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Linux/Documentation/trace/ftrace.txt

  1                 ftrace - Function Tracer
  2                 ========================
  3 
  4 Copyright 2008 Red Hat Inc.
  5    Author:   Steven Rostedt <srostedt@redhat.com>
  6   License:   The GNU Free Documentation License, Version 1.2
  7                (dual licensed under the GPL v2)
  8 Reviewers:   Elias Oltmanns, Randy Dunlap, Andrew Morton,
  9              John Kacur, and David Teigland.
 10 Written for: 2.6.28-rc2
 11 Updated for: 3.10
 12 
 13 Introduction
 14 ------------
 15 
 16 Ftrace is an internal tracer designed to help out developers and
 17 designers of systems to find what is going on inside the kernel.
 18 It can be used for debugging or analyzing latencies and
 19 performance issues that take place outside of user-space.
 20 
 21 Although ftrace is typically considered the function tracer, it
 22 is really a frame work of several assorted tracing utilities.
 23 There's latency tracing to examine what occurs between interrupts
 24 disabled and enabled, as well as for preemption and from a time
 25 a task is woken to the task is actually scheduled in.
 26 
 27 One of the most common uses of ftrace is the event tracing.
 28 Through out the kernel is hundreds of static event points that
 29 can be enabled via the debugfs file system to see what is
 30 going on in certain parts of the kernel.
 31 
 32 
 33 Implementation Details
 34 ----------------------
 35 
 36 See ftrace-design.txt for details for arch porters and such.
 37 
 38 
 39 The File System
 40 ---------------
 41 
 42 Ftrace uses the debugfs file system to hold the control files as
 43 well as the files to display output.
 44 
 45 When debugfs is configured into the kernel (which selecting any ftrace
 46 option will do) the directory /sys/kernel/debug will be created. To mount
 47 this directory, you can add to your /etc/fstab file:
 48 
 49  debugfs       /sys/kernel/debug          debugfs defaults        0       0
 50 
 51 Or you can mount it at run time with:
 52 
 53  mount -t debugfs nodev /sys/kernel/debug
 54 
 55 For quicker access to that directory you may want to make a soft link to
 56 it:
 57 
 58  ln -s /sys/kernel/debug /debug
 59 
 60 Any selected ftrace option will also create a directory called tracing
 61 within the debugfs. The rest of the document will assume that you are in
 62 the ftrace directory (cd /sys/kernel/debug/tracing) and will only concentrate
 63 on the files within that directory and not distract from the content with
 64 the extended "/sys/kernel/debug/tracing" path name.
 65 
 66 That's it! (assuming that you have ftrace configured into your kernel)
 67 
 68 After mounting debugfs, you can see a directory called
 69 "tracing".  This directory contains the control and output files
 70 of ftrace. Here is a list of some of the key files:
 71 
 72 
 73  Note: all time values are in microseconds.
 74 
 75   current_tracer:
 76 
 77         This is used to set or display the current tracer
 78         that is configured.
 79 
 80   available_tracers:
 81 
 82         This holds the different types of tracers that
 83         have been compiled into the kernel. The
 84         tracers listed here can be configured by
 85         echoing their name into current_tracer.
 86 
 87   tracing_on:
 88 
 89         This sets or displays whether writing to the trace
 90         ring buffer is enabled. Echo 0 into this file to disable
 91         the tracer or 1 to enable it. Note, this only disables
 92         writing to the ring buffer, the tracing overhead may
 93         still be occurring.
 94 
 95   trace:
 96 
 97         This file holds the output of the trace in a human
 98         readable format (described below).
 99 
100   trace_pipe:
101 
102         The output is the same as the "trace" file but this
103         file is meant to be streamed with live tracing.
104         Reads from this file will block until new data is
105         retrieved.  Unlike the "trace" file, this file is a
106         consumer. This means reading from this file causes
107         sequential reads to display more current data. Once
108         data is read from this file, it is consumed, and
109         will not be read again with a sequential read. The
110         "trace" file is static, and if the tracer is not
111         adding more data, it will display the same
112         information every time it is read.
113 
114   trace_options:
115 
116         This file lets the user control the amount of data
117         that is displayed in one of the above output
118         files. Options also exist to modify how a tracer
119         or events work (stack traces, timestamps, etc).
120 
121   options:
122 
123         This is a directory that has a file for every available
124         trace option (also in trace_options). Options may also be set
125         or cleared by writing a "1" or "0" respectively into the
126         corresponding file with the option name.
127 
128   tracing_max_latency:
129 
130         Some of the tracers record the max latency.
131         For example, the time interrupts are disabled.
132         This time is saved in this file. The max trace
133         will also be stored, and displayed by "trace".
134         A new max trace will only be recorded if the
135         latency is greater than the value in this
136         file. (in microseconds)
137 
138   tracing_thresh:
139 
140         Some latency tracers will record a trace whenever the
141         latency is greater than the number in this file.
142         Only active when the file contains a number greater than 0.
143         (in microseconds)
144 
145   buffer_size_kb:
146 
147         This sets or displays the number of kilobytes each CPU
148         buffer holds. By default, the trace buffers are the same size
149         for each CPU. The displayed number is the size of the
150         CPU buffer and not total size of all buffers. The
151         trace buffers are allocated in pages (blocks of memory
152         that the kernel uses for allocation, usually 4 KB in size).
153         If the last page allocated has room for more bytes
154         than requested, the rest of the page will be used,
155         making the actual allocation bigger than requested.
156         ( Note, the size may not be a multiple of the page size
157           due to buffer management meta-data. )
158 
159   buffer_total_size_kb:
160 
161         This displays the total combined size of all the trace buffers.
162 
163   free_buffer:
164 
165         If a process is performing the tracing, and the ring buffer
166         should be shrunk "freed" when the process is finished, even
167         if it were to be killed by a signal, this file can be used
168         for that purpose. On close of this file, the ring buffer will
169         be resized to its minimum size. Having a process that is tracing
170         also open this file, when the process exits its file descriptor
171         for this file will be closed, and in doing so, the ring buffer
172         will be "freed".
173 
174         It may also stop tracing if disable_on_free option is set.
175 
176   tracing_cpumask:
177 
178         This is a mask that lets the user only trace
179         on specified CPUs. The format is a hex string
180         representing the CPUs.
181 
182   set_ftrace_filter:
183 
184         When dynamic ftrace is configured in (see the
185         section below "dynamic ftrace"), the code is dynamically
186         modified (code text rewrite) to disable calling of the
187         function profiler (mcount). This lets tracing be configured
188         in with practically no overhead in performance.  This also
189         has a side effect of enabling or disabling specific functions
190         to be traced. Echoing names of functions into this file
191         will limit the trace to only those functions.
192 
193         This interface also allows for commands to be used. See the
194         "Filter commands" section for more details.
195 
196   set_ftrace_notrace:
197 
198         This has an effect opposite to that of
199         set_ftrace_filter. Any function that is added here will not
200         be traced. If a function exists in both set_ftrace_filter
201         and set_ftrace_notrace, the function will _not_ be traced.
202 
203   set_ftrace_pid:
204 
205         Have the function tracer only trace a single thread.
206 
207   set_event_pid:
208 
209         Have the events only trace a task with a PID listed in this file.
210         Note, sched_switch and sched_wake_up will also trace events
211         listed in this file.
212 
213   set_graph_function:
214 
215         Set a "trigger" function where tracing should start
216         with the function graph tracer (See the section
217         "dynamic ftrace" for more details).
218 
219   available_filter_functions:
220 
221         This lists the functions that ftrace
222         has processed and can trace. These are the function
223         names that you can pass to "set_ftrace_filter" or
224         "set_ftrace_notrace". (See the section "dynamic ftrace"
225         below for more details.)
226 
227   enabled_functions:
228 
229         This file is more for debugging ftrace, but can also be useful
230         in seeing if any function has a callback attached to it.
231         Not only does the trace infrastructure use ftrace function
232         trace utility, but other subsystems might too. This file
233         displays all functions that have a callback attached to them
234         as well as the number of callbacks that have been attached.
235         Note, a callback may also call multiple functions which will
236         not be listed in this count.
237 
238         If the callback registered to be traced by a function with
239         the "save regs" attribute (thus even more overhead), a 'R'
240         will be displayed on the same line as the function that
241         is returning registers.
242 
243         If the callback registered to be traced by a function with
244         the "ip modify" attribute (thus the regs->ip can be changed),
245         an 'I' will be displayed on the same line as the function that
246         can be overridden.
247 
248   function_profile_enabled:
249 
250         When set it will enable all functions with either the function
251         tracer, or if enabled, the function graph tracer. It will
252         keep a histogram of the number of functions that were called
253         and if run with the function graph tracer, it will also keep
254         track of the time spent in those functions. The histogram
255         content can be displayed in the files:
256 
257         trace_stats/function<cpu> ( function0, function1, etc).
258 
259   trace_stats:
260 
261         A directory that holds different tracing stats.
262 
263   kprobe_events:
264  
265         Enable dynamic trace points. See kprobetrace.txt.
266 
267   kprobe_profile:
268 
269         Dynamic trace points stats. See kprobetrace.txt.
270 
271   max_graph_depth:
272 
273         Used with the function graph tracer. This is the max depth
274         it will trace into a function. Setting this to a value of
275         one will show only the first kernel function that is called
276         from user space.
277 
278   printk_formats:
279 
280         This is for tools that read the raw format files. If an event in
281         the ring buffer references a string (currently only trace_printk()
282         does this), only a pointer to the string is recorded into the buffer
283         and not the string itself. This prevents tools from knowing what
284         that string was. This file displays the string and address for
285         the string allowing tools to map the pointers to what the
286         strings were.
287 
288   saved_cmdlines:
289 
290         Only the pid of the task is recorded in a trace event unless
291         the event specifically saves the task comm as well. Ftrace
292         makes a cache of pid mappings to comms to try to display
293         comms for events. If a pid for a comm is not listed, then
294         "<...>" is displayed in the output.
295 
296   snapshot:
297 
298         This displays the "snapshot" buffer and also lets the user
299         take a snapshot of the current running trace.
300         See the "Snapshot" section below for more details.
301 
302   stack_max_size:
303 
304         When the stack tracer is activated, this will display the
305         maximum stack size it has encountered.
306         See the "Stack Trace" section below.
307 
308   stack_trace:
309 
310         This displays the stack back trace of the largest stack
311         that was encountered when the stack tracer is activated.
312         See the "Stack Trace" section below.
313 
314   stack_trace_filter:
315 
316         This is similar to "set_ftrace_filter" but it limits what
317         functions the stack tracer will check.
318 
319   trace_clock:
320 
321         Whenever an event is recorded into the ring buffer, a
322         "timestamp" is added. This stamp comes from a specified
323         clock. By default, ftrace uses the "local" clock. This
324         clock is very fast and strictly per cpu, but on some
325         systems it may not be monotonic with respect to other
326         CPUs. In other words, the local clocks may not be in sync
327         with local clocks on other CPUs.
328 
329         Usual clocks for tracing:
330 
331           # cat trace_clock
332           [local] global counter x86-tsc
333 
334           local: Default clock, but may not be in sync across CPUs
335 
336           global: This clock is in sync with all CPUs but may
337                   be a bit slower than the local clock.
338 
339           counter: This is not a clock at all, but literally an atomic
340                    counter. It counts up one by one, but is in sync
341                    with all CPUs. This is useful when you need to
342                    know exactly the order events occurred with respect to
343                    each other on different CPUs.
344 
345           uptime: This uses the jiffies counter and the time stamp
346                   is relative to the time since boot up.
347 
348           perf: This makes ftrace use the same clock that perf uses.
349                 Eventually perf will be able to read ftrace buffers
350                 and this will help out in interleaving the data.
351 
352           x86-tsc: Architectures may define their own clocks. For
353                    example, x86 uses its own TSC cycle clock here.
354 
355           ppc-tb: This uses the powerpc timebase register value.
356                   This is in sync across CPUs and can also be used
357                   to correlate events across hypervisor/guest if
358                   tb_offset is known.
359 
360         To set a clock, simply echo the clock name into this file.
361 
362           echo global > trace_clock
363 
364   trace_marker:
365 
366         This is a very useful file for synchronizing user space
367         with events happening in the kernel. Writing strings into
368         this file will be written into the ftrace buffer.
369 
370         It is useful in applications to open this file at the start
371         of the application and just reference the file descriptor
372         for the file.
373 
374         void trace_write(const char *fmt, ...)
375         {
376                 va_list ap;
377                 char buf[256];
378                 int n;
379 
380                 if (trace_fd < 0)
381                         return;
382 
383                 va_start(ap, fmt);
384                 n = vsnprintf(buf, 256, fmt, ap);
385                 va_end(ap);
386 
387                 write(trace_fd, buf, n);
388         }
389 
390         start:
391 
392                 trace_fd = open("trace_marker", WR_ONLY);
393 
394   uprobe_events:
395  
396         Add dynamic tracepoints in programs.
397         See uprobetracer.txt
398 
399   uprobe_profile:
400 
401         Uprobe statistics. See uprobetrace.txt
402 
403   instances:
404 
405         This is a way to make multiple trace buffers where different
406         events can be recorded in different buffers.
407         See "Instances" section below.
408 
409   events:
410 
411         This is the trace event directory. It holds event tracepoints
412         (also known as static tracepoints) that have been compiled
413         into the kernel. It shows what event tracepoints exist
414         and how they are grouped by system. There are "enable"
415         files at various levels that can enable the tracepoints
416         when a "1" is written to them.
417 
418         See events.txt for more information.
419 
420   per_cpu:
421 
422         This is a directory that contains the trace per_cpu information.
423 
424   per_cpu/cpu0/buffer_size_kb:
425 
426         The ftrace buffer is defined per_cpu. That is, there's a separate
427         buffer for each CPU to allow writes to be done atomically,
428         and free from cache bouncing. These buffers may have different
429         size buffers. This file is similar to the buffer_size_kb
430         file, but it only displays or sets the buffer size for the
431         specific CPU. (here cpu0).
432 
433   per_cpu/cpu0/trace:
434 
435         This is similar to the "trace" file, but it will only display
436         the data specific for the CPU. If written to, it only clears
437         the specific CPU buffer.
