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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,they will display the same
112         information every time they are 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_graph_function:
208 
209         Set a "trigger" function where tracing should start
210         with the function graph tracer (See the section
211         "dynamic ftrace" for more details).
212 
213   available_filter_functions:
214 
215         This lists the functions that ftrace
216         has processed and can trace. These are the function
217         names that you can pass to "set_ftrace_filter" or
218         "set_ftrace_notrace". (See the section "dynamic ftrace"
219         below for more details.)
220 
221   enabled_functions:
222 
223         This file is more for debugging ftrace, but can also be useful
224         in seeing if any function has a callback attached to it.
225         Not only does the trace infrastructure use ftrace function
226         trace utility, but other subsystems might too. This file
227         displays all functions that have a callback attached to them
228         as well as the number of callbacks that have been attached.
229         Note, a callback may also call multiple functions which will
230         not be listed in this count.
231 
232         If the callback registered to be traced by a function with
233         the "save regs" attribute (thus even more overhead), a 'R'
234         will be displayed on the same line as the function that
235         is returning registers.
236 
237         If the callback registered to be traced by a function with
238         the "ip modify" attribute (thus the regs->ip can be changed),
239         an 'I' will be displayed on the same line as the function that
240         can be overridden.
241 
242   function_profile_enabled:
243 
244         When set it will enable all functions with either the function
245         tracer, or if enabled, the function graph tracer. It will
246         keep a histogram of the number of functions that were called
247         and if run with the function graph tracer, it will also keep
248         track of the time spent in those functions. The histogram
249         content can be displayed in the files:
250 
251         trace_stats/function<cpu> ( function0, function1, etc).
252 
253   trace_stats:
254 
255         A directory that holds different tracing stats.
256 
257   kprobe_events:
258  
259         Enable dynamic trace points. See kprobetrace.txt.
260 
261   kprobe_profile:
262 
263         Dynamic trace points stats. See kprobetrace.txt.
264 
265   max_graph_depth:
266 
267         Used with the function graph tracer. This is the max depth
268         it will trace into a function. Setting this to a value of
269         one will show only the first kernel function that is called
270         from user space.
271 
272   printk_formats:
273 
274         This is for tools that read the raw format files. If an event in
275         the ring buffer references a string (currently only trace_printk()
276         does this), only a pointer to the string is recorded into the buffer
277         and not the string itself. This prevents tools from knowing what
278         that string was. This file displays the string and address for
279         the string allowing tools to map the pointers to what the
280         strings were.
281 
282   saved_cmdlines:
283 
284         Only the pid of the task is recorded in a trace event unless
285         the event specifically saves the task comm as well. Ftrace
286         makes a cache of pid mappings to comms to try to display
287         comms for events. If a pid for a comm is not listed, then
288         "<...>" is displayed in the output.
289 
290   snapshot:
291 
292         This displays the "snapshot" buffer and also lets the user
293         take a snapshot of the current running trace.
294         See the "Snapshot" section below for more details.
295 
296   stack_max_size:
297 
298         When the stack tracer is activated, this will display the
299         maximum stack size it has encountered.
300         See the "Stack Trace" section below.
301 
302   stack_trace:
303 
304         This displays the stack back trace of the largest stack
305         that was encountered when the stack tracer is activated.
306         See the "Stack Trace" section below.
307 
308   stack_trace_filter:
309 
310         This is similar to "set_ftrace_filter" but it limits what
311         functions the stack tracer will check.
312 
313   trace_clock:
314 
315         Whenever an event is recorded into the ring buffer, a
316         "timestamp" is added. This stamp comes from a specified
317         clock. By default, ftrace uses the "local" clock. This
318         clock is very fast and strictly per cpu, but on some
319         systems it may not be monotonic with respect to other
320         CPUs. In other words, the local clocks may not be in sync
321         with local clocks on other CPUs.
322 
323         Usual clocks for tracing:
324 
325           # cat trace_clock
326           [local] global counter x86-tsc
327 
328           local: Default clock, but may not be in sync across CPUs
329 
330           global: This clock is in sync with all CPUs but may
331                   be a bit slower than the local clock.
332 
333           counter: This is not a clock at all, but literally an atomic
334                    counter. It counts up one by one, but is in sync
335                    with all CPUs. This is useful when you need to
336                    know exactly the order events occurred with respect to
337                    each other on different CPUs.
338 
339           uptime: This uses the jiffies counter and the time stamp
340                   is relative to the time since boot up.
341 
342           perf: This makes ftrace use the same clock that perf uses.
343                 Eventually perf will be able to read ftrace buffers
344                 and this will help out in interleaving the data.
345 
346           x86-tsc: Architectures may define their own clocks. For
347                    example, x86 uses its own TSC cycle clock here.
348 
349         To set a clock, simply echo the clock name into this file.
350 
351           echo global > trace_clock
352 
353   trace_marker:
354 
355         This is a very useful file for synchronizing user space
356         with events happening in the kernel. Writing strings into
357         this file will be written into the ftrace buffer.
358 
359         It is useful in applications to open this file at the start
360         of the application and just reference the file descriptor
361         for the file.
362 
363         void trace_write(const char *fmt, ...)
364         {
365                 va_list ap;
366                 char buf[256];
367                 int n;
368 
369                 if (trace_fd < 0)
370                         return;
371 
372                 va_start(ap, fmt);
373                 n = vsnprintf(buf, 256, fmt, ap);
374                 va_end(ap);
375 
376                 write(trace_fd, buf, n);
377         }
378 
379         start:
380 
381                 trace_fd = open("trace_marker", WR_ONLY);
382 
383   uprobe_events:
384  
385         Add dynamic tracepoints in programs.
386         See uprobetracer.txt
387 
388   uprobe_profile:
389 
390         Uprobe statistics. See uprobetrace.txt
391 
392   instances:
393 
394         This is a way to make multiple trace buffers where different
395         events can be recorded in different buffers.
396         See "Instances" section below.
397 
398   events:
399 
400         This is the trace event directory. It holds event tracepoints
401         (also known as static tracepoints) that have been compiled
402         into the kernel. It shows what event tracepoints exist
403         and how they are grouped by system. There are "enable"
404         files at various levels that can enable the tracepoints
405         when a "1" is written to them.
406 
407         See events.txt for more information.
408 
409   per_cpu:
410 
411         This is a directory that contains the trace per_cpu information.
412 
413   per_cpu/cpu0/buffer_size_kb:
414 
415         The ftrace buffer is defined per_cpu. That is, there's a separate
416         buffer for each CPU to allow writes to be done atomically,
417         and free from cache bouncing. These buffers may have different
418         size buffers. This file is similar to the buffer_size_kb
419         file, but it only displays or sets the buffer size for the
420         specific CPU. (here cpu0).
421 
422   per_cpu/cpu0/trace:
423 
424         This is similar to the "trace" file, but it will only display
425         the data specific for the CPU. If written to, it only clears
426         the specific CPU buffer.
427 
428   per_cpu/cpu0/trace_pipe
429 
430         This is similar to the "trace_pipe" file, and is a consuming
431         read, but it will only display (and consume) the data specific
432         for the CPU.
433 
434   per_cpu/cpu0/trace_pipe_raw
435 
436         For tools that can parse the ftrace ring buffer binary format,
437         the trace_pipe_raw file can be used to extract the data
438         from the ring buffer directly. With the use of the splice()
439         system call, the buffer data can be quickly transferred to
440         a file or to the network where a server is collecting the
441         data.
442 
443         Like trace_pipe, this is a consuming reader, where multiple
444         reads will always produce different data.
445 
446   per_cpu/cpu0/snapshot:
447 
448         This is similar to the main "snapshot" file, but will only
449         snapshot the current CPU (if supported). It only displays
450         the content of the snapshot for a given CPU, and if
451         written to, only clears this CPU buffer.
452 
453   per_cpu/cpu0/snapshot_raw:
454 
455         Similar to the trace_pipe_raw, but will read the binary format
456         from the snapshot buffer for the given CPU.
457 
458   per_cpu/cpu0/stats:
459 
460         This displays certain stats about the ring buffer:
461 
462          entries: The number of events that are still in the buffer.
463 
464          overrun: The number of lost events due to overwriting when
465                   the buffer was full.
466 
467          commit overrun: Should always be zero.
468                 This gets set if so many events happened within a nested
469                 event (ring buffer is re-entrant), that it fills the
470                 buffer and starts dropping events.
471 
472          bytes: Bytes actually read (not overwritten).
473 
474          oldest event ts: The oldest timestamp in the buffer
475 
476          now ts: The current timestamp
477 
478          dropped events: Events lost due to overwrite option being off.
