	This is used to take a snapshot and to read the output
3478	of the snapshot. Echo 1 into this file to allocate a
3479	spare buffer and to take a snapshot (swap), then read
3480	the snapshot from this file in the same format as
3481	"trace" (described above in the section "The File
3482	System"). Both reads snapshot and tracing are executable
3483	in parallel. When the spare buffer is allocated, echoing
3484	0 frees it, and echoing else (positive) values clear the
3485	snapshot contents.
3486	More details are shown in the table below.
3487
3488	+--------------+------------+------------+------------+
3489	|status\\input |     0      |     1      |    else    |
3490	+==============+============+============+============+
3491	|not allocated |(do nothing)| alloc+swap |(do nothing)|
3492	+--------------+------------+------------+------------+
3493	|allocated     |    free    |    swap    |   clear    |
3494	+--------------+------------+------------+------------+
3495
3496Here is an example of using the snapshot feature.
3497::
3498
3499  # echo 1 > events/sched/enable
3500  # echo 1 > snapshot
3501  # cat snapshot
3502  # tracer: nop
3503  #
3504  # entries-in-buffer/entries-written: 71/71   #P:8
3505  #
3506  #                              _-----=> irqs-off
3507  #                             / _----=> need-resched
3508  #                            | / _---=> hardirq/softirq
3509  #                            || / _--=> preempt-depth
3510  #                            ||| /     delay
3511  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
3512  #              | |       |   ||||       |         |
3513            <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
3514             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
3515  [...]
3516          <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
3517
3518  # cat trace
3519  # tracer: nop
3520  #
3521  # entries-in-buffer/entries-written: 77/77   #P:8
3522  #
3523  #                              _-----=> irqs-off
3524  #                             / _----=> need-resched
3525  #                            | / _---=> hardirq/softirq
3526  #                            || / _--=> preempt-depth
3527  #                            ||| /     delay
3528  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
3529  #              | |       |   ||||       |         |
3530            <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
3531   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
3532  [...]
3533
3534
3535If you try to use this snapshot feature when current tracer is
3536one of the latency tracers, you will get the following results.
3537::
3538
3539  # echo wakeup > current_tracer
3540  # echo 1 > snapshot
3541  bash: echo: write error: Device or resource busy
3542  # cat snapshot
3543  cat: snapshot: Device or resource busy
3544
3545
3546Instances
3547---------
3548In the tracefs tracing directory, there is a directory called "instances".
3549This directory can have new directories created inside of it using
3550mkdir, and removing directories with rmdir. The directory created
3551with mkdir in this directory will already contain files and other
3552directories after it is created.
3553::
3554
3555  # mkdir instances/foo
3556  # ls instances/foo
3557  buffer_size_kb  buffer_total_size_kb  events  free_buffer  per_cpu
3558  set_event  snapshot  trace  trace_clock  trace_marker  trace_options
3559  trace_pipe  tracing_on
3560
3561As you can see, the new directory looks similar to the tracing directory
3562itself. In fact, it is very similar, except that the buffer and
3563events are agnostic from the main directory, or from any other
3564instances that are created.
3565
3566The files in the new directory work just like the files with the
3567same name in the tracing directory except the buffer that is used
3568is a separate and new buffer. The files affect that buffer but do not
3569affect the main buffer with the exception of trace_options. Currently,
3570the trace_options affect all instances and the top level buffer
3571the same, but this may change in future releases. That is, options
3572may become specific to the instance they reside in.
3573
3574Notice that none of the function tracer files are there, nor is
3575current_tracer and available_tracers. This is because the buffers
3576can currently only have events enabled for them.
3577::
3578
3579  # mkdir instances/foo
3580  # mkdir instances/bar
3581  # mkdir instances/zoot
3582  # echo 100000 > buffer_size_kb
3583  # echo 1000 > instances/foo/buffer_size_kb
3584  # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
3585  # echo function > current_trace
3586  # echo 1 > instances/foo/events/sched/sched_wakeup/enable
3587  # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
3588  # echo 1 > instances/foo/events/sched/sched_switch/enable
3589  # echo 1 > instances/bar/events/irq/enable
3590  # echo 1 > instances/zoot/events/syscalls/enable
3591  # cat trace_pipe
3592  CPU:2 [LOST 11745 EVENTS]
3593              bash-2044  [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
3594              bash-2044  [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
3595              bash-2044  [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
3596              bash-2044  [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
3597              bash-2044  [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
3598              bash-2044  [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
3599              bash-2044  [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
3600              bash-2044  [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
3601              bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
3602              bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
3603              bash-2044  [002] .... 10594.481035: arch_dup_task_struct <-copy_process
3604  [...]
3605
3606  # cat instances/foo/trace_pipe
3607              bash-1998  [000] d..4   136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
3608              bash-1998  [000] dN.4   136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
3609            <idle>-0     [003] d.h3   136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
3610            <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
3611       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
3612              bash-1998  [000] d..4   136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
3613              bash-1998  [000] dN.4   136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
3614              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
3615       kworker/0:1-59    [000] d..4   136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
3616       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
3617  [...]