438 
439   per_cpu/cpu0/trace_pipe
440 
441         This is similar to the "trace_pipe" file, and is a consuming
442         read, but it will only display (and consume) the data specific
443         for the CPU.
444 
445   per_cpu/cpu0/trace_pipe_raw
446 
447         For tools that can parse the ftrace ring buffer binary format,
448         the trace_pipe_raw file can be used to extract the data
449         from the ring buffer directly. With the use of the splice()
450         system call, the buffer data can be quickly transferred to
451         a file or to the network where a server is collecting the
452         data.
453 
454         Like trace_pipe, this is a consuming reader, where multiple
455         reads will always produce different data.
456 
457   per_cpu/cpu0/snapshot:
458 
459         This is similar to the main "snapshot" file, but will only
460         snapshot the current CPU (if supported). It only displays
461         the content of the snapshot for a given CPU, and if
462         written to, only clears this CPU buffer.
463 
464   per_cpu/cpu0/snapshot_raw:
465 
466         Similar to the trace_pipe_raw, but will read the binary format
467         from the snapshot buffer for the given CPU.
468 
469   per_cpu/cpu0/stats:
470 
471         This displays certain stats about the ring buffer:
472 
473          entries: The number of events that are still in the buffer.
474 
475          overrun: The number of lost events due to overwriting when
476                   the buffer was full.
477 
478          commit overrun: Should always be zero.
479                 This gets set if so many events happened within a nested
480                 event (ring buffer is re-entrant), that it fills the
481                 buffer and starts dropping events.
482 
483          bytes: Bytes actually read (not overwritten).
484 
485          oldest event ts: The oldest timestamp in the buffer
486 
487          now ts: The current timestamp
488 
489          dropped events: Events lost due to overwrite option being off.
490 
491          read events: The number of events read.
492 
493 The Tracers
494 -----------
495 
496 Here is the list of current tracers that may be configured.
497 
498   "function"
499 
500         Function call tracer to trace all kernel functions.
501 
502   "function_graph"
503 
504         Similar to the function tracer except that the
505         function tracer probes the functions on their entry
506         whereas the function graph tracer traces on both entry
507         and exit of the functions. It then provides the ability
508         to draw a graph of function calls similar to C code
509         source.
510 
511   "irqsoff"
512 
513         Traces the areas that disable interrupts and saves
514         the trace with the longest max latency.
515         See tracing_max_latency. When a new max is recorded,
516         it replaces the old trace. It is best to view this
517         trace with the latency-format option enabled.
518 
519   "preemptoff"
520 
521         Similar to irqsoff but traces and records the amount of
522         time for which preemption is disabled.
523 
524   "preemptirqsoff"
525 
526         Similar to irqsoff and preemptoff, but traces and
527         records the largest time for which irqs and/or preemption
528         is disabled.
529 
530   "wakeup"
531 
532         Traces and records the max latency that it takes for
533         the highest priority task to get scheduled after
534         it has been woken up.
535         Traces all tasks as an average developer would expect.
536 
537   "wakeup_rt"
538 
539         Traces and records the max latency that it takes for just
540         RT tasks (as the current "wakeup" does). This is useful
541         for those interested in wake up timings of RT tasks.
542 
543   "nop"
544 
545         This is the "trace nothing" tracer. To remove all
546         tracers from tracing simply echo "nop" into
547         current_tracer.
548 
549 
550 Examples of using the tracer
551 ----------------------------
552 
553 Here are typical examples of using the tracers when controlling
554 them only with the debugfs interface (without using any
555 user-land utilities).
556 
557 Output format:
558 --------------
559 
560 Here is an example of the output format of the file "trace"
561 
562                              --------
563 # tracer: function
564 #
565 # entries-in-buffer/entries-written: 140080/250280   #P:4
566 #
567 #                              _-----=> irqs-off
568 #                             / _----=> need-resched
569 #                            | / _---=> hardirq/softirq
570 #                            || / _--=> preempt-depth
571 #                            ||| /     delay
572 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
573 #              | |       |   ||||       |         |
574             bash-1977  [000] .... 17284.993652: sys_close <-system_call_fastpath
575             bash-1977  [000] .... 17284.993653: __close_fd <-sys_close
576             bash-1977  [000] .... 17284.993653: _raw_spin_lock <-__close_fd
577             sshd-1974  [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
578             bash-1977  [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
579             bash-1977  [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
580             bash-1977  [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
581             bash-1977  [000] .... 17284.993657: filp_close <-__close_fd
582             bash-1977  [000] .... 17284.993657: dnotify_flush <-filp_close
583             sshd-1974  [003] .... 17284.993658: sys_select <-system_call_fastpath
584                              --------
585 
586 A header is printed with the tracer name that is represented by
587 the trace. In this case the tracer is "function". Then it shows the
588 number of events in the buffer as well as the total number of entries
589 that were written. The difference is the number of entries that were
590 lost due to the buffer filling up (250280 - 140080 = 110200 events
591 lost).
592 
593 The header explains the content of the events. Task name "bash", the task
594 PID "1977", the CPU that it was running on "000", the latency format
595 (explained below), the timestamp in <secs>.<usecs> format, the
596 function name that was traced "sys_close" and the parent function that
597 called this function "system_call_fastpath". The timestamp is the time
598 at which the function was entered.
599 
600 Latency trace format
601 --------------------
602 
603 When the latency-format option is enabled or when one of the latency
604 tracers is set, the trace file gives somewhat more information to see
605 why a latency happened. Here is a typical trace.
606 
607 # tracer: irqsoff
608 #
609 # irqsoff latency trace v1.1.5 on 3.8.0-test+
610 # --------------------------------------------------------------------
611 # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
612 #    -----------------
613 #    | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
614 #    -----------------
615 #  => started at: __lock_task_sighand
616 #  => ended at:   _raw_spin_unlock_irqrestore
617 #
618 #
619 #                  _------=> CPU#            
620 #                 / _-----=> irqs-off        
621 #                | / _----=> need-resched    
622 #                || / _---=> hardirq/softirq 
623 #                ||| / _--=> preempt-depth   
624 #                |||| /     delay             
625 #  cmd     pid   ||||| time  |   caller      
626 #     \   /      |||||  \    |   /           
627       ps-6143    2d...    0us!: trace_hardirqs_off <-__lock_task_sighand
628       ps-6143    2d..1  259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
629       ps-6143    2d..1  263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
630       ps-6143    2d..1  306us : <stack trace>
631  => trace_hardirqs_on_caller
632  => trace_hardirqs_on
633  => _raw_spin_unlock_irqrestore
634  => do_task_stat
635  => proc_tgid_stat
636  => proc_single_show
637  => seq_read
638  => vfs_read
639  => sys_read
640  => system_call_fastpath
641 
642 
643 This shows that the current tracer is "irqsoff" tracing the time
644 for which interrupts were disabled. It gives the trace version (which
645 never changes) and the version of the kernel upon which this was executed on
646 (3.10). Then it displays the max latency in microseconds (259 us). The number
647 of trace entries displayed and the total number (both are four: #4/4).
648 VP, KP, SP, and HP are always zero and are reserved for later use.
649 #P is the number of online CPUs (#P:4).
650 
651 The task is the process that was running when the latency
652 occurred. (ps pid: 6143).
653 
654 The start and stop (the functions in which the interrupts were
655 disabled and enabled respectively) that caused the latencies:
656 
657  __lock_task_sighand is where the interrupts were disabled.
658  _raw_spin_unlock_irqrestore is where they were enabled again.
659 
660 The next lines after the header are the trace itself. The header
661 explains which is which.
662 
663   cmd: The name of the process in the trace.
664 
665   pid: The PID of that process.
666 
667   CPU#: The CPU which the process was running on.
668 
669   irqs-off: 'd' interrupts are disabled. '.' otherwise.
670             Note: If the architecture does not support a way to
671                   read the irq flags variable, an 'X' will always
672                   be printed here.
673 
674   need-resched:
675         'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
676         'n' only TIF_NEED_RESCHED is set,
677         'p' only PREEMPT_NEED_RESCHED is set,
678         '.' otherwise.
679 
680   hardirq/softirq:
681         'H' - hard irq occurred inside a softirq.
682         'h' - hard irq is running
683         's' - soft irq is running
684         '.' - normal context.
685 
686   preempt-depth: The level of preempt_disabled
687 
688 The above is mostly meaningful for kernel developers.
689 
690   time: When the latency-format option is enabled, the trace file
691         output includes a timestamp relative to the start of the
692         trace. This differs from the output when latency-format
693         is disabled, which includes an absolute timestamp.
694 
695   delay: This is just to help catch your eye a bit better. And
696          needs to be fixed to be only relative to the same CPU.
697          The marks are determined by the difference between this
698          current trace and the next trace.
699           '$' - greater than 1 second
700           '@' - greater than 100 milisecond
701           '*' - greater than 10 milisecond
702           '#' - greater than 1000 microsecond
703           '!' - greater than 100 microsecond
704           '+' - greater than 10 microsecond
705           ' ' - less than or equal to 10 microsecond.
706 
707   The rest is the same as the 'trace' file.
708 
709   Note, the latency tracers will usually end with a back trace
710   to easily find where the latency occurred.
711 
712 trace_options
713 -------------
714 
715 The trace_options file (or the options directory) is used to control
716 what gets printed in the trace output, or manipulate the tracers.
717 To see what is available, simply cat the file:
718 
719   cat trace_options
720 print-parent
721 nosym-offset
722 nosym-addr
723 noverbose
724 noraw
725 nohex
726 nobin
727 noblock
728 nostacktrace
729 trace_printk
730 noftrace_preempt
731 nobranch
732 annotate
733 nouserstacktrace
734 nosym-userobj
735 noprintk-msg-only
736 context-info
737 latency-format
738 sleep-time
739 graph-time
740 record-cmd
741 overwrite
742 nodisable_on_free
743 irq-info
744 markers
745 function-trace
746 
747 To disable one of the options, echo in the option prepended with
748 "no".
749 
750   echo noprint-parent > trace_options
751 
752 To enable an option, leave off the "no".
753 
754   echo sym-offset > trace_options
755 
756 Here are the available options:
757 
758   print-parent - On function traces, display the calling (parent)
759                  function as well as the function being traced.
760 
761   print-parent:
762    bash-4000  [01]  1477.606694: simple_strtoul <-kstrtoul
763 
764   noprint-parent:
765    bash-4000  [01]  1477.606694: simple_strtoul
766 
767 
768   sym-offset - Display not only the function name, but also the
769                offset in the function. For example, instead of
770                seeing just "ktime_get", you will see
771                "ktime_get+0xb/0x20".
772 
773   sym-offset:
774    bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0
775 
776   sym-addr - this will also display the function address as well
777              as the function name.
778 
779   sym-addr:
780    bash-4000  [01]  1477.606694: simple_strtoul <c0339346>
781 
782   verbose - This deals with the trace file when the
783             latency-format option is enabled.
784 
785     bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
786     (+0.000ms): simple_strtoul (kstrtoul)
787 
788   raw - This will display raw numbers. This option is best for
789         use with user applications that can translate the raw
790         numbers better than having it done in the kernel.
791 
792   hex - Similar to raw, but the numbers will be in a hexadecimal
793         format.
794 
795   bin - This will print out the formats in raw binary.
796 
797   block - When set, reading trace_pipe will not block when polled.
798 
799   stacktrace - This is one of the options that changes the trace
800                itself. When a trace is recorded, so is the stack
801                of functions. This allows for back traces of
802                trace sites.
803 
804   trace_printk - Can disable trace_printk() from writing into the buffer.
805 
806   branch - Enable branch tracing with the tracer.
807 
808   annotate - It is sometimes confusing when the CPU buffers are full
809              and one CPU buffer had a lot of events recently, thus
810              a shorter time frame, were another CPU may have only had
811              a few events, which lets it have older events. When
812              the trace is reported, it shows the oldest events first,
813              and it may look like only one CPU ran (the one with the
814              oldest events). When the annotate option is set, it will
815              display when a new CPU buffer started:
816 
817           <idle>-0     [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
818           <idle>-0     [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
819           <idle>-0     [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
820 ##### CPU 2 buffer started ####
821           <idle>-0     [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
822           <idle>-0     [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
823           <idle>-0     [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
824 
825   userstacktrace - This option changes the trace. It records a
826                    stacktrace of the current userspace thread.
827 
828   sym-userobj - when user stacktrace are enabled, look up which
829                 object the address belongs to, and print a
830                 relative address. This is especially useful when
831                 ASLR is on, otherwise you don't get a chance to
832                 resolve the address to object/file/line after
833                 the app is no longer running
834 
835                 The lookup is performed when you read
836                 trace,trace_pipe. Example:
837 
838                 a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
839 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
840 
841 
842   printk-msg-only - When set, trace_printk()s will only show the format
843                     and not their parameters (if trace_bprintk() or
844                     trace_bputs() was used to save the trace_printk()).
845 
846   context-info - Show only the event data. Hides the comm, PID,
847                  timestamp, CPU, and other useful data.
848 
849   latency-format - This option changes the trace. When
850                    it is enabled, the trace displays
851                    additional information about the
852                    latencies, as described in "Latency
853                    trace format".
854 
855   sleep-time - When running function graph tracer, to include
856                the time a task schedules out in its function.
857                When enabled, it will account time the task has been
858                scheduled out as part of the function call.
859 
860   graph-time - When running function graph tracer, to include the
861                time to call nested functions. When this is not set,
862                the time reported for the function will only include
863                the time the function itself executed for, not the time
864                for functions that it called.
865 
866   record-cmd - When any event or tracer is enabled, a hook is enabled
867                in the sched_switch trace point to fill comm cache
868                with mapped pids and comms. But this may cause some
869                overhead, and if you only care about pids, and not the
870                name of the task, disabling this option can lower the
871                impact of tracing.
872 
873   overwrite - This controls what happens when the trace buffer is
874               full. If "1" (default), the oldest events are
875               discarded and overwritten. If "0", then the newest
876               events are discarded.
877                 (see per_cpu/cpu0/stats for overrun and dropped)
878 
879   disable_on_free - When the free_buffer is closed, tracing will
880                     stop (tracing_on set to 0).