479 
480          read events: The number of events read.
481 
482 The Tracers
483 -----------
484 
485 Here is the list of current tracers that may be configured.
486 
487   "function"
488 
489         Function call tracer to trace all kernel functions.
490 
491   "function_graph"
492 
493         Similar to the function tracer except that the
494         function tracer probes the functions on their entry
495         whereas the function graph tracer traces on both entry
496         and exit of the functions. It then provides the ability
497         to draw a graph of function calls similar to C code
498         source.
499 
500   "irqsoff"
501 
502         Traces the areas that disable interrupts and saves
503         the trace with the longest max latency.
504         See tracing_max_latency. When a new max is recorded,
505         it replaces the old trace. It is best to view this
506         trace with the latency-format option enabled.
507 
508   "preemptoff"
509 
510         Similar to irqsoff but traces and records the amount of
511         time for which preemption is disabled.
512 
513   "preemptirqsoff"
514 
515         Similar to irqsoff and preemptoff, but traces and
516         records the largest time for which irqs and/or preemption
517         is disabled.
518 
519   "wakeup"
520 
521         Traces and records the max latency that it takes for
522         the highest priority task to get scheduled after
523         it has been woken up.
524         Traces all tasks as an average developer would expect.
525 
526   "wakeup_rt"
527 
528         Traces and records the max latency that it takes for just
529         RT tasks (as the current "wakeup" does). This is useful
530         for those interested in wake up timings of RT tasks.
531 
532   "nop"
533 
534         This is the "trace nothing" tracer. To remove all
535         tracers from tracing simply echo "nop" into
536         current_tracer.
537 
538 
539 Examples of using the tracer
540 ----------------------------
541 
542 Here are typical examples of using the tracers when controlling
543 them only with the debugfs interface (without using any
544 user-land utilities).
545 
546 Output format:
547 --------------
548 
549 Here is an example of the output format of the file "trace"
550 
551                              --------
552 # tracer: function
553 #
554 # entries-in-buffer/entries-written: 140080/250280   #P:4
555 #
556 #                              _-----=> irqs-off
557 #                             / _----=> need-resched
558 #                            | / _---=> hardirq/softirq
559 #                            || / _--=> preempt-depth
560 #                            ||| /     delay
561 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
562 #              | |       |   ||||       |         |
563             bash-1977  [000] .... 17284.993652: sys_close <-system_call_fastpath
564             bash-1977  [000] .... 17284.993653: __close_fd <-sys_close
565             bash-1977  [000] .... 17284.993653: _raw_spin_lock <-__close_fd
566             sshd-1974  [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
567             bash-1977  [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
568             bash-1977  [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
569             bash-1977  [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
570             bash-1977  [000] .... 17284.993657: filp_close <-__close_fd
571             bash-1977  [000] .... 17284.993657: dnotify_flush <-filp_close
572             sshd-1974  [003] .... 17284.993658: sys_select <-system_call_fastpath
573                              --------
574 
575 A header is printed with the tracer name that is represented by
576 the trace. In this case the tracer is "function". Then it shows the
577 number of events in the buffer as well as the total number of entries
578 that were written. The difference is the number of entries that were
579 lost due to the buffer filling up (250280 - 140080 = 110200 events
580 lost).
581 
582 The header explains the content of the events. Task name "bash", the task
583 PID "1977", the CPU that it was running on "000", the latency format
584 (explained below), the timestamp in <secs>.<usecs> format, the
585 function name that was traced "sys_close" and the parent function that
586 called this function "system_call_fastpath". The timestamp is the time
587 at which the function was entered.
588 
589 Latency trace format
590 --------------------
591 
592 When the latency-format option is enabled or when one of the latency
593 tracers is set, the trace file gives somewhat more information to see
594 why a latency happened. Here is a typical trace.
595 
596 # tracer: irqsoff
597 #
598 # irqsoff latency trace v1.1.5 on 3.8.0-test+
599 # --------------------------------------------------------------------
600 # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
601 #    -----------------
602 #    | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
603 #    -----------------
604 #  => started at: __lock_task_sighand
605 #  => ended at:   _raw_spin_unlock_irqrestore
606 #
607 #
608 #                  _------=> CPU#            
609 #                 / _-----=> irqs-off        
610 #                | / _----=> need-resched    
611 #                || / _---=> hardirq/softirq 
612 #                ||| / _--=> preempt-depth   
613 #                |||| /     delay             
614 #  cmd     pid   ||||| time  |   caller      
615 #     \   /      |||||  \    |   /           
616       ps-6143    2d...    0us!: trace_hardirqs_off <-__lock_task_sighand
617       ps-6143    2d..1  259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
618       ps-6143    2d..1  263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
619       ps-6143    2d..1  306us : <stack trace>
620  => trace_hardirqs_on_caller
621  => trace_hardirqs_on
622  => _raw_spin_unlock_irqrestore
623  => do_task_stat
624  => proc_tgid_stat
625  => proc_single_show
626  => seq_read
627  => vfs_read
628  => sys_read
629  => system_call_fastpath
630 
631 
632 This shows that the current tracer is "irqsoff" tracing the time
633 for which interrupts were disabled. It gives the trace version (which
634 never changes) and the version of the kernel upon which this was executed on
635 (3.10). Then it displays the max latency in microseconds (259 us). The number
636 of trace entries displayed and the total number (both are four: #4/4).
637 VP, KP, SP, and HP are always zero and are reserved for later use.
638 #P is the number of online CPUs (#P:4).
639 
640 The task is the process that was running when the latency
641 occurred. (ps pid: 6143).
642 
643 The start and stop (the functions in which the interrupts were
644 disabled and enabled respectively) that caused the latencies:
645 
646  __lock_task_sighand is where the interrupts were disabled.
647  _raw_spin_unlock_irqrestore is where they were enabled again.
648 
649 The next lines after the header are the trace itself. The header
650 explains which is which.
651 
652   cmd: The name of the process in the trace.
653 
654   pid: The PID of that process.
655 
656   CPU#: The CPU which the process was running on.
657 
658   irqs-off: 'd' interrupts are disabled. '.' otherwise.
659             Note: If the architecture does not support a way to
660                   read the irq flags variable, an 'X' will always
661                   be printed here.
662 
663   need-resched:
664         'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
665         'n' only TIF_NEED_RESCHED is set,
666         'p' only PREEMPT_NEED_RESCHED is set,
667         '.' otherwise.
668 
669   hardirq/softirq:
670         'H' - hard irq occurred inside a softirq.
671         'h' - hard irq is running
672         's' - soft irq is running
673         '.' - normal context.
674 
675   preempt-depth: The level of preempt_disabled
676 
677 The above is mostly meaningful for kernel developers.
678 
679   time: When the latency-format option is enabled, the trace file
680         output includes a timestamp relative to the start of the
681         trace. This differs from the output when latency-format
682         is disabled, which includes an absolute timestamp.
683 
684   delay: This is just to help catch your eye a bit better. And
685          needs to be fixed to be only relative to the same CPU.
686          The marks are determined by the difference between this
687          current trace and the next trace.
688           '$' - greater than 1 second
689           '#' - greater than 1000 microsecond
690           '!' - greater than 100 microsecond
691           '+' - greater than 10 microsecond
692           ' ' - less than or equal to 10 microsecond.
693 
694   The rest is the same as the 'trace' file.
695 
696   Note, the latency tracers will usually end with a back trace
697   to easily find where the latency occurred.
698 
699 trace_options
700 -------------
701 
702 The trace_options file (or the options directory) is used to control
703 what gets printed in the trace output, or manipulate the tracers.
704 To see what is available, simply cat the file:
705 
706   cat trace_options
707 print-parent
708 nosym-offset
709 nosym-addr
710 noverbose
711 noraw
712 nohex
713 nobin
714 noblock
715 nostacktrace
716 trace_printk
717 noftrace_preempt
718 nobranch
719 annotate
720 nouserstacktrace
721 nosym-userobj
722 noprintk-msg-only
723 context-info
724 latency-format
725 sleep-time
726 graph-time
727 record-cmd
728 overwrite
729 nodisable_on_free
730 irq-info
731 markers
732 function-trace
733 
734 To disable one of the options, echo in the option prepended with
735 "no".
736 
737   echo noprint-parent > trace_options
738 
739 To enable an option, leave off the "no".
740 
741   echo sym-offset > trace_options
742 
743 Here are the available options:
744 
745   print-parent - On function traces, display the calling (parent)
746                  function as well as the function being traced.