3618
3619  # cat instances/bar/trace_pipe
3620       migration/1-14    [001] d.h3   138.732674: softirq_raise: vec=3 [action=NET_RX]
3621            <idle>-0     [001] dNh3   138.732725: softirq_raise: vec=3 [action=NET_RX]
3622              bash-1998  [000] d.h1   138.733101: softirq_raise: vec=1 [action=TIMER]
3623              bash-1998  [000] d.h1   138.733102: softirq_raise: vec=9 [action=RCU]
3624              bash-1998  [000] ..s2   138.733105: softirq_entry: vec=1 [action=TIMER]
3625              bash-1998  [000] ..s2   138.733106: softirq_exit: vec=1 [action=TIMER]
3626              bash-1998  [000] ..s2   138.733106: softirq_entry: vec=9 [action=RCU]
3627              bash-1998  [000] ..s2   138.733109: softirq_exit: vec=9 [action=RCU]
3628              sshd-1995  [001] d.h1   138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
3629              sshd-1995  [001] d.h1   138.733280: irq_handler_exit: irq=21 ret=unhandled
3630              sshd-1995  [001] d.h1   138.733281: irq_handler_entry: irq=21 name=eth0
3631              sshd-1995  [001] d.h1   138.733283: irq_handler_exit: irq=21 ret=handled
3632  [...]
3633
3634  # cat instances/zoot/trace
3635  # tracer: nop
3636  #
3637  # entries-in-buffer/entries-written: 18996/18996   #P:4
3638  #
3639  #                              _-----=> irqs-off
3640  #                             / _----=> need-resched
3641  #                            | / _---=> hardirq/softirq
3642  #                            || / _--=> preempt-depth
3643  #                            ||| /     delay
3644  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
3645  #              | |       |   ||||       |         |
3646              bash-1998  [000] d...   140.733501: sys_write -> 0x2
3647              bash-1998  [000] d...   140.733504: sys_dup2(oldfd: a, newfd: 1)
3648              bash-1998  [000] d...   140.733506: sys_dup2 -> 0x1
3649              bash-1998  [000] d...   140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
3650              bash-1998  [000] d...   140.733509: sys_fcntl -> 0x1
3651              bash-1998  [000] d...   140.733510: sys_close(fd: a)
3652              bash-1998  [000] d...   140.733510: sys_close -> 0x0
3653              bash-1998  [000] d...   140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
3654              bash-1998  [000] d...   140.733515: sys_rt_sigprocmask -> 0x0
3655              bash-1998  [000] d...   140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
3656              bash-1998  [000] d...   140.733516: sys_rt_sigaction -> 0x0
3657
3658You can see that the trace of the top most trace buffer shows only
3659the function tracing. The foo instance displays wakeups and task
3660switches.
3661
3662To remove the instances, simply delete their directories:
3663::
3664
3665  # rmdir instances/foo
3666  # rmdir instances/bar
3667  # rmdir instances/zoot
3668
3669Note, if a process has a trace file open in one of the instance
3670directories, the rmdir will fail with EBUSY.
3671
3672
3673Stack trace
3674-----------
3675Since the kernel has a fixed sized stack, it is important not to
3676waste it in functions. A kernel developer must be conscious of
3677what they allocate on the stack. If they add too much, the system
3678can be in danger of a stack overflow, and corruption will occur,
3679usually leading to a system panic.
3680
3681There are some tools that check this, usually with interrupts
3682periodically checking usage. But if you can perform a check
3683at every function call that will become very useful. As ftrace provides
3684a function tracer, it makes it convenient to check the stack size
3685at every function call. This is enabled via the stack tracer.
3686
3687CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
3688To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
3689::
3690
3691 # echo 1 > /proc/sys/kernel/stack_tracer_enabled
3692
3693You can also enable it from the kernel command line to trace
3694the stack size of the kernel during boot up, by adding "stacktrace"
3695to the kernel command line parameter.
3696
3697After running it for a few minutes, the output looks like:
3698::
3699
3700  # cat stack_max_size
3701  2928
3702
3703  # cat stack_trace
3704          Depth    Size   Location    (18 entries)
3705          -----    ----   --------
3706    0)     2928     224   update_sd_lb_stats+0xbc/0x4ac
3707    1)     2704     160   find_busiest_group+0x31/0x1f1
3708    2)     2544     256   load_balance+0xd9/0x662
3709    3)     2288      80   idle_balance+0xbb/0x130
3710    4)     2208     128   __schedule+0x26e/0x5b9
3711    5)     2080      16   schedule+0x64/0x66
3712    6)     2064     128   schedule_timeout+0x34/0xe0
3713    7)     1936     112   wait_for_common+0x97/0xf1
3714    8)     1824      16   wait_for_completion+0x1d/0x1f
3715    9)     1808     128   flush_work+0xfe/0x119
3716   10)     1680      16   tty_flush_to_ldisc+0x1e/0x20
3717   11)     1664      48   input_available_p+0x1d/0x5c
3718   12)     1616      48   n_tty_poll+0x6d/0x134
3719   13)     1568      64   tty_poll+0x64/0x7f
3720   14)     1504     880   do_select+0x31e/0x511
3721   15)      624     400   core_sys_select+0x177/0x216
3722   16)      224      96   sys_select+0x91/0xb9
3723   17)      128     128   system_call_fastpath+0x16/0x1b
3724
3725Note, if -mfentry is being used by gcc, functions get traced before
3726they set up the stack frame. This means that leaf level functions
3727are not tested by the stack tracer when -mfentry is used.
3728
3729Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
3730
3731More
3732----
3733More details can be found in the source code, in the `kernel/trace/*.c` files.
3734