881 
882   irq-info - Shows the interrupt, preempt count, need resched data.
883              When disabled, the trace looks like:
884 
885 # tracer: function
886 #
887 # entries-in-buffer/entries-written: 144405/9452052   #P:4
888 #
889 #           TASK-PID   CPU#      TIMESTAMP  FUNCTION
890 #              | |       |          |         |
891           <idle>-0     [002]  23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
892           <idle>-0     [002]  23636.756054: activate_task <-ttwu_do_activate.constprop.89
893           <idle>-0     [002]  23636.756055: enqueue_task <-activate_task
894 
895 
896   markers - When set, the trace_marker is writable (only by root).
897             When disabled, the trace_marker will error with EINVAL
898             on write.
899 
900 
901   function-trace - The latency tracers will enable function tracing
902             if this option is enabled (default it is). When
903             it is disabled, the latency tracers do not trace
904             functions. This keeps the overhead of the tracer down
905             when performing latency tests.
906 
907  Note: Some tracers have their own options. They only appear
908        when the tracer is active.
909 
910 
911 
912 irqsoff
913 -------
914 
915 When interrupts are disabled, the CPU can not react to any other
916 external event (besides NMIs and SMIs). This prevents the timer
917 interrupt from triggering or the mouse interrupt from letting
918 the kernel know of a new mouse event. The result is a latency
919 with the reaction time.
920 
921 The irqsoff tracer tracks the time for which interrupts are
922 disabled. When a new maximum latency is hit, the tracer saves
923 the trace leading up to that latency point so that every time a
924 new maximum is reached, the old saved trace is discarded and the
925 new trace is saved.
926 
927 To reset the maximum, echo 0 into tracing_max_latency. Here is
928 an example:
929 
930  # echo 0 > options/function-trace
931  # echo irqsoff > current_tracer
932  # echo 1 > tracing_on
933  # echo 0 > tracing_max_latency
934  # ls -ltr
935  [...]
936  # echo 0 > tracing_on
937  # cat trace
938 # tracer: irqsoff
939 #
940 # irqsoff latency trace v1.1.5 on 3.8.0-test+
941 # --------------------------------------------------------------------
942 # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
943 #    -----------------
944 #    | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
945 #    -----------------
946 #  => started at: run_timer_softirq
947 #  => ended at:   run_timer_softirq
948 #
949 #
950 #                  _------=> CPU#            
951 #                 / _-----=> irqs-off        
952 #                | / _----=> need-resched    
953 #                || / _---=> hardirq/softirq 
954 #                ||| / _--=> preempt-depth   
955 #                |||| /     delay             
956 #  cmd     pid   ||||| time  |   caller      
957 #     \   /      |||||  \    |   /           
958   <idle>-0       0d.s2    0us+: _raw_spin_lock_irq <-run_timer_softirq
959   <idle>-0       0dNs3   17us : _raw_spin_unlock_irq <-run_timer_softirq
960   <idle>-0       0dNs3   17us+: trace_hardirqs_on <-run_timer_softirq
961   <idle>-0       0dNs3   25us : <stack trace>
962  => _raw_spin_unlock_irq
963  => run_timer_softirq
964  => __do_softirq
965  => call_softirq
966  => do_softirq
967  => irq_exit
968  => smp_apic_timer_interrupt
969  => apic_timer_interrupt
970  => rcu_idle_exit
971  => cpu_idle
972  => rest_init
973  => start_kernel
974  => x86_64_start_reservations
975  => x86_64_start_kernel
976 
977 Here we see that that we had a latency of 16 microseconds (which is
978 very good). The _raw_spin_lock_irq in run_timer_softirq disabled
979 interrupts. The difference between the 16 and the displayed
980 timestamp 25us occurred because the clock was incremented
981 between the time of recording the max latency and the time of
982 recording the function that had that latency.
983 
984 Note the above example had function-trace not set. If we set
985 function-trace, we get a much larger output:
986 
987  with echo 1 > options/function-trace
988 
989 # tracer: irqsoff
990 #
991 # irqsoff latency trace v1.1.5 on 3.8.0-test+
992 # --------------------------------------------------------------------
993 # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
994 #    -----------------
995 #    | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
996 #    -----------------
997 #  => started at: ata_scsi_queuecmd
998 #  => ended at:   ata_scsi_queuecmd
999 #
1000 #
1001 #                  _------=> CPU#            
1002 #                 / _-----=> irqs-off        
1003 #                | / _----=> need-resched    
1004 #                || / _---=> hardirq/softirq 
1005 #                ||| / _--=> preempt-depth   
1006 #                |||| /     delay             
1007 #  cmd     pid   ||||| time  |   caller      
1008 #     \   /      |||||  \    |   /           
1009     bash-2042    3d...    0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1010     bash-2042    3d...    0us : add_preempt_count <-_raw_spin_lock_irqsave
1011     bash-2042    3d..1    1us : ata_scsi_find_dev <-ata_scsi_queuecmd
1012     bash-2042    3d..1    1us : __ata_scsi_find_dev <-ata_scsi_find_dev
1013     bash-2042    3d..1    2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
1014     bash-2042    3d..1    2us : ata_qc_new_init <-__ata_scsi_queuecmd
1015     bash-2042    3d..1    3us : ata_sg_init <-__ata_scsi_queuecmd
1016     bash-2042    3d..1    4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
1017     bash-2042    3d..1    4us : ata_build_rw_tf <-ata_scsi_rw_xlat
1018 [...]
1019     bash-2042    3d..1   67us : delay_tsc <-__delay
1020     bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
1021     bash-2042    3d..2   67us : sub_preempt_count <-delay_tsc
1022     bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
1023     bash-2042    3d..2   68us : sub_preempt_count <-delay_tsc
1024     bash-2042    3d..1   68us+: ata_bmdma_start <-ata_bmdma_qc_issue
1025     bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1026     bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1027     bash-2042    3d..1   72us+: trace_hardirqs_on <-ata_scsi_queuecmd
1028     bash-2042    3d..1  120us : <stack trace>
1029  => _raw_spin_unlock_irqrestore
1030  => ata_scsi_queuecmd
1031  => scsi_dispatch_cmd
1032  => scsi_request_fn
1033  => __blk_run_queue_uncond
1034  => __blk_run_queue
1035  => blk_queue_bio
1036  => generic_make_request
1037  => submit_bio
1038  => submit_bh
1039  => __ext3_get_inode_loc
1040  => ext3_iget
1041  => ext3_lookup
1042  => lookup_real
1043  => __lookup_hash
1044  => walk_component
1045  => lookup_last
1046  => path_lookupat
1047  => filename_lookup
1048  => user_path_at_empty
1049  => user_path_at
1050  => vfs_fstatat
1051  => vfs_stat
1052  => sys_newstat
1053  => system_call_fastpath
1054 
1055 
1056 Here we traced a 71 microsecond latency. But we also see all the
1057 functions that were called during that time. Note that by
1058 enabling function tracing, we incur an added overhead. This
1059 overhead may extend the latency times. But nevertheless, this
1060 trace has provided some very helpful debugging information.
1061 
1062 
1063 preemptoff
1064 ----------
1065 
1066 When preemption is disabled, we may be able to receive
1067 interrupts but the task cannot be preempted and a higher
1068 priority task must wait for preemption to be enabled again
1069 before it can preempt a lower priority task.
1070 
1071 The preemptoff tracer traces the places that disable preemption.
1072 Like the irqsoff tracer, it records the maximum latency for
1073 which preemption was disabled. The control of preemptoff tracer
1074 is much like the irqsoff tracer.
1075 
1076  # echo 0 > options/function-trace
1077  # echo preemptoff > current_tracer
1078  # echo 1 > tracing_on
1079  # echo 0 > tracing_max_latency
1080  # ls -ltr
1081  [...]
1082  # echo 0 > tracing_on
1083  # cat trace
1084 # tracer: preemptoff
1085 #
1086 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1087 # --------------------------------------------------------------------
1088 # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1089 #    -----------------
1090 #    | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
1091 #    -----------------
1092 #  => started at: do_IRQ
1093 #  => ended at:   do_IRQ
1094 #
1095 #
1096 #                  _------=> CPU#            
1097 #                 / _-----=> irqs-off        
1098 #                | / _----=> need-resched    
1099 #                || / _---=> hardirq/softirq 
1100 #                ||| / _--=> preempt-depth   
1101 #                |||| /     delay             
1102 #  cmd     pid   ||||| time  |   caller      
1103 #     \   /      |||||  \    |   /           
1104     sshd-1991    1d.h.    0us+: irq_enter <-do_IRQ
1105     sshd-1991    1d..1   46us : irq_exit <-do_IRQ
1106     sshd-1991    1d..1   47us+: trace_preempt_on <-do_IRQ
1107     sshd-1991    1d..1   52us : <stack trace>
1108  => sub_preempt_count
1109  => irq_exit
1110  => do_IRQ
1111  => ret_from_intr
1112 
1113 
1114 This has some more changes. Preemption was disabled when an
1115 interrupt came in (notice the 'h'), and was enabled on exit.
1116 But we also see that interrupts have been disabled when entering
1117 the preempt off section and leaving it (the 'd'). We do not know if
1118 interrupts were enabled in the mean time or shortly after this
1119 was over.
1120 
1121 # tracer: preemptoff
1122 #
1123 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1124 # --------------------------------------------------------------------
1125 # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1126 #    -----------------
1127 #    | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
1128 #    -----------------
1129 #  => started at: wake_up_new_task
1130 #  => ended at:   task_rq_unlock
1131 #
1132 #
1133 #                  _------=> CPU#            
1134 #                 / _-----=> irqs-off        
1135 #                | / _----=> need-resched    
1136 #                || / _---=> hardirq/softirq 
1137 #                ||| / _--=> preempt-depth   
1138 #                |||| /     delay             
1139 #  cmd     pid   ||||| time  |   caller      
1140 #     \   /      |||||  \    |   /           
1141     bash-1994    1d..1    0us : _raw_spin_lock_irqsave <-wake_up_new_task
1142     bash-1994    1d..1    0us : select_task_rq_fair <-select_task_rq
1143     bash-1994    1d..1    1us : __rcu_read_lock <-select_task_rq_fair
1144     bash-1994    1d..1    1us : source_load <-select_task_rq_fair
1145     bash-1994    1d..1    1us : source_load <-select_task_rq_fair
1146 [...]
1147     bash-1994    1d..1   12us : irq_enter <-smp_apic_timer_interrupt
1148     bash-1994    1d..1   12us : rcu_irq_enter <-irq_enter
1149     bash-1994    1d..1   13us : add_preempt_count <-irq_enter
1150     bash-1994    1d.h1   13us : exit_idle <-smp_apic_timer_interrupt
1151     bash-1994    1d.h1   13us : hrtimer_interrupt <-smp_apic_timer_interrupt
1152     bash-1994    1d.h1   13us : _raw_spin_lock <-hrtimer_interrupt
1153     bash-1994    1d.h1   14us : add_preempt_count <-_raw_spin_lock
1154     bash-1994    1d.h2   14us : ktime_get_update_offsets <-hrtimer_interrupt
1155 [...]
1156     bash-1994    1d.h1   35us : lapic_next_event <-clockevents_program_event
1157     bash-1994    1d.h1   35us : irq_exit <-smp_apic_timer_interrupt
1158     bash-1994    1d.h1   36us : sub_preempt_count <-irq_exit
1159     bash-1994    1d..2   36us : do_softirq <-irq_exit
1160     bash-1994    1d..2   36us : __do_softirq <-call_softirq
1161     bash-1994    1d..2   36us : __local_bh_disable <-__do_softirq
1162     bash-1994    1d.s2   37us : add_preempt_count <-_raw_spin_lock_irq
1163     bash-1994    1d.s3   38us : _raw_spin_unlock <-run_timer_softirq
1164     bash-1994    1d.s3   39us : sub_preempt_count <-_raw_spin_unlock
1165     bash-1994    1d.s2   39us : call_timer_fn <-run_timer_softirq
1166 [...]
1167     bash-1994    1dNs2   81us : cpu_needs_another_gp <-rcu_process_callbacks
1168     bash-1994    1dNs2   82us : __local_bh_enable <-__do_softirq
1169     bash-1994    1dNs2   82us : sub_preempt_count <-__local_bh_enable
1170     bash-1994    1dN.2   82us : idle_cpu <-irq_exit
1171     bash-1994    1dN.2   83us : rcu_irq_exit <-irq_exit
1172     bash-1994    1dN.2   83us : sub_preempt_count <-irq_exit
1173     bash-1994    1.N.1   84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
1174     bash-1994    1.N.1   84us+: trace_preempt_on <-task_rq_unlock
1175     bash-1994    1.N.1  104us : <stack trace>
1176  => sub_preempt_count
1177  => _raw_spin_unlock_irqrestore
1178  => task_rq_unlock
1179  => wake_up_new_task
1180  => do_fork
1181  => sys_clone
1182  => stub_clone
1183 
1184 
1185 The above is an example of the preemptoff trace with
1186 function-trace set. Here we see that interrupts were not disabled
1187 the entire time. The irq_enter code lets us know that we entered
1188 an interrupt 'h'. Before that, the functions being traced still
1189 show that it is not in an interrupt, but we can see from the
1190 functions themselves that this is not the case.
1191 
1192 preemptirqsoff
1193 --------------
1194 
1195 Knowing the locations that have interrupts disabled or
1196 preemption disabled for the longest times is helpful. But
1197 sometimes we would like to know when either preemption and/or
1198 interrupts are disabled.
1199 
1200 Consider the following code:
1201 
1202     local_irq_disable();
1203     call_function_with_irqs_off();
1204     preempt_disable();
1205     call_function_with_irqs_and_preemption_off();
1206     local_irq_enable();
1207     call_function_with_preemption_off();
1208     preempt_enable();
1209 
1210 The irqsoff tracer will record the total length of
1211 call_function_with_irqs_off() and
1212 call_function_with_irqs_and_preemption_off().
1213 
1214 The preemptoff tracer will record the total length of
1215 call_function_with_irqs_and_preemption_off() and
1216 call_function_with_preemption_off().