747 
748   print-parent:
749    bash-4000  [01]  1477.606694: simple_strtoul <-kstrtoul
750 
751   noprint-parent:
752    bash-4000  [01]  1477.606694: simple_strtoul
753 
754 
755   sym-offset - Display not only the function name, but also the
756                offset in the function. For example, instead of
757                seeing just "ktime_get", you will see
758                "ktime_get+0xb/0x20".
759 
760   sym-offset:
761    bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0
762 
763   sym-addr - this will also display the function address as well
764              as the function name.
765 
766   sym-addr:
767    bash-4000  [01]  1477.606694: simple_strtoul <c0339346>
768 
769   verbose - This deals with the trace file when the
770             latency-format option is enabled.
771 
772     bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
773     (+0.000ms): simple_strtoul (kstrtoul)
774 
775   raw - This will display raw numbers. This option is best for
776         use with user applications that can translate the raw
777         numbers better than having it done in the kernel.
778 
779   hex - Similar to raw, but the numbers will be in a hexadecimal
780         format.
781 
782   bin - This will print out the formats in raw binary.
783 
784   block - When set, reading trace_pipe will not block when polled.
785 
786   stacktrace - This is one of the options that changes the trace
787                itself. When a trace is recorded, so is the stack
788                of functions. This allows for back traces of
789                trace sites.
790 
791   trace_printk - Can disable trace_printk() from writing into the buffer.
792 
793   branch - Enable branch tracing with the tracer.
794 
795   annotate - It is sometimes confusing when the CPU buffers are full
796              and one CPU buffer had a lot of events recently, thus
797              a shorter time frame, were another CPU may have only had
798              a few events, which lets it have older events. When
799              the trace is reported, it shows the oldest events first,
800              and it may look like only one CPU ran (the one with the
801              oldest events). When the annotate option is set, it will
802              display when a new CPU buffer started:
803 
804           <idle>-0     [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
805           <idle>-0     [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
806           <idle>-0     [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
807 ##### CPU 2 buffer started ####
808           <idle>-0     [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
809           <idle>-0     [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
810           <idle>-0     [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
811 
812   userstacktrace - This option changes the trace. It records a
813                    stacktrace of the current userspace thread.
814 
815   sym-userobj - when user stacktrace are enabled, look up which
816                 object the address belongs to, and print a
817                 relative address. This is especially useful when
818                 ASLR is on, otherwise you don't get a chance to
819                 resolve the address to object/file/line after
820                 the app is no longer running
821 
822                 The lookup is performed when you read
823                 trace,trace_pipe. Example:
824 
825                 a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
826 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
827 
828 
829   printk-msg-only - When set, trace_printk()s will only show the format
830                     and not their parameters (if trace_bprintk() or
831                     trace_bputs() was used to save the trace_printk()).
832 
833   context-info - Show only the event data. Hides the comm, PID,
834                  timestamp, CPU, and other useful data.
835 
836   latency-format - This option changes the trace. When
837                    it is enabled, the trace displays
838                    additional information about the
839                    latencies, as described in "Latency
840                    trace format".
841 
842   sleep-time - When running function graph tracer, to include
843                the time a task schedules out in its function.
844                When enabled, it will account time the task has been
845                scheduled out as part of the function call.
846 
847   graph-time - When running function graph tracer, to include the
848                time to call nested functions. When this is not set,
849                the time reported for the function will only include
850                the time the function itself executed for, not the time
851                for functions that it called.
852 
853   record-cmd - When any event or tracer is enabled, a hook is enabled
854                in the sched_switch trace point to fill comm cache
855                with mapped pids and comms. But this may cause some
856                overhead, and if you only care about pids, and not the
857                name of the task, disabling this option can lower the
858                impact of tracing.
859 
860   overwrite - This controls what happens when the trace buffer is
861               full. If "1" (default), the oldest events are
862               discarded and overwritten. If "0", then the newest
863               events are discarded.
864                 (see per_cpu/cpu0/stats for overrun and dropped)
865 
866   disable_on_free - When the free_buffer is closed, tracing will
867                     stop (tracing_on set to 0).
868 
869   irq-info - Shows the interrupt, preempt count, need resched data.
870              When disabled, the trace looks like:
871 
872 # tracer: function
873 #
874 # entries-in-buffer/entries-written: 144405/9452052   #P:4
875 #
876 #           TASK-PID   CPU#      TIMESTAMP  FUNCTION
877 #              | |       |          |         |
878           <idle>-0     [002]  23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
879           <idle>-0     [002]  23636.756054: activate_task <-ttwu_do_activate.constprop.89
880           <idle>-0     [002]  23636.756055: enqueue_task <-activate_task
881 
882 
883   markers - When set, the trace_marker is writable (only by root).
884             When disabled, the trace_marker will error with EINVAL
885             on write.
886 
887 
888   function-trace - The latency tracers will enable function tracing
889             if this option is enabled (default it is). When
890             it is disabled, the latency tracers do not trace
891             functions. This keeps the overhead of the tracer down
892             when performing latency tests.
893 
894  Note: Some tracers have their own options. They only appear
895        when the tracer is active.
896 
897 
898 
899 irqsoff
900 -------
901 
902 When interrupts are disabled, the CPU can not react to any other
903 external event (besides NMIs and SMIs). This prevents the timer
904 interrupt from triggering or the mouse interrupt from letting
905 the kernel know of a new mouse event. The result is a latency
906 with the reaction time.
907 
908 The irqsoff tracer tracks the time for which interrupts are
909 disabled. When a new maximum latency is hit, the tracer saves
910 the trace leading up to that latency point so that every time a
911 new maximum is reached, the old saved trace is discarded and the
912 new trace is saved.
913 
914 To reset the maximum, echo 0 into tracing_max_latency. Here is
915 an example:
916 
917  # echo 0 > options/function-trace
918  # echo irqsoff > current_tracer
919  # echo 1 > tracing_on
920  # echo 0 > tracing_max_latency
921  # ls -ltr
922  [...]
923  # echo 0 > tracing_on
924  # cat trace
925 # tracer: irqsoff
926 #
927 # irqsoff latency trace v1.1.5 on 3.8.0-test+
928 # --------------------------------------------------------------------
929 # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
930 #    -----------------
931 #    | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
932 #    -----------------
933 #  => started at: run_timer_softirq
934 #  => ended at:   run_timer_softirq
935 #
936 #
937 #                  _------=> CPU#            
938 #                 / _-----=> irqs-off        
939 #                | / _----=> need-resched    
940 #                || / _---=> hardirq/softirq 
941 #                ||| / _--=> preempt-depth   
942 #                |||| /     delay             
943 #  cmd     pid   ||||| time  |   caller      
944 #     \   /      |||||  \    |   /           
945   <idle>-0       0d.s2    0us+: _raw_spin_lock_irq <-run_timer_softirq
946   <idle>-0       0dNs3   17us : _raw_spin_unlock_irq <-run_timer_softirq
947   <idle>-0       0dNs3   17us+: trace_hardirqs_on <-run_timer_softirq
948   <idle>-0       0dNs3   25us : <stack trace>
949  => _raw_spin_unlock_irq
950  => run_timer_softirq
951  => __do_softirq
952  => call_softirq
953  => do_softirq
954  => irq_exit
955  => smp_apic_timer_interrupt
956  => apic_timer_interrupt
957  => rcu_idle_exit
958  => cpu_idle
959  => rest_init
960  => start_kernel
961  => x86_64_start_reservations
962  => x86_64_start_kernel
963 
964 Here we see that that we had a latency of 16 microseconds (which is
965 very good). The _raw_spin_lock_irq in run_timer_softirq disabled
966 interrupts. The difference between the 16 and the displayed
967 timestamp 25us occurred because the clock was incremented
968 between the time of recording the max latency and the time of
969 recording the function that had that latency.
970 
971 Note the above example had function-trace not set. If we set
972 function-trace, we get a much larger output:
973 
974  with echo 1 > options/function-trace
975 
976 # tracer: irqsoff
977 #
978 # irqsoff latency trace v1.1.5 on 3.8.0-test+
979 # --------------------------------------------------------------------
980 # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
981 #    -----------------
982 #    | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
983 #    -----------------
984 #  => started at: ata_scsi_queuecmd
985 #  => ended at:   ata_scsi_queuecmd
986 #
987 #
988 #                  _------=> CPU#            
989 #                 / _-----=> irqs-off        
990 #                | / _----=> need-resched    
991 #                || / _---=> hardirq/softirq 
992 #                ||| / _--=> preempt-depth   
993 #                |||| /     delay             
994 #  cmd     pid   ||||| time  |   caller      
995 #     \   /      |||||  \    |   /           
996     bash-2042    3d...    0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
997     bash-2042    3d...    0us : add_preempt_count <-_raw_spin_lock_irqsave
998     bash-2042    3d..1    1us : ata_scsi_find_dev <-ata_scsi_queuecmd
999     bash-2042    3d..1    1us : __ata_scsi_find_dev <-ata_scsi_find_dev
1000     bash-2042    3d..1    2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
1001     bash-2042    3d..1    2us : ata_qc_new_init <-__ata_scsi_queuecmd
1002     bash-2042    3d..1    3us : ata_sg_init <-__ata_scsi_queuecmd
1003     bash-2042    3d..1    4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
1004     bash-2042    3d..1    4us : ata_build_rw_tf <-ata_scsi_rw_xlat
1005 [...]