1217 
1218 But neither will trace the time that interrupts and/or
1219 preemption is disabled. This total time is the time that we can
1220 not schedule. To record this time, use the preemptirqsoff
1221 tracer.
1222 
1223 Again, using this trace is much like the irqsoff and preemptoff
1224 tracers.
1225 
1226  # echo 0 > options/function-trace
1227  # echo preemptirqsoff > current_tracer
1228  # echo 1 > tracing_on
1229  # echo 0 > tracing_max_latency
1230  # ls -ltr
1231  [...]
1232  # echo 0 > tracing_on
1233  # cat trace
1234 # tracer: preemptirqsoff
1235 #
1236 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1237 # --------------------------------------------------------------------
1238 # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1239 #    -----------------
1240 #    | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
1241 #    -----------------
1242 #  => started at: ata_scsi_queuecmd
1243 #  => ended at:   ata_scsi_queuecmd
1244 #
1245 #
1246 #                  _------=> CPU#            
1247 #                 / _-----=> irqs-off        
1248 #                | / _----=> need-resched    
1249 #                || / _---=> hardirq/softirq 
1250 #                ||| / _--=> preempt-depth   
1251 #                |||| /     delay             
1252 #  cmd     pid   ||||| time  |   caller      
1253 #     \   /      |||||  \    |   /           
1254       ls-2230    3d...    0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1255       ls-2230    3...1  100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1256       ls-2230    3...1  101us+: trace_preempt_on <-ata_scsi_queuecmd
1257       ls-2230    3...1  111us : <stack trace>
1258  => sub_preempt_count
1259  => _raw_spin_unlock_irqrestore
1260  => ata_scsi_queuecmd
1261  => scsi_dispatch_cmd
1262  => scsi_request_fn
1263  => __blk_run_queue_uncond
1264  => __blk_run_queue
1265  => blk_queue_bio
1266  => generic_make_request
1267  => submit_bio
1268  => submit_bh
1269  => ext3_bread
1270  => ext3_dir_bread
1271  => htree_dirblock_to_tree
1272  => ext3_htree_fill_tree
1273  => ext3_readdir
1274  => vfs_readdir
1275  => sys_getdents
1276  => system_call_fastpath
1277 
1278 
1279 The trace_hardirqs_off_thunk is called from assembly on x86 when
1280 interrupts are disabled in the assembly code. Without the
1281 function tracing, we do not know if interrupts were enabled
1282 within the preemption points. We do see that it started with
1283 preemption enabled.
1284 
1285 Here is a trace with function-trace set:
1286 
1287 # tracer: preemptirqsoff
1288 #
1289 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1290 # --------------------------------------------------------------------
1291 # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1292 #    -----------------
1293 #    | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
1294 #    -----------------
1295 #  => started at: schedule
1296 #  => ended at:   mutex_unlock
1297 #
1298 #
1299 #                  _------=> CPU#            
1300 #                 / _-----=> irqs-off        
1301 #                | / _----=> need-resched    
1302 #                || / _---=> hardirq/softirq 
1303 #                ||| / _--=> preempt-depth   
1304 #                |||| /     delay             
1305 #  cmd     pid   ||||| time  |   caller      
1306 #     \   /      |||||  \    |   /           
1307 kworker/-59      3...1    0us : __schedule <-schedule
1308 kworker/-59      3d..1    0us : rcu_preempt_qs <-rcu_note_context_switch
1309 kworker/-59      3d..1    1us : add_preempt_count <-_raw_spin_lock_irq
1310 kworker/-59      3d..2    1us : deactivate_task <-__schedule
1311 kworker/-59      3d..2    1us : dequeue_task <-deactivate_task
1312 kworker/-59      3d..2    2us : update_rq_clock <-dequeue_task
1313 kworker/-59      3d..2    2us : dequeue_task_fair <-dequeue_task
1314 kworker/-59      3d..2    2us : update_curr <-dequeue_task_fair
1315 kworker/-59      3d..2    2us : update_min_vruntime <-update_curr
1316 kworker/-59      3d..2    3us : cpuacct_charge <-update_curr
1317 kworker/-59      3d..2    3us : __rcu_read_lock <-cpuacct_charge
1318 kworker/-59      3d..2    3us : __rcu_read_unlock <-cpuacct_charge
1319 kworker/-59      3d..2    3us : update_cfs_rq_blocked_load <-dequeue_task_fair
1320 kworker/-59      3d..2    4us : clear_buddies <-dequeue_task_fair
1321 kworker/-59      3d..2    4us : account_entity_dequeue <-dequeue_task_fair
1322 kworker/-59      3d..2    4us : update_min_vruntime <-dequeue_task_fair
1323 kworker/-59      3d..2    4us : update_cfs_shares <-dequeue_task_fair
1324 kworker/-59      3d..2    5us : hrtick_update <-dequeue_task_fair
1325 kworker/-59      3d..2    5us : wq_worker_sleeping <-__schedule
1326 kworker/-59      3d..2    5us : kthread_data <-wq_worker_sleeping
1327 kworker/-59      3d..2    5us : put_prev_task_fair <-__schedule
1328 kworker/-59      3d..2    6us : pick_next_task_fair <-pick_next_task
1329 kworker/-59      3d..2    6us : clear_buddies <-pick_next_task_fair
1330 kworker/-59      3d..2    6us : set_next_entity <-pick_next_task_fair
1331 kworker/-59      3d..2    6us : update_stats_wait_end <-set_next_entity
1332       ls-2269    3d..2    7us : finish_task_switch <-__schedule
1333       ls-2269    3d..2    7us : _raw_spin_unlock_irq <-finish_task_switch
1334       ls-2269    3d..2    8us : do_IRQ <-ret_from_intr
1335       ls-2269    3d..2    8us : irq_enter <-do_IRQ
1336       ls-2269    3d..2    8us : rcu_irq_enter <-irq_enter
1337       ls-2269    3d..2    9us : add_preempt_count <-irq_enter
1338       ls-2269    3d.h2    9us : exit_idle <-do_IRQ
1339 [...]
1340       ls-2269    3d.h3   20us : sub_preempt_count <-_raw_spin_unlock
1341       ls-2269    3d.h2   20us : irq_exit <-do_IRQ
1342       ls-2269    3d.h2   21us : sub_preempt_count <-irq_exit
1343       ls-2269    3d..3   21us : do_softirq <-irq_exit
1344       ls-2269    3d..3   21us : __do_softirq <-call_softirq
1345       ls-2269    3d..3   21us+: __local_bh_disable <-__do_softirq
1346       ls-2269    3d.s4   29us : sub_preempt_count <-_local_bh_enable_ip
1347       ls-2269    3d.s5   29us : sub_preempt_count <-_local_bh_enable_ip
1348       ls-2269    3d.s5   31us : do_IRQ <-ret_from_intr
1349       ls-2269    3d.s5   31us : irq_enter <-do_IRQ
1350       ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
1351 [...]
1352       ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
1353       ls-2269    3d.s5   32us : add_preempt_count <-irq_enter
1354       ls-2269    3d.H5   32us : exit_idle <-do_IRQ
1355       ls-2269    3d.H5   32us : handle_irq <-do_IRQ
1356       ls-2269    3d.H5   32us : irq_to_desc <-handle_irq
1357       ls-2269    3d.H5   33us : handle_fasteoi_irq <-handle_irq
1358 [...]
1359       ls-2269    3d.s5  158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
1360       ls-2269    3d.s3  158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
1361       ls-2269    3d.s3  159us : __local_bh_enable <-__do_softirq
1362       ls-2269    3d.s3  159us : sub_preempt_count <-__local_bh_enable
1363       ls-2269    3d..3  159us : idle_cpu <-irq_exit
1364       ls-2269    3d..3  159us : rcu_irq_exit <-irq_exit
1365       ls-2269    3d..3  160us : sub_preempt_count <-irq_exit
1366       ls-2269    3d...  161us : __mutex_unlock_slowpath <-mutex_unlock
1367       ls-2269    3d...  162us+: trace_hardirqs_on <-mutex_unlock
1368       ls-2269    3d...  186us : <stack trace>
1369  => __mutex_unlock_slowpath
1370  => mutex_unlock
1371  => process_output
1372  => n_tty_write
1373  => tty_write
1374  => vfs_write
1375  => sys_write
1376  => system_call_fastpath
1377 
1378 This is an interesting trace. It started with kworker running and
1379 scheduling out and ls taking over. But as soon as ls released the
1380 rq lock and enabled interrupts (but not preemption) an interrupt
1381 triggered. When the interrupt finished, it started running softirqs.
1382 But while the softirq was running, another interrupt triggered.
1383 When an interrupt is running inside a softirq, the annotation is 'H'.
1384 
1385 
1386 wakeup
1387 ------
1388 
1389 One common case that people are interested in tracing is the
1390 time it takes for a task that is woken to actually wake up.
1391 Now for non Real-Time tasks, this can be arbitrary. But tracing
1392 it none the less can be interesting. 
1393 
1394 Without function tracing:
1395 
1396  # echo 0 > options/function-trace
1397  # echo wakeup > current_tracer
1398  # echo 1 > tracing_on
1399  # echo 0 > tracing_max_latency
1400  # chrt -f 5 sleep 1
1401  # echo 0 > tracing_on
1402  # cat trace
1403 # tracer: wakeup
1404 #
1405 # wakeup latency trace v1.1.5 on 3.8.0-test+
1406 # --------------------------------------------------------------------
1407 # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1408 #    -----------------
1409 #    | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
1410 #    -----------------
1411 #
1412 #                  _------=> CPU#            
1413 #                 / _-----=> irqs-off        
1414 #                | / _----=> need-resched    
1415 #                || / _---=> hardirq/softirq 
1416 #                ||| / _--=> preempt-depth   
1417 #                |||| /     delay             
1418 #  cmd     pid   ||||| time  |   caller      
1419 #     \   /      |||||  \    |   /           
1420   <idle>-0       3dNs7    0us :      0:120:R   + [003]   312:100:R kworker/3:1H
1421   <idle>-0       3dNs7    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1422   <idle>-0       3d..3   15us : __schedule <-schedule
1423   <idle>-0       3d..3   15us :      0:120:R ==> [003]   312:100:R kworker/3:1H
1424 
1425 The tracer only traces the highest priority task in the system
1426 to avoid tracing the normal circumstances. Here we see that
1427 the kworker with a nice priority of -20 (not very nice), took
1428 just 15 microseconds from the time it woke up, to the time it
1429 ran.
1430 
1431 Non Real-Time tasks are not that interesting. A more interesting
1432 trace is to concentrate only on Real-Time tasks.
1433 
1434 wakeup_rt
1435 ---------
1436 
1437 In a Real-Time environment it is very important to know the
1438 wakeup time it takes for the highest priority task that is woken
1439 up to the time that it executes. This is also known as "schedule
1440 latency". I stress the point that this is about RT tasks. It is
1441 also important to know the scheduling latency of non-RT tasks,
1442 but the average schedule latency is better for non-RT tasks.
1443 Tools like LatencyTop are more appropriate for such
1444 measurements.
1445 
1446 Real-Time environments are interested in the worst case latency.
1447 That is the longest latency it takes for something to happen,
1448 and not the average. We can have a very fast scheduler that may
1449 only have a large latency once in a while, but that would not
1450 work well with Real-Time tasks.  The wakeup_rt tracer was designed
1451 to record the worst case wakeups of RT tasks. Non-RT tasks are
1452 not recorded because the tracer only records one worst case and
1453 tracing non-RT tasks that are unpredictable will overwrite the
1454 worst case latency of RT tasks (just run the normal wakeup
1455 tracer for a while to see that effect).
1456 
1457 Since this tracer only deals with RT tasks, we will run this
1458 slightly differently than we did with the previous tracers.
1459 Instead of performing an 'ls', we will run 'sleep 1' under
1460 'chrt' which changes the priority of the task.
1461 
1462  # echo 0 > options/function-trace
1463  # echo wakeup_rt > current_tracer
1464  # echo 1 > tracing_on
1465  # echo 0 > tracing_max_latency
1466  # chrt -f 5 sleep 1
1467  # echo 0 > tracing_on
1468  # cat trace
1469 # tracer: wakeup
1470 #
1471 # tracer: wakeup_rt
1472 #
1473 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1474 # --------------------------------------------------------------------
1475 # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1476 #    -----------------
1477 #    | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
1478 #    -----------------
1479 #
1480 #                  _------=> CPU#            
1481 #                 / _-----=> irqs-off        
1482 #                | / _----=> need-resched    
1483 #                || / _---=> hardirq/softirq 
1484 #                ||| / _--=> preempt-depth   
1485 #                |||| /     delay             
1486 #  cmd     pid   ||||| time  |   caller      
1487 #     \   /      |||||  \    |   /           
1488   <idle>-0       3d.h4    0us :      0:120:R   + [003]  2389: 94:R sleep
1489   <idle>-0       3d.h4    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1490   <idle>-0       3d..3    5us : __schedule <-schedule
1491   <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
1492 
1493 
1494 Running this on an idle system, we see that it only took 5 microseconds
1495 to perform the task switch.  Note, since the trace point in the schedule
1496 is before the actual "switch", we stop the tracing when the recorded task
1497 is about to schedule in. This may change if we add a new marker at the
1498 end of the scheduler.
1499 
1500 Notice that the recorded task is 'sleep' with the PID of 2389
1501 and it has an rt_prio of 5. This priority is user-space priority
1502 and not the internal kernel priority. The policy is 1 for
1503 SCHED_FIFO and 2 for SCHED_RR.
1504 
1505 Note, that the trace data shows the internal priority (99 - rtprio).
1506 
1507   <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
1508 
1509 The 0:120:R means idle was running with a nice priority of 0 (120 - 20)
1510 and in the running state 'R'. The sleep task was scheduled in with
1511 2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
1512 and it too is in the running state.
1513 
1514 Doing the same with chrt -r 5 and function-trace set.
1515 
1516   echo 1 > options/function-trace
1517 
1518 # tracer: wakeup_rt
1519 #
1520 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1521 # --------------------------------------------------------------------
1522 # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1523 #    -----------------
1524 #    | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
1525 #    -----------------
1526 #
1527 #                  _------=> CPU#            
1528 #                 / _-----=> irqs-off        
1529 #                | / _----=> need-resched    
1530 #                || / _---=> hardirq/softirq 
1531 #                ||| / _--=> preempt-depth   
1532 #                |||| /     delay             
1533 #  cmd     pid   ||||| time  |   caller      
1534 #     \   /      |||||  \    |   /           
1535   <idle>-0       3d.h4    1us+:      0:120:R   + [003]  2448: 94:R sleep
1536   <idle>-0       3d.h4    2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
1537   <idle>-0       3d.h3    3us : check_preempt_curr <-ttwu_do_wakeup
1538   <idle>-0       3d.h3    3us : resched_curr <-check_preempt_curr
1539   <idle>-0       3dNh3    4us : task_woken_rt <-ttwu_do_wakeup
1540   <idle>-0       3dNh3    4us : _raw_spin_unlock <-try_to_wake_up
1541   <idle>-0       3dNh3    4us : sub_preempt_count <-_raw_spin_unlock
1542   <idle>-0       3dNh2    5us : ttwu_stat <-try_to_wake_up
1543   <idle>-0       3dNh2    5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
1544   <idle>-0       3dNh2    6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1545   <idle>-0       3dNh1    6us : _raw_spin_lock <-__run_hrtimer
1546   <idle>-0       3dNh1    6us : add_preempt_count <-_raw_spin_lock
1547   <idle>-0       3dNh2    7us : _raw_spin_unlock <-hrtimer_interrupt
1548   <idle>-0       3dNh2    7us : sub_preempt_count <-_raw_spin_unlock
1549   <idle>-0       3dNh1    7us : tick_program_event <-hrtimer_interrupt
1550   <idle>-0       3dNh1    7us : clockevents_program_event <-tick_program_event
1551   <idle>-0       3dNh1    8us : ktime_get <-clockevents_program_event
1552   <idle>-0       3dNh1    8us : lapic_next_event <-clockevents_program_event
1553   <idle>-0       3dNh1    8us : irq_exit <-smp_apic_timer_interrupt
1554   <idle>-0       3dNh1    9us : sub_preempt_count <-irq_exit
1555   <idle>-0       3dN.2    9us : idle_cpu <-irq_exit
1556   <idle>-0       3dN.2    9us : rcu_irq_exit <-irq_exit
1557   <idle>-0       3dN.2   10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
1558   <idle>-0       3dN.2   10us : sub_preempt_count <-irq_exit
1559   <idle>-0       3.N.1   11us : rcu_idle_exit <-cpu_idle
1560   <idle>-0       3dN.1   11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
1561   <idle>-0       3.N.1   11us : tick_nohz_idle_exit <-cpu_idle
1562   <idle>-0       3dN.1   12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
1563   <idle>-0       3dN.1   12us : ktime_get <-tick_nohz_idle_exit
1564   <idle>-0       3dN.1   12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
1565   <idle>-0       3dN.1   13us : update_cpu_load_nohz <-tick_nohz_idle_exit
1566   <idle>-0       3dN.1   13us : _raw_spin_lock <-update_cpu_load_nohz
1567   <idle>-0       3dN.1   13us : add_preempt_count <-_raw_spin_lock
1568   <idle>-0       3dN.2   13us : __update_cpu_load <-update_cpu_load_nohz
1569   <idle>-0       3dN.2   14us : sched_avg_update <-__update_cpu_load
1570   <idle>-0       3dN.2   14us : _raw_spin_unlock <-update_cpu_load_nohz
1571   <idle>-0       3dN.2   14us : sub_preempt_count <-_raw_spin_unlock
1572   <idle>-0       3dN.1   15us : calc_load_exit_idle <-tick_nohz_idle_exit
1573   <idle>-0       3dN.1   15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
1574   <idle>-0       3dN.1   15us : hrtimer_cancel <-tick_nohz_idle_exit
1575   <idle>-0       3dN.1   15us : hrtimer_try_to_cancel <-hrtimer_cancel
1576   <idle>-0       3dN.1   16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
1577   <idle>-0       3dN.1   16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
1578   <idle>-0       3dN.1   16us : add_preempt_count <-_raw_spin_lock_irqsave
1579   <idle>-0       3dN.2   17us : __remove_hrtimer <-remove_hrtimer.part.16
1580   <idle>-0       3dN.2   17us : hrtimer_force_reprogram <-__remove_hrtimer
1581   <idle>-0       3dN.2   17us : tick_program_event <-hrtimer_force_reprogram
1582   <idle>-0       3dN.2   18us : clockevents_program_event <-tick_program_event
1583   <idle>-0       3dN.2   18us : ktime_get <-clockevents_program_event
1584   <idle>-0       3dN.2   18us : lapic_next_event <-clockevents_program_event
1585   <idle>-0       3dN.2   19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
1586   <idle>-0       3dN.2   19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1587   <idle>-0       3dN.1   19us : hrtimer_forward <-tick_nohz_idle_exit
1588   <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
1589   <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
1590   <idle>-0       3dN.1   20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
1591   <idle>-0       3dN.1   20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
1592   <idle>-0       3dN.1   21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
1593   <idle>-0       3dN.1   21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
1594   <idle>-0       3dN.1   21us : add_preempt_count <-_raw_spin_lock_irqsave
1595   <idle>-0       3dN.2   22us : ktime_add_safe <-__hrtimer_start_range_ns
1596   <idle>-0       3dN.2   22us : enqueue_hrtimer <-__hrtimer_start_range_ns
1597   <idle>-0       3dN.2   22us : tick_program_event <-__hrtimer_start_range_ns
1598   <idle>-0       3dN.2   23us : clockevents_program_event <-tick_program_event
1599   <idle>-0       3dN.2   23us : ktime_get <-clockevents_program_event
1600   <idle>-0       3dN.2   23us : lapic_next_event <-clockevents_program_event
1601   <idle>-0       3dN.2   24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
1602   <idle>-0       3dN.2   24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1603   <idle>-0       3dN.1   24us : account_idle_ticks <-tick_nohz_idle_exit
1604   <idle>-0       3dN.1   24us : account_idle_time <-account_idle_ticks
1605   <idle>-0       3.N.1   25us : sub_preempt_count <-cpu_idle
1606   <idle>-0       3.N..   25us : schedule <-cpu_idle
1607   <idle>-0       3.N..   25us : __schedule <-preempt_schedule
1608   <idle>-0       3.N..   26us : add_preempt_count <-__schedule
1609   <idle>-0       3.N.1   26us : rcu_note_context_switch <-__schedule
1610   <idle>-0       3.N.1   26us : rcu_sched_qs <-rcu_note_context_switch
1611   <idle>-0       3dN.1   27us : rcu_preempt_qs <-rcu_note_context_switch
1612   <idle>-0       3.N.1   27us : _raw_spin_lock_irq <-__schedule
1613   <idle>-0       3dN.1   27us : add_preempt_count <-_raw_spin_lock_irq
1614   <idle>-0       3dN.2   28us : put_prev_task_idle <-__schedule
1615   <idle>-0       3dN.2   28us : pick_next_task_stop <-pick_next_task
1616   <idle>-0       3dN.2   28us : pick_next_task_rt <-pick_next_task
1617   <idle>-0       3dN.2   29us : dequeue_pushable_task <-pick_next_task_rt
1618   <idle>-0       3d..3   29us : __schedule <-preempt_schedule
1619   <idle>-0       3d..3   30us :      0:120:R ==> [003]  2448: 94:R sleep
1620 
1621 This isn't that big of a trace, even with function tracing enabled,
1622 so I included the entire trace.
1623 
1624 The interrupt went off while when the system was idle. Somewhere
1625 before task_woken_rt() was called, the NEED_RESCHED flag was set,
1626 this is indicated by the first occurrence of the 'N' flag.
1627 
1628 Latency tracing and events
1629 --------------------------
1630 As function tracing can induce a much larger latency, but without
1631 seeing what happens within the latency it is hard to know what
1632 caused it. There is a middle ground, and that is with enabling
1633 events.
1634 
1635  # echo 0 > options/function-trace
1636  # echo wakeup_rt > current_tracer
1637  # echo 1 > events/enable
1638  # echo 1 > tracing_on
1639  # echo 0 > tracing_max_latency
1640  # chrt -f 5 sleep 1
1641  # echo 0 > tracing_on
1642  # cat trace
1643 # tracer: wakeup_rt
1644 #
1645 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1646 # --------------------------------------------------------------------
1647 # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1648 #    -----------------
1649 #    | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
1650 #    -----------------
1651 #
1652 #                  _------=> CPU#            
1653 #                 / _-----=> irqs-off        
1654 #                | / _----=> need-resched    
1655 #                || / _---=> hardirq/softirq 
1656 #                ||| / _--=> preempt-depth   
1657 #                |||| /     delay             
1658 #  cmd     pid   ||||| time  |   caller      
1659 #     \   /      |||||  \    |   /           
1660   <idle>-0       2d.h4    0us :      0:120:R   + [002]  5882: 94:R sleep
1661   <idle>-0       2d.h4    0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
1662   <idle>-0       2d.h4    1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
1663   <idle>-0       2dNh2    1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
1664   <idle>-0       2.N.2    2us : power_end: cpu_id=2
1665   <idle>-0       2.N.2    3us : cpu_idle: state=4294967295 cpu_id=2
1666   <idle>-0       2dN.3    4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
1667   <idle>-0       2dN.3    4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
1668   <idle>-0       2.N.2    5us : rcu_utilization: Start context switch
1669   <idle>-0       2.N.2    5us : rcu_utilization: End context switch
1670   <idle>-0       2d..3    6us : __schedule <-schedule
1671   <idle>-0       2d..3    6us :      0:120:R ==> [002]  5882: 94:R sleep
1672 
1673 
1674 function
1675 --------
1676 
1677 This tracer is the function tracer. Enabling the function tracer
1678 can be done from the debug file system. Make sure the
1679 ftrace_enabled is set; otherwise this tracer is a nop.
1680 See the "ftrace_enabled" section below.
1681 
1682  # sysctl kernel.ftrace_enabled=1
1683  # echo function > current_tracer
1684  # echo 1 > tracing_on
1685  # usleep 1
1686  # echo 0 > tracing_on
1687  # cat trace
1688 # tracer: function
1689 #
1690 # entries-in-buffer/entries-written: 24799/24799   #P:4
1691 #
1692 #                              _-----=> irqs-off
1693 #                             / _----=> need-resched
1694 #                            | / _---=> hardirq/softirq
1695 #                            || / _--=> preempt-depth
1696 #                            ||| /     delay
1697 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
1698 #              | |       |   ||||       |         |
1699             bash-1994  [002] ....  3082.063030: mutex_unlock <-rb_simple_write
1700             bash-1994  [002] ....  3082.063031: __mutex_unlock_slowpath <-mutex_unlock
1701             bash-1994  [002] ....  3082.063031: __fsnotify_parent <-fsnotify_modify
1702             bash-1994  [002] ....  3082.063032: fsnotify <-fsnotify_modify
1703             bash-1994  [002] ....  3082.063032: __srcu_read_lock <-fsnotify
1704             bash-1994  [002] ....  3082.063032: add_preempt_count <-__srcu_read_lock
1705             bash-1994  [002] ...1  3082.063032: sub_preempt_count <-__srcu_read_lock
1706             bash-1994  [002] ....  3082.063033: __srcu_read_unlock <-fsnotify
1707 [...]
1708 
1709 
1710 Note: function tracer uses ring buffers to store the above
1711 entries. The newest data may overwrite the oldest data.
1712 Sometimes using echo to stop the trace is not sufficient because
1713 the tracing could have overwritten the data that you wanted to
1714 record. For this reason, it is sometimes better to disable
1715 tracing directly from a program. This allows you to stop the
1716 tracing at the point that you hit the part that you are
1717 interested in. To disable the tracing directly from a C program,
1718 something like following code snippet can be used:
1719 
1720 int trace_fd;
1721 [...]
1722 int main(int argc, char *argv[]) {
1723         [...]
1724         trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
1725         [...]
1726         if (condition_hit()) {
1727                 write(trace_fd, "0", 1);
1728         }
1729         [...]