1006     bash-2042    3d..1   67us : delay_tsc <-__delay
1007     bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
1008     bash-2042    3d..2   67us : sub_preempt_count <-delay_tsc
1009     bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
1010     bash-2042    3d..2   68us : sub_preempt_count <-delay_tsc
1011     bash-2042    3d..1   68us+: ata_bmdma_start <-ata_bmdma_qc_issue
1012     bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1013     bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1014     bash-2042    3d..1   72us+: trace_hardirqs_on <-ata_scsi_queuecmd
1015     bash-2042    3d..1  120us : <stack trace>
1016  => _raw_spin_unlock_irqrestore
1017  => ata_scsi_queuecmd
1018  => scsi_dispatch_cmd
1019  => scsi_request_fn
1020  => __blk_run_queue_uncond
1021  => __blk_run_queue
1022  => blk_queue_bio
1023  => generic_make_request
1024  => submit_bio
1025  => submit_bh
1026  => __ext3_get_inode_loc
1027  => ext3_iget
1028  => ext3_lookup
1029  => lookup_real
1030  => __lookup_hash
1031  => walk_component
1032  => lookup_last
1033  => path_lookupat
1034  => filename_lookup
1035  => user_path_at_empty
1036  => user_path_at
1037  => vfs_fstatat
1038  => vfs_stat
1039  => sys_newstat
1040  => system_call_fastpath
1041 
1042 
1043 Here we traced a 71 microsecond latency. But we also see all the
1044 functions that were called during that time. Note that by
1045 enabling function tracing, we incur an added overhead. This
1046 overhead may extend the latency times. But nevertheless, this
1047 trace has provided some very helpful debugging information.
1048 
1049 
1050 preemptoff
1051 ----------
1052 
1053 When preemption is disabled, we may be able to receive
1054 interrupts but the task cannot be preempted and a higher
1055 priority task must wait for preemption to be enabled again
1056 before it can preempt a lower priority task.
1057 
1058 The preemptoff tracer traces the places that disable preemption.
1059 Like the irqsoff tracer, it records the maximum latency for
1060 which preemption was disabled. The control of preemptoff tracer
1061 is much like the irqsoff tracer.
1062 
1063  # echo 0 > options/function-trace
1064  # echo preemptoff > current_tracer
1065  # echo 1 > tracing_on
1066  # echo 0 > tracing_max_latency
1067  # ls -ltr
1068  [...]
1069  # echo 0 > tracing_on
1070  # cat trace
1071 # tracer: preemptoff
1072 #
1073 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1074 # --------------------------------------------------------------------
1075 # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1076 #    -----------------
1077 #    | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
1078 #    -----------------
1079 #  => started at: do_IRQ
1080 #  => ended at:   do_IRQ
1081 #
1082 #
1083 #                  _------=> CPU#            
1084 #                 / _-----=> irqs-off        
1085 #                | / _----=> need-resched    
1086 #                || / _---=> hardirq/softirq 
1087 #                ||| / _--=> preempt-depth   
1088 #                |||| /     delay             
1089 #  cmd     pid   ||||| time  |   caller      
1090 #     \   /      |||||  \    |   /           
1091     sshd-1991    1d.h.    0us+: irq_enter <-do_IRQ
1092     sshd-1991    1d..1   46us : irq_exit <-do_IRQ
1093     sshd-1991    1d..1   47us+: trace_preempt_on <-do_IRQ
1094     sshd-1991    1d..1   52us : <stack trace>
1095  => sub_preempt_count
1096  => irq_exit
1097  => do_IRQ
1098  => ret_from_intr
1099 
1100 
1101 This has some more changes. Preemption was disabled when an
1102 interrupt came in (notice the 'h'), and was enabled on exit.
1103 But we also see that interrupts have been disabled when entering
1104 the preempt off section and leaving it (the 'd'). We do not know if
1105 interrupts were enabled in the mean time or shortly after this
1106 was over.
1107 
1108 # tracer: preemptoff
1109 #
1110 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1111 # --------------------------------------------------------------------
1112 # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1113 #    -----------------
1114 #    | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
1115 #    -----------------
1116 #  => started at: wake_up_new_task
1117 #  => ended at:   task_rq_unlock
1118 #
1119 #
1120 #                  _------=> CPU#            
1121 #                 / _-----=> irqs-off        
1122 #                | / _----=> need-resched    
1123 #                || / _---=> hardirq/softirq 
1124 #                ||| / _--=> preempt-depth   
1125 #                |||| /     delay             
1126 #  cmd     pid   ||||| time  |   caller      
1127 #     \   /      |||||  \    |   /           
1128     bash-1994    1d..1    0us : _raw_spin_lock_irqsave <-wake_up_new_task
1129     bash-1994    1d..1    0us : select_task_rq_fair <-select_task_rq
1130     bash-1994    1d..1    1us : __rcu_read_lock <-select_task_rq_fair
1131     bash-1994    1d..1    1us : source_load <-select_task_rq_fair
1132     bash-1994    1d..1    1us : source_load <-select_task_rq_fair
1133 [...]
1134     bash-1994    1d..1   12us : irq_enter <-smp_apic_timer_interrupt
1135     bash-1994    1d..1   12us : rcu_irq_enter <-irq_enter
1136     bash-1994    1d..1   13us : add_preempt_count <-irq_enter
1137     bash-1994    1d.h1   13us : exit_idle <-smp_apic_timer_interrupt
1138     bash-1994    1d.h1   13us : hrtimer_interrupt <-smp_apic_timer_interrupt
1139     bash-1994    1d.h1   13us : _raw_spin_lock <-hrtimer_interrupt
1140     bash-1994    1d.h1   14us : add_preempt_count <-_raw_spin_lock
1141     bash-1994    1d.h2   14us : ktime_get_update_offsets <-hrtimer_interrupt
1142 [...]
1143     bash-1994    1d.h1   35us : lapic_next_event <-clockevents_program_event
1144     bash-1994    1d.h1   35us : irq_exit <-smp_apic_timer_interrupt
1145     bash-1994    1d.h1   36us : sub_preempt_count <-irq_exit
1146     bash-1994    1d..2   36us : do_softirq <-irq_exit
1147     bash-1994    1d..2   36us : __do_softirq <-call_softirq
1148     bash-1994    1d..2   36us : __local_bh_disable <-__do_softirq
1149     bash-1994    1d.s2   37us : add_preempt_count <-_raw_spin_lock_irq
1150     bash-1994    1d.s3   38us : _raw_spin_unlock <-run_timer_softirq
1151     bash-1994    1d.s3   39us : sub_preempt_count <-_raw_spin_unlock
1152     bash-1994    1d.s2   39us : call_timer_fn <-run_timer_softirq
1153 [...]
1154     bash-1994    1dNs2   81us : cpu_needs_another_gp <-rcu_process_callbacks
1155     bash-1994    1dNs2   82us : __local_bh_enable <-__do_softirq
1156     bash-1994    1dNs2   82us : sub_preempt_count <-__local_bh_enable
1157     bash-1994    1dN.2   82us : idle_cpu <-irq_exit
1158     bash-1994    1dN.2   83us : rcu_irq_exit <-irq_exit
1159     bash-1994    1dN.2   83us : sub_preempt_count <-irq_exit
1160     bash-1994    1.N.1   84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
1161     bash-1994    1.N.1   84us+: trace_preempt_on <-task_rq_unlock
1162     bash-1994    1.N.1  104us : <stack trace>
1163  => sub_preempt_count
1164  => _raw_spin_unlock_irqrestore
1165  => task_rq_unlock
1166  => wake_up_new_task
1167  => do_fork
1168  => sys_clone
1169  => stub_clone
1170 
1171 
1172 The above is an example of the preemptoff trace with
1173 function-trace set. Here we see that interrupts were not disabled
1174 the entire time. The irq_enter code lets us know that we entered
1175 an interrupt 'h'. Before that, the functions being traced still
1176 show that it is not in an interrupt, but we can see from the
1177 functions themselves that this is not the case.
1178 
1179 preemptirqsoff
1180 --------------
1181 
1182 Knowing the locations that have interrupts disabled or
1183 preemption disabled for the longest times is helpful. But
1184 sometimes we would like to know when either preemption and/or
1185 interrupts are disabled.
1186 
1187 Consider the following code:
1188 
1189     local_irq_disable();
1190     call_function_with_irqs_off();
1191     preempt_disable();
1192     call_function_with_irqs_and_preemption_off();
1193     local_irq_enable();
1194     call_function_with_preemption_off();
1195     preempt_enable();
1196 
1197 The irqsoff tracer will record the total length of
1198 call_function_with_irqs_off() and
1199 call_function_with_irqs_and_preemption_off().
1200 
1201 The preemptoff tracer will record the total length of
1202 call_function_with_irqs_and_preemption_off() and
1203 call_function_with_preemption_off().
1204 
1205 But neither will trace the time that interrupts and/or
1206 preemption is disabled. This total time is the time that we can
1207 not schedule. To record this time, use the preemptirqsoff
1208 tracer.
1209 
1210 Again, using this trace is much like the irqsoff and preemptoff
1211 tracers.
1212 
1213  # echo 0 > options/function-trace
1214  # echo preemptirqsoff > current_tracer
1215  # echo 1 > tracing_on
1216  # echo 0 > tracing_max_latency
1217  # ls -ltr
1218  [...]
1219  # echo 0 > tracing_on
1220  # cat trace
1221 # tracer: preemptirqsoff
1222 #
1223 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1224 # --------------------------------------------------------------------
1225 # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1226 #    -----------------
1227 #    | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
1228 #    -----------------
1229 #  => started at: ata_scsi_queuecmd
1230 #  => ended at:   ata_scsi_queuecmd
1231 #
1232 #
1233 #                  _------=> CPU#            
1234 #                 / _-----=> irqs-off        
1235 #                | / _----=> need-resched    
1236 #                || / _---=> hardirq/softirq 
1237 #                ||| / _--=> preempt-depth   
1238 #                |||| /     delay             
1239 #  cmd     pid   ||||| time  |   caller      
1240 #     \   /      |||||  \    |   /           
1241       ls-2230    3d...    0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1242       ls-2230    3...1  100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1243       ls-2230    3...1  101us+: trace_preempt_on <-ata_scsi_queuecmd
1244       ls-2230    3...1  111us : <stack trace>
1245  => sub_preempt_count
1246  => _raw_spin_unlock_irqrestore
1247  => ata_scsi_queuecmd
1248  => scsi_dispatch_cmd
1249  => scsi_request_fn
1250  => __blk_run_queue_uncond
1251  => __blk_run_queue
1252  => blk_queue_bio
1253  => generic_make_request
1254  => submit_bio
1255  => submit_bh
1256  => ext3_bread
1257  => ext3_dir_bread
1258  => htree_dirblock_to_tree
1259  => ext3_htree_fill_tree
1260  => ext3_readdir
1261  => vfs_readdir
1262  => sys_getdents
1263  => system_call_fastpath
1264 
1265 
1266 The trace_hardirqs_off_thunk is called from assembly on x86 when
1267 interrupts are disabled in the assembly code. Without the
1268 function tracing, we do not know if interrupts were enabled
1269 within the preemption points. We do see that it started with
1270 preemption enabled.
1271 
1272 Here is a trace with function-trace set:
1273 
1274 # tracer: preemptirqsoff
1275 #
1276 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1277 # --------------------------------------------------------------------
1278 # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1279 #    -----------------
1280 #    | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
1281 #    -----------------
1282 #  => started at: schedule
1283 #  => ended at:   mutex_unlock
1284 #
1285 #
1286 #                  _------=> CPU#            
1287 #                 / _-----=> irqs-off        
1288 #                | / _----=> need-resched    
1289 #                || / _---=> hardirq/softirq 
1290 #                ||| / _--=> preempt-depth   
1291 #                |||| /     delay             
1292 #  cmd     pid   ||||| time  |   caller      
1293 #     \   /      |||||  \    |   /           
1294 kworker/-59      3...1    0us : __schedule <-schedule
1295 kworker/-59      3d..1    0us : rcu_preempt_qs <-rcu_note_context_switch
1296 kworker/-59      3d..1    1us : add_preempt_count <-_raw_spin_lock_irq
1297 kworker/-59      3d..2    1us : deactivate_task <-__schedule
1298 kworker/-59      3d..2    1us : dequeue_task <-deactivate_task
1299 kworker/-59      3d..2    2us : update_rq_clock <-dequeue_task
1300 kworker/-59      3d..2    2us : dequeue_task_fair <-dequeue_task
1301 kworker/-59      3d..2    2us : update_curr <-dequeue_task_fair
1302 kworker/-59      3d..2    2us : update_min_vruntime <-update_curr
1303 kworker/-59      3d..2    3us : cpuacct_charge <-update_curr
1304 kworker/-59      3d..2    3us : __rcu_read_lock <-cpuacct_charge
1305 kworker/-59      3d..2    3us : __rcu_read_unlock <-cpuacct_charge
1306 kworker/-59      3d..2    3us : update_cfs_rq_blocked_load <-dequeue_task_fair
1307 kworker/-59      3d..2    4us : clear_buddies <-dequeue_task_fair
1308 kworker/-59      3d..2    4us : account_entity_dequeue <-dequeue_task_fair
1309 kworker/-59      3d..2    4us : update_min_vruntime <-dequeue_task_fair
1310 kworker/-59      3d..2    4us : update_cfs_shares <-dequeue_task_fair
1311 kworker/-59      3d..2    5us : hrtick_update <-dequeue_task_fair
1312 kworker/-59      3d..2    5us : wq_worker_sleeping <-__schedule
1313 kworker/-59      3d..2    5us : kthread_data <-wq_worker_sleeping
1314 kworker/-59      3d..2    5us : put_prev_task_fair <-__schedule
1315 kworker/-59      3d..2    6us : pick_next_task_fair <-pick_next_task
1316 kworker/-59      3d..2    6us : clear_buddies <-pick_next_task_fair
1317 kworker/-59      3d..2    6us : set_next_entity <-pick_next_task_fair
1318 kworker/-59      3d..2    6us : update_stats_wait_end <-set_next_entity
1319       ls-2269    3d..2    7us : finish_task_switch <-__schedule
1320       ls-2269    3d..2    7us : _raw_spin_unlock_irq <-finish_task_switch
1321       ls-2269    3d..2    8us : do_IRQ <-ret_from_intr
1322       ls-2269    3d..2    8us : irq_enter <-do_IRQ
1323       ls-2269    3d..2    8us : rcu_irq_enter <-irq_enter
1324       ls-2269    3d..2    9us : add_preempt_count <-irq_enter
1325       ls-2269    3d.h2    9us : exit_idle <-do_IRQ
1326 [...]
1327       ls-2269    3d.h3   20us : sub_preempt_count <-_raw_spin_unlock
1328       ls-2269    3d.h2   20us : irq_exit <-do_IRQ
1329       ls-2269    3d.h2   21us : sub_preempt_count <-irq_exit
1330       ls-2269    3d..3   21us : do_softirq <-irq_exit
1331       ls-2269    3d..3   21us : __do_softirq <-call_softirq
1332       ls-2269    3d..3   21us+: __local_bh_disable <-__do_softirq
1333       ls-2269    3d.s4   29us : sub_preempt_count <-_local_bh_enable_ip
1334       ls-2269    3d.s5   29us : sub_preempt_count <-_local_bh_enable_ip
1335       ls-2269    3d.s5   31us : do_IRQ <-ret_from_intr
1336       ls-2269    3d.s5   31us : irq_enter <-do_IRQ
1337       ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
1338 [...]
1339       ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
1340       ls-2269    3d.s5   32us : add_preempt_count <-irq_enter
1341       ls-2269    3d.H5   32us : exit_idle <-do_IRQ
1342       ls-2269    3d.H5   32us : handle_irq <-do_IRQ
1343       ls-2269    3d.H5   32us : irq_to_desc <-handle_irq
1344       ls-2269    3d.H5   33us : handle_fasteoi_irq <-handle_irq
1345 [...]