1730 }
1731 
1732 
1733 Single thread tracing
1734 ---------------------
1735 
1736 By writing into set_ftrace_pid you can trace a
1737 single thread. For example:
1738 
1739 # cat set_ftrace_pid
1740 no pid
1741 # echo 3111 > set_ftrace_pid
1742 # cat set_ftrace_pid
1743 3111
1744 # echo function > current_tracer
1745 # cat trace | head
1746  # tracer: function
1747  #
1748  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
1749  #              | |       |          |         |
1750      yum-updatesd-3111  [003]  1637.254676: finish_task_switch <-thread_return
1751      yum-updatesd-3111  [003]  1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
1752      yum-updatesd-3111  [003]  1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
1753      yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
1754      yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
1755      yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll
1756 # echo > set_ftrace_pid
1757 # cat trace |head
1758  # tracer: function
1759  #
1760  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
1761  #              | |       |          |         |
1762  ##### CPU 3 buffer started ####
1763      yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
1764      yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
1765      yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
1766      yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
1767      yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit
1768 
1769 If you want to trace a function when executing, you could use
1770 something like this simple program:
1771 
1772 #include <stdio.h>
1773 #include <stdlib.h>
1774 #include <sys/types.h>
1775 #include <sys/stat.h>
1776 #include <fcntl.h>
1777 #include <unistd.h>
1778 #include <string.h>
1779 
1780 #define _STR(x) #x
1781 #define STR(x) _STR(x)
1782 #define MAX_PATH 256
1783 
1784 const char *find_debugfs(void)
1785 {
1786        static char debugfs[MAX_PATH+1];
1787        static int debugfs_found;
1788        char type[100];
1789        FILE *fp;
1790 
1791        if (debugfs_found)
1792                return debugfs;
1793 
1794        if ((fp = fopen("/proc/mounts","r")) == NULL) {
1795                perror("/proc/mounts");
1796                return NULL;
1797        }
1798 
1799        while (fscanf(fp, "%*s %"
1800                      STR(MAX_PATH)
1801                      "s %99s %*s %*d %*d\n",
1802                      debugfs, type) == 2) {
1803                if (strcmp(type, "debugfs") == 0)
1804                        break;
1805        }
1806        fclose(fp);
1807 
1808        if (strcmp(type, "debugfs") != 0) {
1809                fprintf(stderr, "debugfs not mounted");
1810                return NULL;
1811        }
1812 
1813        strcat(debugfs, "/tracing/");
1814        debugfs_found = 1;
1815 
1816        return debugfs;
1817 }
1818 
1819 const char *tracing_file(const char *file_name)
1820 {
1821        static char trace_file[MAX_PATH+1];
1822        snprintf(trace_file, MAX_PATH, "%s/%s", find_debugfs(), file_name);
1823        return trace_file;
1824 }
1825 
1826 int main (int argc, char **argv)
1827 {
1828         if (argc < 1)
1829                 exit(-1);
1830 
1831         if (fork() > 0) {
1832                 int fd, ffd;
1833                 char line[64];
1834                 int s;
1835 
1836                 ffd = open(tracing_file("current_tracer"), O_WRONLY);
1837                 if (ffd < 0)
1838                         exit(-1);
1839                 write(ffd, "nop", 3);
1840 
1841                 fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
1842                 s = sprintf(line, "%d\n", getpid());
1843                 write(fd, line, s);
1844 
1845                 write(ffd, "function", 8);
1846 
1847                 close(fd);
1848                 close(ffd);
1849 
1850                 execvp(argv[1], argv+1);
1851         }
1852 
1853         return 0;
1854 }
1855 
1856 Or this simple script!
1857 
1858 ------
1859 #!/bin/bash
1860 
1861 debugfs=`sed -ne 's/^debugfs \(.*\) debugfs.*/\1/p' /proc/mounts`
1862 echo nop > $debugfs/tracing/current_tracer
1863 echo 0 > $debugfs/tracing/tracing_on
1864 echo $$ > $debugfs/tracing/set_ftrace_pid
1865 echo function > $debugfs/tracing/current_tracer
1866 echo 1 > $debugfs/tracing/tracing_on
1867 exec "$@"
1868 ------
1869 
1870 
1871 function graph tracer
1872 ---------------------------
1873 
1874 This tracer is similar to the function tracer except that it
1875 probes a function on its entry and its exit. This is done by
1876 using a dynamically allocated stack of return addresses in each
1877 task_struct. On function entry the tracer overwrites the return
1878 address of each function traced to set a custom probe. Thus the
1879 original return address is stored on the stack of return address
1880 in the task_struct.
1881 
1882 Probing on both ends of a function leads to special features
1883 such as:
1884 
1885 - measure of a function's time execution
1886 - having a reliable call stack to draw function calls graph
1887 
1888 This tracer is useful in several situations:
1889 
1890 - you want to find the reason of a strange kernel behavior and
1891   need to see what happens in detail on any areas (or specific
1892   ones).
1893 
1894 - you are experiencing weird latencies but it's difficult to
1895   find its origin.
1896 
1897 - you want to find quickly which path is taken by a specific
1898   function
1899 
1900 - you just want to peek inside a working kernel and want to see
1901   what happens there.
1902 
1903 # tracer: function_graph
1904 #
1905 # CPU  DURATION                  FUNCTION CALLS
1906 # |     |   |                     |   |   |   |
1907 
1908  0)               |  sys_open() {
1909  0)               |    do_sys_open() {
1910  0)               |      getname() {
1911  0)               |        kmem_cache_alloc() {
1912  0)   1.382 us    |          __might_sleep();
1913  0)   2.478 us    |        }
1914  0)               |        strncpy_from_user() {
1915  0)               |          might_fault() {
1916  0)   1.389 us    |            __might_sleep();
1917  0)   2.553 us    |          }
1918  0)   3.807 us    |        }
1919  0)   7.876 us    |      }
1920  0)               |      alloc_fd() {
1921  0)   0.668 us    |        _spin_lock();
1922  0)   0.570 us    |        expand_files();
1923  0)   0.586 us    |        _spin_unlock();
1924 
1925 
1926 There are several columns that can be dynamically
1927 enabled/disabled. You can use every combination of options you
1928 want, depending on your needs.
1929 
1930 - The cpu number on which the function executed is default
1931   enabled.  It is sometimes better to only trace one cpu (see
1932   tracing_cpu_mask file) or you might sometimes see unordered
1933   function calls while cpu tracing switch.
1934 
1935         hide: echo nofuncgraph-cpu > trace_options
1936         show: echo funcgraph-cpu > trace_options
1937 
1938 - The duration (function's time of execution) is displayed on
1939   the closing bracket line of a function or on the same line
1940   than the current function in case of a leaf one. It is default
1941   enabled.
1942 
1943         hide: echo nofuncgraph-duration > trace_options
1944         show: echo funcgraph-duration > trace_options
1945 
1946 - The overhead field precedes the duration field in case of
1947   reached duration thresholds.
1948 
1949         hide: echo nofuncgraph-overhead > trace_options
1950         show: echo funcgraph-overhead > trace_options
1951         depends on: funcgraph-duration
1952 
1953   ie:
1954 
1955   3) # 1837.709 us |          } /* __switch_to */
1956   3)               |          finish_task_switch() {
1957   3)   0.313 us    |            _raw_spin_unlock_irq();
1958   3)   3.177 us    |          }
1959   3) # 1889.063 us |        } /* __schedule */
1960   3) ! 140.417 us  |      } /* __schedule */
1961   3) # 2034.948 us |    } /* schedule */
1962   3) * 33998.59 us |  } /* schedule_preempt_disabled */
1963 
1964   [...]
1965 
1966   1)   0.260 us    |              msecs_to_jiffies();
1967   1)   0.313 us    |              __rcu_read_unlock();
1968   1) + 61.770 us   |            }
1969   1) + 64.479 us   |          }
1970   1)   0.313 us    |          rcu_bh_qs();
1971   1)   0.313 us    |          __local_bh_enable();
1972   1) ! 217.240 us  |        }
1973   1)   0.365 us    |        idle_cpu();
1974   1)               |        rcu_irq_exit() {
1975   1)   0.417 us    |          rcu_eqs_enter_common.isra.47();
1976   1)   3.125 us    |        }
1977   1) ! 227.812 us  |      }
1978   1) ! 457.395 us  |    }
1979   1) @ 119760.2 us |  }
1980 
1981   [...]
1982 
1983   2)               |    handle_IPI() {
1984   1)   6.979 us    |                  }
1985   2)   0.417 us    |      scheduler_ipi();
1986   1)   9.791 us    |                }
1987   1) + 12.917 us   |              }
1988   2)   3.490 us    |    }
1989   1) + 15.729 us   |            }
1990   1) + 18.542 us   |          }
1991   2) $ 3594274 us  |  }
1992 
1993   + means that the function exceeded 10 usecs.
1994   ! means that the function exceeded 100 usecs.
1995   # means that the function exceeded 1000 usecs.
1996   * means that the function exceeded 10 msecs.
1997   @ means that the function exceeded 100 msecs.
1998   $ means that the function exceeded 1 sec.
1999 
2000 
2001 - The task/pid field displays the thread cmdline and pid which
2002   executed the function. It is default disabled.
2003 
2004         hide: echo nofuncgraph-proc > trace_options
2005         show: echo funcgraph-proc > trace_options
2006 
2007   ie:
2008 
2009   # tracer: function_graph
2010   #
2011   # CPU  TASK/PID        DURATION                  FUNCTION CALLS
2012   # |    |    |           |   |                     |   |   |   |
2013   0)    sh-4802     |               |                  d_free() {
2014   0)    sh-4802     |               |                    call_rcu() {
2015   0)    sh-4802     |               |                      __call_rcu() {
2016   0)    sh-4802     |   0.616 us    |                        rcu_process_gp_end();
2017   0)    sh-4802     |   0.586 us    |                        check_for_new_grace_period();
2018   0)    sh-4802     |   2.899 us    |                      }
2019   0)    sh-4802     |   4.040 us    |                    }
2020   0)    sh-4802     |   5.151 us    |                  }
2021   0)    sh-4802     | + 49.370 us   |                }
2022 
2023 
2024 - The absolute time field is an absolute timestamp given by the
2025   system clock since it started. A snapshot of this time is
2026   given on each entry/exit of functions
2027 
2028         hide: echo nofuncgraph-abstime > trace_options
2029         show: echo funcgraph-abstime > trace_options
2030 
2031   ie:
2032 
2033   #
2034   #      TIME       CPU  DURATION                  FUNCTION CALLS
2035   #       |         |     |   |                     |   |   |   |
2036   360.774522 |   1)   0.541 us    |                                          }
2037   360.774522 |   1)   4.663 us    |                                        }
2038   360.774523 |   1)   0.541 us    |                                        __wake_up_bit();
2039   360.774524 |   1)   6.796 us    |                                      }
2040   360.774524 |   1)   7.952 us    |                                    }
2041   360.774525 |   1)   9.063 us    |                                  }
2042   360.774525 |   1)   0.615 us    |                                  journal_mark_dirty();
2043   360.774527 |   1)   0.578 us    |                                  __brelse();
2044   360.774528 |   1)               |                                  reiserfs_prepare_for_journal() {
2045   360.774528 |   1)               |                                    unlock_buffer() {
2046   360.774529 |   1)               |                                      wake_up_bit() {
2047   360.774529 |   1)               |                                        bit_waitqueue() {
2048   360.774530 |   1)   0.594 us    |                                          __phys_addr();
2049 
2050 
2051 The function name is always displayed after the closing bracket
2052 for a function if the start of that function is not in the
2053 trace buffer.
2054 
2055 Display of the function name after the closing bracket may be
2056 enabled for functions whose start is in the trace buffer,
2057 allowing easier searching with grep for function durations.
2058 It is default disabled.
2059 
2060         hide: echo nofuncgraph-tail > trace_options
2061         show: echo funcgraph-tail > trace_options
2062 
2063   Example with nofuncgraph-tail (default):
2064   0)               |      putname() {
2065   0)               |        kmem_cache_free() {
2066   0)   0.518 us    |          __phys_addr();
2067   0)   1.757 us    |        }
2068   0)   2.861 us    |      }
2069 
2070   Example with funcgraph-tail:
2071   0)               |      putname() {
2072   0)               |        kmem_cache_free() {
2073   0)   0.518 us    |          __phys_addr();
2074   0)   1.757 us    |        } /* kmem_cache_free() */
2075   0)   2.861 us    |      } /* putname() */
2076 
2077 You can put some comments on specific functions by using
2078 trace_printk() For example, if you want to put a comment inside
2079 the __might_sleep() function, you just have to include
2080 <linux/ftrace.h> and call trace_printk() inside __might_sleep()
2081 
2082 trace_printk("I'm a comment!\n")
2083 
2084 will produce:
2085 
2086  1)               |             __might_sleep() {
2087  1)               |                /* I'm a comment! */
2088  1)   1.449 us    |             }
2089 
2090 
2091 You might find other useful features for this tracer in the
2092 following "dynamic ftrace" section such as tracing only specific
2093 functions or tasks.
2094 
2095 dynamic ftrace
2096 --------------
2097 
2098 If CONFIG_DYNAMIC_FTRACE is set, the system will run with
2099 virtually no overhead when function tracing is disabled. The way
2100 this works is the mcount function call (placed at the start of
2101 every kernel function, produced by the -pg switch in gcc),
2102 starts of pointing to a simple return. (Enabling FTRACE will
2103 include the -pg switch in the compiling of the kernel.)
2104 
2105 At compile time every C file object is run through the
2106 recordmcount program (located in the scripts directory). This
2107 program will parse the ELF headers in the C object to find all
2108 the locations in the .text section that call mcount. (Note, only
2109 white listed .text sections are processed, since processing other
2110 sections like .init.text may cause races due to those sections
2111 being freed unexpectedly).
2112 
2113 A new section called "__mcount_loc" is created that holds
2114 references to all the mcount call sites in the .text section.
2115 The recordmcount program re-links this section back into the
2116 original object. The final linking stage of the kernel will add all these
2117 references into a single table.
2118 
2119 On boot up, before SMP is initialized, the dynamic ftrace code
2120 scans this table and updates all the locations into nops. It
2121 also records the locations, which are added to the
2122 available_filter_functions list.  Modules are processed as they
2123 are loaded and before they are executed.  When a module is
2124 unloaded, it also removes its functions from the ftrace function
2125 list. This is automatic in the module unload code, and the
2126 module author does not need to worry about it.
2127 
2128 When tracing is enabled, the process of modifying the function
2129 tracepoints is dependent on architecture. The old method is to use
2130 kstop_machine to prevent races with the CPUs executing code being
2131 modified (which can cause the CPU to do undesirable things, especially
2132 if the modified code crosses cache (or page) boundaries), and the nops are
2133 patched back to calls. But this time, they do not call mcount
2134 (which is just a function stub). They now call into the ftrace
2135 infrastructure.
2136 
2137 The new method of modifying the function tracepoints is to place
2138 a breakpoint at the location to be modified, sync all CPUs, modify
2139 the rest of the instruction not covered by the breakpoint. Sync
2140 all CPUs again, and then remove the breakpoint with the finished
2141 version to the ftrace call site.
2142 
2143 Some archs do not even need to monkey around with the synchronization,
2144 and can just slap the new code on top of the old without any
2145 problems with other CPUs executing it at the same time.
2146 
2147 One special side-effect to the recording of the functions being
2148 traced is that we can now selectively choose which functions we
2149 wish to trace and which ones we want the mcount calls to remain
2150 as nops.
2151 
2152 Two files are used, one for enabling and one for disabling the
2153 tracing of specified functions. They are:
2154 
2155   set_ftrace_filter
2156 
2157 and
2158 
2159   set_ftrace_notrace
2160 
2161 A list of available functions that you can add to these files is
2162 listed in:
2163 
2164    available_filter_functions
2165 
2166  # cat available_filter_functions
2167 put_prev_task_idle
2168 kmem_cache_create
2169 pick_next_task_rt
2170 get_online_cpus
2171 pick_next_task_fair
2172 mutex_lock
2173 [...]
2174 
2175 If I am only interested in sys_nanosleep and hrtimer_interrupt:
2176 
2177  # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
2178  # echo function > current_tracer
2179  # echo 1 > tracing_on
2180  # usleep 1
2181  # echo 0 > tracing_on
2182  # cat trace
2183 # tracer: function
2184 #
2185 # entries-in-buffer/entries-written: 5/5   #P:4
2186 #
2187 #                              _-----=> irqs-off
2188 #                             / _----=> need-resched
2189 #                            | / _---=> hardirq/softirq
2190 #                            || / _--=> preempt-depth
2191 #                            ||| /     delay
2192 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2193 #              | |       |   ||||       |         |
2194           usleep-2665  [001] ....  4186.475355: sys_nanosleep <-system_call_fastpath
2195           <idle>-0     [001] d.h1  4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
2196           usleep-2665  [001] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2197           <idle>-0     [003] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2198           <idle>-0     [002] d.h1  4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
2199 
2200 To see which functions are being traced, you can cat the file:
2201 
2202  # cat set_ftrace_filter
2203 hrtimer_interrupt
2204 sys_nanosleep
2205 
2206 
2207 Perhaps this is not enough. The filters also allow simple wild
2208 cards. Only the following are currently available
2209 
2210   <match>*  - will match functions that begin with <match>
2211   *<match>  - will match functions that end with <match>
2212   *<match>* - will match functions that have <match> in it
2213 
2214 These are the only wild cards which are supported.
2215 
2216   <match>*<match> will not work.
2217 
2218 Note: It is better to use quotes to enclose the wild cards,
2219       otherwise the shell may expand the parameters into names
2220       of files in the local directory.
2221 
2222  # echo 'hrtimer_*' > set_ftrace_filter
2223 
2224 Produces:
2225 
2226 # tracer: function
2227 #
2228 # entries-in-buffer/entries-written: 897/897   #P:4
2229 #
2230 #                              _-----=> irqs-off
2231 #                             / _----=> need-resched
2232 #                            | / _---=> hardirq/softirq
2233 #                            || / _--=> preempt-depth
2234 #                            ||| /     delay
2235 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2236 #              | |       |   ||||       |         |
2237           <idle>-0     [003] dN.1  4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
2238           <idle>-0     [003] dN.1  4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
2239           <idle>-0     [003] dN.2  4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
2240           <idle>-0     [003] dN.1  4228.547805: hrtimer_forward <-tick_nohz_idle_exit
2241           <idle>-0     [003] dN.1  4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
2242           <idle>-0     [003] d..1  4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
2243           <idle>-0     [003] d..1  4228.547859: hrtimer_start <-__tick_nohz_idle_enter
2244           <idle>-0     [003] d..2  4228.547860: hrtimer_force_reprogram <-__rem
2245 
2246 Notice that we lost the sys_nanosleep.
2247 
2248  # cat set_ftrace_filter
2249 hrtimer_run_queues
2250 hrtimer_run_pending
2251 hrtimer_init
2252 hrtimer_cancel
2253 hrtimer_try_to_cancel
2254 hrtimer_forward
2255 hrtimer_start
2256 hrtimer_reprogram
2257 hrtimer_force_reprogram
2258 hrtimer_get_next_event
2259 hrtimer_interrupt
2260 hrtimer_nanosleep
2261 hrtimer_wakeup
2262 hrtimer_get_remaining
2263 hrtimer_get_res
2264 hrtimer_init_sleeper
2265 
2266 
2267 This is because the '>' and '>>' act just like they do in bash.
2268 To rewrite the filters, use '>'
2269 To append to the filters, use '>>'
2270 
2271 To clear out a filter so that all functions will be recorded
2272 again:
2273 
2274  # echo > set_ftrace_filter
2275  # cat set_ftrace_filter
2276  #
2277 
2278 Again, now we want to append.
2279 
2280  # echo sys_nanosleep > set_ftrace_filter
2281  # cat set_ftrace_filter
2282 sys_nanosleep
2283  # echo 'hrtimer_*' >> set_ftrace_filter
2284  # cat set_ftrace_filter
2285 hrtimer_run_queues
2286 hrtimer_run_pending
2287 hrtimer_init
2288 hrtimer_cancel
2289 hrtimer_try_to_cancel
2290 hrtimer_forward
2291 hrtimer_start
2292 hrtimer_reprogram
2293 hrtimer_force_reprogram
2294 hrtimer_get_next_event
2295 hrtimer_interrupt
2296 sys_nanosleep
2297 hrtimer_nanosleep
2298 hrtimer_wakeup
2299 hrtimer_get_remaining
2300 hrtimer_get_res
2301 hrtimer_init_sleeper
2302 
2303 
2304 The set_ftrace_notrace prevents those functions from being
2305 traced.
2306 
2307  # echo '*preempt*' '*lock*' > set_ftrace_notrace
2308 
2309 Produces:
2310 
2311 # tracer: function
2312 #
2313 # entries-in-buffer/entries-written: 39608/39608   #P:4
2314 #
2315 #                              _-----=> irqs-off
2316 #                             / _----=> need-resched
2317 #                            | / _---=> hardirq/softirq
2318 #                            || / _--=> preempt-depth
2319 #                            ||| /     delay
2320 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2321 #              | |       |   ||||       |         |
2322             bash-1994  [000] ....  4342.324896: file_ra_state_init <-do_dentry_open
2323             bash-1994  [000] ....  4342.324897: open_check_o_direct <-do_last
2324             bash-1994  [000] ....  4342.324897: ima_file_check <-do_last
2325             bash-1994  [000] ....  4342.324898: process_measurement <-ima_file_check
2326             bash-1994  [000] ....  4342.324898: ima_get_action <-process_measurement
2327             bash-1994  [000] ....  4342.324898: ima_match_policy <-ima_get_action
2328             bash-1994  [000] ....  4342.324899: do_truncate <-do_last
2329             bash-1994  [000] ....  4342.324899: should_remove_suid <-do_truncate
2330             bash-1994  [000] ....  4342.324899: notify_change <-do_truncate
2331             bash-1994  [000] ....  4342.324900: current_fs_time <-notify_change
2332             bash-1994  [000] ....  4342.324900: current_kernel_time <-current_fs_time
2333             bash-1994  [000] ....  4342.324900: timespec_trunc <-current_fs_time
2334 
2335 We can see that there's no more lock or preempt tracing.
2336 
2337 
2338 Dynamic ftrace with the function graph tracer
2339 ---------------------------------------------
2340 
2341 Although what has been explained above concerns both the
2342 function tracer and the function-graph-tracer, there are some
2343 special features only available in the function-graph tracer.
2344 
2345 If you want to trace only one function and all of its children,
2346 you just have to echo its name into set_graph_function:
2347 
2348  echo __do_fault > set_graph_function
2349 
2350 will produce the following "expanded" trace of the __do_fault()
2351 function:
2352 
2353  0)               |  __do_fault() {
2354  0)               |    filemap_fault() {
2355  0)               |      find_lock_page() {
2356  0)   0.804 us    |        find_get_page();
2357  0)               |        __might_sleep() {
2358  0)   1.329 us    |        }
2359  0)   3.904 us    |      }
2360  0)   4.979 us    |    }
2361  0)   0.653 us    |    _spin_lock();
2362  0)   0.578 us    |    page_add_file_rmap();
2363  0)   0.525 us    |    native_set_pte_at();
2364  0)   0.585 us    |    _spin_unlock();
2365  0)               |    unlock_page() {
2366  0)   0.541 us    |      page_waitqueue();
2367  0)   0.639 us    |      __wake_up_bit();
2368  0)   2.786 us    |    }
2369  0) + 14.237 us   |  }
2370  0)               |  __do_fault() {
2371  0)               |    filemap_fault() {
2372  0)               |      find_lock_page() {
2373  0)   0.698 us    |        find_get_page();
2374  0)               |        __might_sleep() {
2375  0)   1.412 us    |        }
2376  0)   3.950 us    |      }
2377  0)   5.098 us    |    }
2378  0)   0.631 us    |    _spin_lock();
2379  0)   0.571 us    |    page_add_file_rmap();
2380  0)   0.526 us    |    native_set_pte_at();
2381  0)   0.586 us    |    _spin_unlock();
2382  0)               |    unlock_page() {
2383  0)   0.533 us    |      page_waitqueue();
2384  0)   0.638 us    |      __wake_up_bit();
2385  0)   2.793 us    |    }
2386  0) + 14.012 us   |  }
2387 
2388 You can also expand several functions at once:
2389 
2390  echo sys_open > set_graph_function
2391  echo sys_close >> set_graph_function
2392 
2393 Now if you want to go back to trace all functions you can clear
2394 this special filter via:
2395 
2396  echo > set_graph_function
2397 
2398 
2399 ftrace_enabled
2400 --------------
2401 
2402 Note, the proc sysctl ftrace_enable is a big on/off switch for the
2403 function tracer. By default it is enabled (when function tracing is
2404 enabled in the kernel). If it is disabled, all function tracing is
2405 disabled. This includes not only the function tracers for ftrace, but
2406 also for any other uses (perf, kprobes, stack tracing, profiling, etc).
2407 
2408 Please disable this with care.
2409 
2410 This can be disable (and enabled) with:
2411 
2412   sysctl kernel.ftrace_enabled=0
2413   sysctl kernel.ftrace_enabled=1
2414 
2415  or
2416 
2417   echo 0 > /proc/sys/kernel/ftrace_enabled
2418   echo 1 > /proc/sys/kernel/ftrace_enabled
2419 
2420 
2421 Filter commands
2422 ---------------
2423 
2424 A few commands are supported by the set_ftrace_filter interface.
2425 Trace commands have the following format:
2426 
2427 <function>:<command>:<parameter>
2428 
2429 The following commands are supported:
2430 
2431 - mod
2432   This command enables function filtering per module. The
2433   parameter defines the module. For example, if only the write*
2434   functions in the ext3 module are desired, run:
2435 
2436    echo 'write*:mod:ext3' > set_ftrace_filter
2437 
2438   This command interacts with the filter in the same way as
2439   filtering based on function names. Thus, adding more functions
2440   in a different module is accomplished by appending (>>) to the
2441   filter file. Remove specific module functions by prepending
2442   '!':
2443 
2444    echo '!writeback*:mod:ext3' >> set_ftrace_filter
2445 
2446   Mod command supports module globbing. Disable tracing for all
2447   functions except a specific module:
2448 
2449    echo '!*:mod:!ext3' >> set_ftrace_filter
2450 
2451   Disable tracing for all modules, but still trace kernel:
2452 
2453    echo '!*:mod:*' >> set_ftrace_filter
2454 
2455   Enable filter only for kernel:
2456 
2457    echo '*write*:mod:!*' >> set_ftrace_filter
2458 
2459   Enable filter for module globbing:
2460 
2461    echo '*write*:mod:*snd*' >> set_ftrace_filter
2462 
2463 - traceon/traceoff
2464   These commands turn tracing on and off when the specified
2465   functions are hit. The parameter determines how many times the
2466   tracing system is turned on and off. If unspecified, there is
2467   no limit. For example, to disable tracing when a schedule bug
2468   is hit the first 5 times, run:
2469 
2470    echo '__schedule_bug:traceoff:5' > set_ftrace_filter
2471 
2472   To always disable tracing when __schedule_bug is hit:
2473 
2474    echo '__schedule_bug:traceoff' > set_ftrace_filter
2475 
2476   These commands are cumulative whether or not they are appended
2477   to set_ftrace_filter. To remove a command, prepend it by '!'
2478   and drop the parameter:
2479 
2480    echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
2481 
2482     The above removes the traceoff command for __schedule_bug
2483     that have a counter. To remove commands without counters:
2484 
2485    echo '!__schedule_bug:traceoff' > set_ftrace_filter
2486 
2487 - snapshot
2488   Will cause a snapshot to be triggered when the function is hit.
2489 
2490    echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
2491 
2492   To only snapshot once:
2493 
2494    echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
2495 
2496   To remove the above commands:
2497 
2498    echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
2499    echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
2500 
2501 - enable_event/disable_event
2502   These commands can enable or disable a trace event. Note, because
2503   function tracing callbacks are very sensitive, when these commands
2504   are registered, the trace point is activated, but disabled in
2505   a "soft" mode. That is, the tracepoint will be called, but
2506   just will not be traced. The event tracepoint stays in this mode
2507   as long as there's a command that triggers it.
2508 
2509    echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
2510          set_ftrace_filter
2511 
2512   The format is:
2513 
2514     <function>:enable_event:<system>:<event>[:count]
2515     <function>:disable_event:<system>:<event>[:count]
2516 
2517   To remove the events commands:
2518 
2519 
2520    echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
2521          set_ftrace_filter
2522    echo '!schedule:disable_event:sched:sched_switch' > \
2523          set_ftrace_filter
2524 
2525 - dump
2526   When the function is hit, it will dump the contents of the ftrace
2527   ring buffer to the console. This is useful if you need to debug
2528   something, and want to dump the trace when a certain function
2529   is hit. Perhaps its a function that is called before a tripple
2530   fault happens and does not allow you to get a regular dump.
2531 
2532 - cpudump
2533   When the function is hit, it will dump the contents of the ftrace
2534   ring buffer for the current CPU to the console. Unlike the "dump"
2535   command, it only prints out the contents of the ring buffer for the
2536   CPU that executed the function that triggered the dump.
2537 
2538 trace_pipe
2539 ----------
2540 
2541 The trace_pipe outputs the same content as the trace file, but
2542 the effect on the tracing is different. Every read from
2543 trace_pipe is consumed. This means that subsequent reads will be
2544 different. The trace is live.