1346       ls-2269    3d.s5  158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
1347       ls-2269    3d.s3  158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
1348       ls-2269    3d.s3  159us : __local_bh_enable <-__do_softirq
1349       ls-2269    3d.s3  159us : sub_preempt_count <-__local_bh_enable
1350       ls-2269    3d..3  159us : idle_cpu <-irq_exit
1351       ls-2269    3d..3  159us : rcu_irq_exit <-irq_exit
1352       ls-2269    3d..3  160us : sub_preempt_count <-irq_exit
1353       ls-2269    3d...  161us : __mutex_unlock_slowpath <-mutex_unlock
1354       ls-2269    3d...  162us+: trace_hardirqs_on <-mutex_unlock
1355       ls-2269    3d...  186us : <stack trace>
1356  => __mutex_unlock_slowpath
1357  => mutex_unlock
1358  => process_output
1359  => n_tty_write
1360  => tty_write
1361  => vfs_write
1362  => sys_write
1363  => system_call_fastpath
1364 
1365 This is an interesting trace. It started with kworker running and
1366 scheduling out and ls taking over. But as soon as ls released the
1367 rq lock and enabled interrupts (but not preemption) an interrupt
1368 triggered. When the interrupt finished, it started running softirqs.
1369 But while the softirq was running, another interrupt triggered.
1370 When an interrupt is running inside a softirq, the annotation is 'H'.
1371 
1372 
1373 wakeup
1374 ------
1375 
1376 One common case that people are interested in tracing is the
1377 time it takes for a task that is woken to actually wake up.
1378 Now for non Real-Time tasks, this can be arbitrary. But tracing
1379 it none the less can be interesting. 
1380 
1381 Without function tracing:
1382 
1383  # echo 0 > options/function-trace
1384  # echo wakeup > current_tracer
1385  # echo 1 > tracing_on
1386  # echo 0 > tracing_max_latency
1387  # chrt -f 5 sleep 1
1388  # echo 0 > tracing_on
1389  # cat trace
1390 # tracer: wakeup
1391 #
1392 # wakeup latency trace v1.1.5 on 3.8.0-test+
1393 # --------------------------------------------------------------------
1394 # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1395 #    -----------------
1396 #    | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
1397 #    -----------------
1398 #
1399 #                  _------=> CPU#            
1400 #                 / _-----=> irqs-off        
1401 #                | / _----=> need-resched    
1402 #                || / _---=> hardirq/softirq 
1403 #                ||| / _--=> preempt-depth   
1404 #                |||| /     delay             
1405 #  cmd     pid   ||||| time  |   caller      
1406 #     \   /      |||||  \    |   /           
1407   <idle>-0       3dNs7    0us :      0:120:R   + [003]   312:100:R kworker/3:1H
1408   <idle>-0       3dNs7    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1409   <idle>-0       3d..3   15us : __schedule <-schedule
1410   <idle>-0       3d..3   15us :      0:120:R ==> [003]   312:100:R kworker/3:1H
1411 
1412 The tracer only traces the highest priority task in the system
1413 to avoid tracing the normal circumstances. Here we see that
1414 the kworker with a nice priority of -20 (not very nice), took
1415 just 15 microseconds from the time it woke up, to the time it
1416 ran.
1417 
1418 Non Real-Time tasks are not that interesting. A more interesting
1419 trace is to concentrate only on Real-Time tasks.
1420 
1421 wakeup_rt
1422 ---------
1423 
1424 In a Real-Time environment it is very important to know the
1425 wakeup time it takes for the highest priority task that is woken
1426 up to the time that it executes. This is also known as "schedule
1427 latency". I stress the point that this is about RT tasks. It is
1428 also important to know the scheduling latency of non-RT tasks,
1429 but the average schedule latency is better for non-RT tasks.
1430 Tools like LatencyTop are more appropriate for such
1431 measurements.
1432 
1433 Real-Time environments are interested in the worst case latency.
1434 That is the longest latency it takes for something to happen,
1435 and not the average. We can have a very fast scheduler that may
1436 only have a large latency once in a while, but that would not
1437 work well with Real-Time tasks.  The wakeup_rt tracer was designed
1438 to record the worst case wakeups of RT tasks. Non-RT tasks are
1439 not recorded because the tracer only records one worst case and
1440 tracing non-RT tasks that are unpredictable will overwrite the
1441 worst case latency of RT tasks (just run the normal wakeup
1442 tracer for a while to see that effect).
1443 
1444 Since this tracer only deals with RT tasks, we will run this
1445 slightly differently than we did with the previous tracers.
1446 Instead of performing an 'ls', we will run 'sleep 1' under
1447 'chrt' which changes the priority of the task.
1448 
1449  # echo 0 > options/function-trace
1450  # echo wakeup_rt > current_tracer
1451  # echo 1 > tracing_on
1452  # echo 0 > tracing_max_latency
1453  # chrt -f 5 sleep 1
1454  # echo 0 > tracing_on
1455  # cat trace
1456 # tracer: wakeup
1457 #
1458 # tracer: wakeup_rt
1459 #
1460 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1461 # --------------------------------------------------------------------
1462 # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1463 #    -----------------
1464 #    | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
1465 #    -----------------
1466 #
1467 #                  _------=> CPU#            
1468 #                 / _-----=> irqs-off        
1469 #                | / _----=> need-resched    
1470 #                || / _---=> hardirq/softirq 
1471 #                ||| / _--=> preempt-depth   
1472 #                |||| /     delay             
1473 #  cmd     pid   ||||| time  |   caller      
1474 #     \   /      |||||  \    |   /           
1475   <idle>-0       3d.h4    0us :      0:120:R   + [003]  2389: 94:R sleep
1476   <idle>-0       3d.h4    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1477   <idle>-0       3d..3    5us : __schedule <-schedule
1478   <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
1479 
1480 
1481 Running this on an idle system, we see that it only took 5 microseconds
1482 to perform the task switch.  Note, since the trace point in the schedule
1483 is before the actual "switch", we stop the tracing when the recorded task
1484 is about to schedule in. This may change if we add a new marker at the
1485 end of the scheduler.
1486 
1487 Notice that the recorded task is 'sleep' with the PID of 2389
1488 and it has an rt_prio of 5. This priority is user-space priority
1489 and not the internal kernel priority. The policy is 1 for
1490 SCHED_FIFO and 2 for SCHED_RR.
1491 
1492 Note, that the trace data shows the internal priority (99 - rtprio).
1493 
1494   <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
1495 
1496 The 0:120:R means idle was running with a nice priority of 0 (120 - 20)
1497 and in the running state 'R'. The sleep task was scheduled in with
1498 2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
1499 and it too is in the running state.
1500 
1501 Doing the same with chrt -r 5 and function-trace set.
1502 
1503   echo 1 > options/function-trace
1504 
1505 # tracer: wakeup_rt
1506 #
1507 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1508 # --------------------------------------------------------------------
1509 # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1510 #    -----------------
1511 #    | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
1512 #    -----------------
1513 #
1514 #                  _------=> CPU#            
1515 #                 / _-----=> irqs-off        
1516 #                | / _----=> need-resched    
1517 #                || / _---=> hardirq/softirq 
1518 #                ||| / _--=> preempt-depth   
1519 #                |||| /     delay             
1520 #  cmd     pid   ||||| time  |   caller      
1521 #     \   /      |||||  \    |   /           
1522   <idle>-0       3d.h4    1us+:      0:120:R   + [003]  2448: 94:R sleep
1523   <idle>-0       3d.h4    2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
1524   <idle>-0       3d.h3    3us : check_preempt_curr <-ttwu_do_wakeup
1525   <idle>-0       3d.h3    3us : resched_curr <-check_preempt_curr
1526   <idle>-0       3dNh3    4us : task_woken_rt <-ttwu_do_wakeup
1527   <idle>-0       3dNh3    4us : _raw_spin_unlock <-try_to_wake_up
1528   <idle>-0       3dNh3    4us : sub_preempt_count <-_raw_spin_unlock
1529   <idle>-0       3dNh2    5us : ttwu_stat <-try_to_wake_up
1530   <idle>-0       3dNh2    5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
1531   <idle>-0       3dNh2    6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1532   <idle>-0       3dNh1    6us : _raw_spin_lock <-__run_hrtimer
1533   <idle>-0       3dNh1    6us : add_preempt_count <-_raw_spin_lock
1534   <idle>-0       3dNh2    7us : _raw_spin_unlock <-hrtimer_interrupt
1535   <idle>-0       3dNh2    7us : sub_preempt_count <-_raw_spin_unlock
1536   <idle>-0       3dNh1    7us : tick_program_event <-hrtimer_interrupt
1537   <idle>-0       3dNh1    7us : clockevents_program_event <-tick_program_event
1538   <idle>-0       3dNh1    8us : ktime_get <-clockevents_program_event
1539   <idle>-0       3dNh1    8us : lapic_next_event <-clockevents_program_event
1540   <idle>-0       3dNh1    8us : irq_exit <-smp_apic_timer_interrupt
1541   <idle>-0       3dNh1    9us : sub_preempt_count <-irq_exit
1542   <idle>-0       3dN.2    9us : idle_cpu <-irq_exit
1543   <idle>-0       3dN.2    9us : rcu_irq_exit <-irq_exit
1544   <idle>-0       3dN.2   10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
1545   <idle>-0       3dN.2   10us : sub_preempt_count <-irq_exit
1546   <idle>-0       3.N.1   11us : rcu_idle_exit <-cpu_idle
1547   <idle>-0       3dN.1   11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
1548   <idle>-0       3.N.1   11us : tick_nohz_idle_exit <-cpu_idle
1549   <idle>-0       3dN.1   12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
1550   <idle>-0       3dN.1   12us : ktime_get <-tick_nohz_idle_exit
1551   <idle>-0       3dN.1   12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
1552   <idle>-0       3dN.1   13us : update_cpu_load_nohz <-tick_nohz_idle_exit
1553   <idle>-0       3dN.1   13us : _raw_spin_lock <-update_cpu_load_nohz
1554   <idle>-0       3dN.1   13us : add_preempt_count <-_raw_spin_lock
1555   <idle>-0       3dN.2   13us : __update_cpu_load <-update_cpu_load_nohz
1556   <idle>-0       3dN.2   14us : sched_avg_update <-__update_cpu_load
1557   <idle>-0       3dN.2   14us : _raw_spin_unlock <-update_cpu_load_nohz
1558   <idle>-0       3dN.2   14us : sub_preempt_count <-_raw_spin_unlock
1559   <idle>-0       3dN.1   15us : calc_load_exit_idle <-tick_nohz_idle_exit
1560   <idle>-0       3dN.1   15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
1561   <idle>-0       3dN.1   15us : hrtimer_cancel <-tick_nohz_idle_exit
1562   <idle>-0       3dN.1   15us : hrtimer_try_to_cancel <-hrtimer_cancel
1563   <idle>-0       3dN.1   16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
1564   <idle>-0       3dN.1   16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
1565   <idle>-0       3dN.1   16us : add_preempt_count <-_raw_spin_lock_irqsave
1566   <idle>-0       3dN.2   17us : __remove_hrtimer <-remove_hrtimer.part.16
1567   <idle>-0       3dN.2   17us : hrtimer_force_reprogram <-__remove_hrtimer
1568   <idle>-0       3dN.2   17us : tick_program_event <-hrtimer_force_reprogram
1569   <idle>-0       3dN.2   18us : clockevents_program_event <-tick_program_event
1570   <idle>-0       3dN.2   18us : ktime_get <-clockevents_program_event
1571   <idle>-0       3dN.2   18us : lapic_next_event <-clockevents_program_event
1572   <idle>-0       3dN.2   19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
1573   <idle>-0       3dN.2   19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1574   <idle>-0       3dN.1   19us : hrtimer_forward <-tick_nohz_idle_exit
1575   <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
1576   <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
1577   <idle>-0       3dN.1   20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
1578   <idle>-0       3dN.1   20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
1579   <idle>-0       3dN.1   21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
1580   <idle>-0       3dN.1   21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
1581   <idle>-0       3dN.1   21us : add_preempt_count <-_raw_spin_lock_irqsave
1582   <idle>-0       3dN.2   22us : ktime_add_safe <-__hrtimer_start_range_ns
1583   <idle>-0       3dN.2   22us : enqueue_hrtimer <-__hrtimer_start_range_ns
1584   <idle>-0       3dN.2   22us : tick_program_event <-__hrtimer_start_range_ns
1585   <idle>-0       3dN.2   23us : clockevents_program_event <-tick_program_event
1586   <idle>-0       3dN.2   23us : ktime_get <-clockevents_program_event
1587   <idle>-0       3dN.2   23us : lapic_next_event <-clockevents_program_event
1588   <idle>-0       3dN.2   24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
1589   <idle>-0       3dN.2   24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1590   <idle>-0       3dN.1   24us : account_idle_ticks <-tick_nohz_idle_exit
1591   <idle>-0       3dN.1   24us : account_idle_time <-account_idle_ticks
1592   <idle>-0       3.N.1   25us : sub_preempt_count <-cpu_idle
1593   <idle>-0       3.N..   25us : schedule <-cpu_idle
1594   <idle>-0       3.N..   25us : __schedule <-preempt_schedule
1595   <idle>-0       3.N..   26us : add_preempt_count <-__schedule
1596   <idle>-0       3.N.1   26us : rcu_note_context_switch <-__schedule
1597   <idle>-0       3.N.1   26us : rcu_sched_qs <-rcu_note_context_switch
1598   <idle>-0       3dN.1   27us : rcu_preempt_qs <-rcu_note_context_switch
1599   <idle>-0       3.N.1   27us : _raw_spin_lock_irq <-__schedule
1600   <idle>-0       3dN.1   27us : add_preempt_count <-_raw_spin_lock_irq
1601   <idle>-0       3dN.2   28us : put_prev_task_idle <-__schedule
1602   <idle>-0       3dN.2   28us : pick_next_task_stop <-pick_next_task
1603   <idle>-0       3dN.2   28us : pick_next_task_rt <-pick_next_task
1604   <idle>-0       3dN.2   29us : dequeue_pushable_task <-pick_next_task_rt
1605   <idle>-0       3d..3   29us : __schedule <-preempt_schedule
1606   <idle>-0       3d..3   30us :      0:120:R ==> [003]  2448: 94:R sleep
1607 
1608 This isn't that big of a trace, even with function tracing enabled,
1609 so I included the entire trace.
1610 
1611 The interrupt went off while when the system was idle. Somewhere
1612 before task_woken_rt() was called, the NEED_RESCHED flag was set,
1613 this is indicated by the first occurrence of the 'N' flag.
1614 
1615 Latency tracing and events
1616 --------------------------
1617 As function tracing can induce a much larger latency, but without
1618 seeing what happens within the latency it is hard to know what
1619 caused it. There is a middle ground, and that is with enabling
1620 events.
1621 
1622  # echo 0 > options/function-trace
1623  # echo wakeup_rt > current_tracer
1624  # echo 1 > events/enable
1625  # echo 1 > tracing_on
1626  # echo 0 > tracing_max_latency
1627  # chrt -f 5 sleep 1
1628  # echo 0 > tracing_on
1629  # cat trace
1630 # tracer: wakeup_rt
1631 #
1632 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1633 # --------------------------------------------------------------------
1634 # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1635 #    -----------------
1636 #    | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
1637 #    -----------------
1638 #
1639 #                  _------=> CPU#            
1640 #                 / _-----=> irqs-off        
1641 #                | / _----=> need-resched    
1642 #                || / _---=> hardirq/softirq 
1643 #                ||| / _--=> preempt-depth   
1644 #                |||| /     delay             
1645 #  cmd     pid   ||||| time  |   caller      
1646 #     \   /      |||||  \    |   /           
1647   <idle>-0       2d.h4    0us :      0:120:R   + [002]  5882: 94:R sleep
1648   <idle>-0       2d.h4    0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
1649   <idle>-0       2d.h4    1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
1650   <idle>-0       2dNh2    1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
1651   <idle>-0       2.N.2    2us : power_end: cpu_id=2
1652   <idle>-0       2.N.2    3us : cpu_idle: state=4294967295 cpu_id=2
1653   <idle>-0       2dN.3    4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
1654   <idle>-0       2dN.3    4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
1655   <idle>-0       2.N.2    5us : rcu_utilization: Start context switch
1656   <idle>-0       2.N.2    5us : rcu_utilization: End context switch
1657   <idle>-0       2d..3    6us : __schedule <-schedule
1658   <idle>-0       2d..3    6us :      0:120:R ==> [002]  5882: 94:R sleep
1659 
1660 
1661 function
1662 --------
1663 
1664 This tracer is the function tracer. Enabling the function tracer
1665 can be done from the debug file system. Make sure the
1666 ftrace_enabled is set; otherwise this tracer is a nop.