2545 
2546  # echo function > current_tracer
2547  # cat trace_pipe > /tmp/trace.out &
2548 [1] 4153
2549  # echo 1 > tracing_on
2550  # usleep 1
2551  # echo 0 > tracing_on
2552  # cat trace
2553 # tracer: function
2554 #
2555 # entries-in-buffer/entries-written: 0/0   #P:4
2556 #
2557 #                              _-----=> irqs-off
2558 #                             / _----=> need-resched
2559 #                            | / _---=> hardirq/softirq
2560 #                            || / _--=> preempt-depth
2561 #                            ||| /     delay
2562 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2563 #              | |       |   ||||       |         |
2564 
2565  #
2566  # cat /tmp/trace.out
2567             bash-1994  [000] ....  5281.568961: mutex_unlock <-rb_simple_write
2568             bash-1994  [000] ....  5281.568963: __mutex_unlock_slowpath <-mutex_unlock
2569             bash-1994  [000] ....  5281.568963: __fsnotify_parent <-fsnotify_modify
2570             bash-1994  [000] ....  5281.568964: fsnotify <-fsnotify_modify
2571             bash-1994  [000] ....  5281.568964: __srcu_read_lock <-fsnotify
2572             bash-1994  [000] ....  5281.568964: add_preempt_count <-__srcu_read_lock
2573             bash-1994  [000] ...1  5281.568965: sub_preempt_count <-__srcu_read_lock
2574             bash-1994  [000] ....  5281.568965: __srcu_read_unlock <-fsnotify
2575             bash-1994  [000] ....  5281.568967: sys_dup2 <-system_call_fastpath
2576 
2577 
2578 Note, reading the trace_pipe file will block until more input is
2579 added.
2580 
2581 trace entries
2582 -------------
2583 
2584 Having too much or not enough data can be troublesome in
2585 diagnosing an issue in the kernel. The file buffer_size_kb is
2586 used to modify the size of the internal trace buffers. The
2587 number listed is the number of entries that can be recorded per
2588 CPU. To know the full size, multiply the number of possible CPUs
2589 with the number of entries.
2590 
2591  # cat buffer_size_kb
2592 1408 (units kilobytes)
2593 
2594 Or simply read buffer_total_size_kb
2595 
2596  # cat buffer_total_size_kb 
2597 5632
2598 
2599 To modify the buffer, simple echo in a number (in 1024 byte segments).
2600 
2601  # echo 10000 > buffer_size_kb
2602  # cat buffer_size_kb
2603 10000 (units kilobytes)
2604 
2605 It will try to allocate as much as possible. If you allocate too
2606 much, it can cause Out-Of-Memory to trigger.
2607 
2608  # echo 1000000000000 > buffer_size_kb
2609 -bash: echo: write error: Cannot allocate memory
2610  # cat buffer_size_kb
2611 85
2612 
2613 The per_cpu buffers can be changed individually as well:
2614 
2615  # echo 10000 > per_cpu/cpu0/buffer_size_kb
2616  # echo 100 > per_cpu/cpu1/buffer_size_kb
2617 
2618 When the per_cpu buffers are not the same, the buffer_size_kb
2619 at the top level will just show an X
2620 
2621  # cat buffer_size_kb
2622 X
2623 
2624 This is where the buffer_total_size_kb is useful:
2625 
2626  # cat buffer_total_size_kb 
2627 12916
2628 
2629 Writing to the top level buffer_size_kb will reset all the buffers
2630 to be the same again.
2631 
2632 Snapshot
2633 --------
2634 CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
2635 available to all non latency tracers. (Latency tracers which
2636 record max latency, such as "irqsoff" or "wakeup", can't use
2637 this feature, since those are already using the snapshot
2638 mechanism internally.)
2639 
2640 Snapshot preserves a current trace buffer at a particular point
2641 in time without stopping tracing. Ftrace swaps the current
2642 buffer with a spare buffer, and tracing continues in the new
2643 current (=previous spare) buffer.
2644 
2645 The following debugfs files in "tracing" are related to this
2646 feature:
2647 
2648   snapshot:
2649 
2650         This is used to take a snapshot and to read the output
2651         of the snapshot. Echo 1 into this file to allocate a
2652         spare buffer and to take a snapshot (swap), then read
2653         the snapshot from this file in the same format as
2654         "trace" (described above in the section "The File
2655         System"). Both reads snapshot and tracing are executable
2656         in parallel. When the spare buffer is allocated, echoing
2657         0 frees it, and echoing else (positive) values clear the
2658         snapshot contents.
2659         More details are shown in the table below.
2660 
2661         status\input  |     0      |     1      |    else    |
2662         --------------+------------+------------+------------+
2663         not allocated |(do nothing)| alloc+swap |(do nothing)|
2664         --------------+------------+------------+------------+
2665         allocated     |    free    |    swap    |   clear    |
2666         --------------+------------+------------+------------+
2667 
2668 Here is an example of using the snapshot feature.
2669 
2670  # echo 1 > events/sched/enable
2671  # echo 1 > snapshot
2672  # cat snapshot
2673 # tracer: nop
2674 #
2675 # entries-in-buffer/entries-written: 71/71   #P:8
2676 #
2677 #                              _-----=> irqs-off
2678 #                             / _----=> need-resched
2679 #                            | / _---=> hardirq/softirq
2680 #                            || / _--=> preempt-depth
2681 #                            ||| /     delay
2682 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2683 #              | |       |   ||||       |         |
2684           <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
2685            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
2686 [...]
2687           <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
2688 
2689  # cat trace
2690 # tracer: nop
2691 #
2692 # entries-in-buffer/entries-written: 77/77   #P:8
2693 #
2694 #                              _-----=> irqs-off
2695 #                             / _----=> need-resched
2696 #                            | / _---=> hardirq/softirq
2697 #                            || / _--=> preempt-depth
2698 #                            ||| /     delay
2699 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2700 #              | |       |   ||||       |         |
2701           <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
2702  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
2703 [...]
2704 
2705 
2706 If you try to use this snapshot feature when current tracer is
2707 one of the latency tracers, you will get the following results.
2708 
2709  # echo wakeup > current_tracer
2710  # echo 1 > snapshot
2711 bash: echo: write error: Device or resource busy
2712  # cat snapshot
2713 cat: snapshot: Device or resource busy
2714 
2715 
2716 Instances
2717 ---------
2718 In the debugfs tracing directory is a directory called "instances".
2719 This directory can have new directories created inside of it using
2720 mkdir, and removing directories with rmdir. The directory created
2721 with mkdir in this directory will already contain files and other
2722 directories after it is created.
2723 
2724  # mkdir instances/foo
2725  # ls instances/foo
2726 buffer_size_kb  buffer_total_size_kb  events  free_buffer  per_cpu
2727 set_event  snapshot  trace  trace_clock  trace_marker  trace_options
2728 trace_pipe  tracing_on
2729 
2730 As you can see, the new directory looks similar to the tracing directory
2731 itself. In fact, it is very similar, except that the buffer and
2732 events are agnostic from the main director, or from any other
2733 instances that are created.
2734 
2735 The files in the new directory work just like the files with the
2736 same name in the tracing directory except the buffer that is used
2737 is a separate and new buffer. The files affect that buffer but do not
2738 affect the main buffer with the exception of trace_options. Currently,
2739 the trace_options affect all instances and the top level buffer
2740 the same, but this may change in future releases. That is, options
2741 may become specific to the instance they reside in.
2742 
2743 Notice that none of the function tracer files are there, nor is
2744 current_tracer and available_tracers. This is because the buffers
2745 can currently only have events enabled for them.
2746 
2747  # mkdir instances/foo
2748  # mkdir instances/bar
2749  # mkdir instances/zoot
2750  # echo 100000 > buffer_size_kb
2751  # echo 1000 > instances/foo/buffer_size_kb
2752  # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
2753  # echo function > current_trace
2754  # echo 1 > instances/foo/events/sched/sched_wakeup/enable
2755  # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
2756  # echo 1 > instances/foo/events/sched/sched_switch/enable
2757  # echo 1 > instances/bar/events/irq/enable
2758  # echo 1 > instances/zoot/events/syscalls/enable
2759  # cat trace_pipe
2760 CPU:2 [LOST 11745 EVENTS]
2761             bash-2044  [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
2762             bash-2044  [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
2763             bash-2044  [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
2764             bash-2044  [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
2765             bash-2044  [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
2766             bash-2044  [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
2767             bash-2044  [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
2768             bash-2044  [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
2769             bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
2770             bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
2771             bash-2044  [002] .... 10594.481035: arch_dup_task_struct <-copy_process
2772 [...]
2773 
2774  # cat instances/foo/trace_pipe
2775             bash-1998  [000] d..4   136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
2776             bash-1998  [000] dN.4   136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
2777           <idle>-0     [003] d.h3   136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
2778           <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
2779      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
2780             bash-1998  [000] d..4   136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
2781             bash-1998  [000] dN.4   136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
2782             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
2783      kworker/0:1-59    [000] d..4   136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
2784      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
2785 [...]
2786 
2787  # cat instances/bar/trace_pipe
2788      migration/1-14    [001] d.h3   138.732674: softirq_raise: vec=3 [action=NET_RX]
2789           <idle>-0     [001] dNh3   138.732725: softirq_raise: vec=3 [action=NET_RX]
2790             bash-1998  [000] d.h1   138.733101: softirq_raise: vec=1 [action=TIMER]
2791             bash-1998  [000] d.h1   138.733102: softirq_raise: vec=9 [action=RCU]
2792             bash-1998  [000] ..s2   138.733105: softirq_entry: vec=1 [action=TIMER]
2793             bash-1998  [000] ..s2   138.733106: softirq_exit: vec=1 [action=TIMER]
2794             bash-1998  [000] ..s2   138.733106: softirq_entry: vec=9 [action=RCU]
2795             bash-1998  [000] ..s2   138.733109: softirq_exit: vec=9 [action=RCU]
2796             sshd-1995  [001] d.h1   138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
2797             sshd-1995  [001] d.h1   138.733280: irq_handler_exit: irq=21 ret=unhandled
2798             sshd-1995  [001] d.h1   138.733281: irq_handler_entry: irq=21 name=eth0
2799             sshd-1995  [001] d.h1   138.733283: irq_handler_exit: irq=21 ret=handled
2800 [...]
2801 
2802  # cat instances/zoot/trace
2803 # tracer: nop
2804 #
2805 # entries-in-buffer/entries-written: 18996/18996   #P:4
2806 #
2807 #                              _-----=> irqs-off
2808 #                             / _----=> need-resched
2809 #                            | / _---=> hardirq/softirq
2810 #                            || / _--=> preempt-depth
2811 #                            ||| /     delay
2812 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2813 #              | |       |   ||||       |         |
2814             bash-1998  [000] d...   140.733501: sys_write -> 0x2
2815             bash-1998  [000] d...   140.733504: sys_dup2(oldfd: a, newfd: 1)
2816             bash-1998  [000] d...   140.733506: sys_dup2 -> 0x1
2817             bash-1998  [000] d...   140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
2818             bash-1998  [000] d...   140.733509: sys_fcntl -> 0x1
2819             bash-1998  [000] d...   140.733510: sys_close(fd: a)
2820             bash-1998  [000] d...   140.733510: sys_close -> 0x0
2821             bash-1998  [000] d...   140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
2822             bash-1998  [000] d...   140.733515: sys_rt_sigprocmask -> 0x0
2823             bash-1998  [000] d...   140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
2824             bash-1998  [000] d...   140.733516: sys_rt_sigaction -> 0x0
2825 
2826 You can see that the trace of the top most trace buffer shows only
2827 the function tracing. The foo instance displays wakeups and task
2828 switches.
2829 
2830 To remove the instances, simply delete their directories:
2831 
2832  # rmdir instances/foo
2833  # rmdir instances/bar
2834  # rmdir instances/zoot
2835 
2836 Note, if a process has a trace file open in one of the instance
2837 directories, the rmdir will fail with EBUSY.
2838 
2839 
2840 Stack trace
2841 -----------
2842 Since the kernel has a fixed sized stack, it is important not to
2843 waste it in functions. A kernel developer must be conscience of
2844 what they allocate on the stack. If they add too much, the system
2845 can be in danger of a stack overflow, and corruption will occur,
2846 usually leading to a system panic.
2847 
2848 There are some tools that check this, usually with interrupts
2849 periodically checking usage. But if you can perform a check
2850 at every function call that will become very useful. As ftrace provides
2851 a function tracer, it makes it convenient to check the stack size
2852 at every function call. This is enabled via the stack tracer.
2853 
2854 CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
2855 To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
2856 
2857  # echo 1 > /proc/sys/kernel/stack_tracer_enabled
2858 
2859 You can also enable it from the kernel command line to trace
2860 the stack size of the kernel during boot up, by adding "stacktrace"
2861 to the kernel command line parameter.
2862 
2863 After running it for a few minutes, the output looks like:
2864 
2865  # cat stack_max_size
2866 2928
2867 
2868  # cat stack_trace
2869         Depth    Size   Location    (18 entries)
2870         -----    ----   --------
2871   0)     2928     224   update_sd_lb_stats+0xbc/0x4ac
2872   1)     2704     160   find_busiest_group+0x31/0x1f1
2873   2)     2544     256   load_balance+0xd9/0x662
2874   3)     2288      80   idle_balance+0xbb/0x130
2875   4)     2208     128   __schedule+0x26e/0x5b9
2876   5)     2080      16   schedule+0x64/0x66
2877   6)     2064     128   schedule_timeout+0x34/0xe0
2878   7)     1936     112   wait_for_common+0x97/0xf1
2879   8)     1824      16   wait_for_completion+0x1d/0x1f
2880   9)     1808     128   flush_work+0xfe/0x119
2881  10)     1680      16   tty_flush_to_ldisc+0x1e/0x20
2882  11)     1664      48   input_available_p+0x1d/0x5c
2883  12)     1616      48   n_tty_poll+0x6d/0x134
2884  13)     1568      64   tty_poll+0x64/0x7f
2885  14)     1504     880   do_select+0x31e/0x511
2886  15)      624     400   core_sys_select+0x177/0x216
2887  16)      224      96   sys_select+0x91/0xb9
2888  17)      128     128   system_call_fastpath+0x16/0x1b
2889 
2890 Note, if -mfentry is being used by gcc, functions get traced before
2891 they set up the stack frame. This means that leaf level functions
2892 are not tested by the stack tracer when -mfentry is used.
2893 
2894 Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
2895 
2896 ---------
2897 
2898 More details can be found in the source code, in the
2899 kernel/trace/*.c files.

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