1667 See the "ftrace_enabled" section below.
1668 
1669  # sysctl kernel.ftrace_enabled=1
1670  # echo function > current_tracer
1671  # echo 1 > tracing_on
1672  # usleep 1
1673  # echo 0 > tracing_on
1674  # cat trace
1675 # tracer: function
1676 #
1677 # entries-in-buffer/entries-written: 24799/24799   #P:4
1678 #
1679 #                              _-----=> irqs-off
1680 #                             / _----=> need-resched
1681 #                            | / _---=> hardirq/softirq
1682 #                            || / _--=> preempt-depth
1683 #                            ||| /     delay
1684 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
1685 #              | |       |   ||||       |         |
1686             bash-1994  [002] ....  3082.063030: mutex_unlock <-rb_simple_write
1687             bash-1994  [002] ....  3082.063031: __mutex_unlock_slowpath <-mutex_unlock
1688             bash-1994  [002] ....  3082.063031: __fsnotify_parent <-fsnotify_modify
1689             bash-1994  [002] ....  3082.063032: fsnotify <-fsnotify_modify
1690             bash-1994  [002] ....  3082.063032: __srcu_read_lock <-fsnotify
1691             bash-1994  [002] ....  3082.063032: add_preempt_count <-__srcu_read_lock
1692             bash-1994  [002] ...1  3082.063032: sub_preempt_count <-__srcu_read_lock
1693             bash-1994  [002] ....  3082.063033: __srcu_read_unlock <-fsnotify
1694 [...]
1695 
1696 
1697 Note: function tracer uses ring buffers to store the above
1698 entries. The newest data may overwrite the oldest data.
1699 Sometimes using echo to stop the trace is not sufficient because
1700 the tracing could have overwritten the data that you wanted to
1701 record. For this reason, it is sometimes better to disable
1702 tracing directly from a program. This allows you to stop the
1703 tracing at the point that you hit the part that you are
1704 interested in. To disable the tracing directly from a C program,
1705 something like following code snippet can be used:
1706 
1707 int trace_fd;
1708 [...]
1709 int main(int argc, char *argv[]) {
1710         [...]
1711         trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
1712         [...]
1713         if (condition_hit()) {
1714                 write(trace_fd, "0", 1);
1715         }
1716         [...]
1717 }
1718 
1719 
1720 Single thread tracing
1721 ---------------------
1722 
1723 By writing into set_ftrace_pid you can trace a
1724 single thread. For example:
1725 
1726 # cat set_ftrace_pid
1727 no pid
1728 # echo 3111 > set_ftrace_pid
1729 # cat set_ftrace_pid
1730 3111
1731 # echo function > current_tracer
1732 # cat trace | head
1733  # tracer: function
1734  #
1735  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
1736  #              | |       |          |         |
1737      yum-updatesd-3111  [003]  1637.254676: finish_task_switch <-thread_return
1738      yum-updatesd-3111  [003]  1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
1739      yum-updatesd-3111  [003]  1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
1740      yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
1741      yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
1742      yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll
1743 # echo > set_ftrace_pid
1744 # cat trace |head
1745  # tracer: function
1746  #
1747  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
1748  #              | |       |          |         |
1749  ##### CPU 3 buffer started ####
1750      yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
1751      yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
1752      yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
1753      yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
1754      yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit
1755 
1756 If you want to trace a function when executing, you could use
1757 something like this simple program:
1758 
1759 #include <stdio.h>
1760 #include <stdlib.h>
1761 #include <sys/types.h>
1762 #include <sys/stat.h>
1763 #include <fcntl.h>
1764 #include <unistd.h>
1765 #include <string.h>
1766 
1767 #define _STR(x) #x
1768 #define STR(x) _STR(x)
1769 #define MAX_PATH 256
1770 
1771 const char *find_debugfs(void)
1772 {
1773        static char debugfs[MAX_PATH+1];
1774        static int debugfs_found;
1775        char type[100];
1776        FILE *fp;
1777 
1778        if (debugfs_found)
1779                return debugfs;
1780 
1781        if ((fp = fopen("/proc/mounts","r")) == NULL) {
1782                perror("/proc/mounts");
1783                return NULL;
1784        }
1785 
1786        while (fscanf(fp, "%*s %"
1787                      STR(MAX_PATH)
1788                      "s %99s %*s %*d %*d\n",
1789                      debugfs, type) == 2) {
1790                if (strcmp(type, "debugfs") == 0)
1791                        break;
1792        }
1793        fclose(fp);
1794 
1795        if (strcmp(type, "debugfs") != 0) {
1796                fprintf(stderr, "debugfs not mounted");
1797                return NULL;
1798        }
1799 
1800        strcat(debugfs, "/tracing/");
1801        debugfs_found = 1;
1802 
1803        return debugfs;
1804 }
1805 
1806 const char *tracing_file(const char *file_name)
1807 {
1808        static char trace_file[MAX_PATH+1];
1809        snprintf(trace_file, MAX_PATH, "%s/%s", find_debugfs(), file_name);
1810        return trace_file;
1811 }
1812 
1813 int main (int argc, char **argv)
1814 {
1815         if (argc < 1)
1816                 exit(-1);
1817 
1818         if (fork() > 0) {
1819                 int fd, ffd;
1820                 char line[64];
1821                 int s;
1822 
1823                 ffd = open(tracing_file("current_tracer"), O_WRONLY);
1824                 if (ffd < 0)
1825                         exit(-1);
1826                 write(ffd, "nop", 3);
1827 
1828                 fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
1829                 s = sprintf(line, "%d\n", getpid());
1830                 write(fd, line, s);
1831 
1832                 write(ffd, "function", 8);
1833 
1834                 close(fd);
1835                 close(ffd);
1836 
1837                 execvp(argv[1], argv+1);
1838         }
1839 
1840         return 0;
1841 }
1842 
1843 Or this simple script!
1844 
1845 ------
1846 #!/bin/bash
1847 
1848 debugfs=`sed -ne 's/^debugfs \(.*\) debugfs.*/\1/p' /proc/mounts`
1849 echo nop > $debugfs/tracing/current_tracer
1850 echo 0 > $debugfs/tracing/tracing_on
1851 echo $$ > $debugfs/tracing/set_ftrace_pid
1852 echo function > $debugfs/tracing/current_tracer
1853 echo 1 > $debugfs/tracing/tracing_on
1854 exec "$@"
1855 ------
1856 
1857 
1858 function graph tracer
1859 ---------------------------
1860 
1861 This tracer is similar to the function tracer except that it
1862 probes a function on its entry and its exit. This is done by
1863 using a dynamically allocated stack of return addresses in each
1864 task_struct. On function entry the tracer overwrites the return
1865 address of each function traced to set a custom probe. Thus the
1866 original return address is stored on the stack of return address
1867 in the task_struct.
1868 
1869 Probing on both ends of a function leads to special features
1870 such as:
1871 
1872 - measure of a function's time execution
1873 - having a reliable call stack to draw function calls graph
1874 
1875 This tracer is useful in several situations:
1876 
1877 - you want to find the reason of a strange kernel behavior and
1878   need to see what happens in detail on any areas (or specific
1879   ones).
1880 
1881 - you are experiencing weird latencies but it's difficult to
1882   find its origin.
1883 
1884 - you want to find quickly which path is taken by a specific
1885   function
1886 
1887 - you just want to peek inside a working kernel and want to see
1888   what happens there.
1889 
1890 # tracer: function_graph
1891 #
1892 # CPU  DURATION                  FUNCTION CALLS
1893 # |     |   |                     |   |   |   |
1894 
1895  0)               |  sys_open() {
1896  0)               |    do_sys_open() {
1897  0)               |      getname() {
1898  0)               |        kmem_cache_alloc() {
1899  0)   1.382 us    |          __might_sleep();
1900  0)   2.478 us    |        }
1901  0)               |        strncpy_from_user() {
1902  0)               |          might_fault() {
1903  0)   1.389 us    |            __might_sleep();
1904  0)   2.553 us    |          }
1905  0)   3.807 us    |        }
1906  0)   7.876 us    |      }
1907  0)               |      alloc_fd() {
1908  0)   0.668 us    |        _spin_lock();
1909  0)   0.570 us    |        expand_files();
1910  0)   0.586 us    |        _spin_unlock();
1911 
1912 
1913 There are several columns that can be dynamically
1914 enabled/disabled. You can use every combination of options you
1915 want, depending on your needs.
1916 
1917 - The cpu number on which the function executed is default
1918   enabled.  It is sometimes better to only trace one cpu (see
1919   tracing_cpu_mask file) or you might sometimes see unordered
1920   function calls while cpu tracing switch.
1921 
1922         hide: echo nofuncgraph-cpu > trace_options
1923         show: echo funcgraph-cpu > trace_options
1924 
1925 - The duration (function's time of execution) is displayed on
1926   the closing bracket line of a function or on the same line
1927   than the current function in case of a leaf one. It is default
1928   enabled.
1929 
1930         hide: echo nofuncgraph-duration > trace_options
1931         show: echo funcgraph-duration > trace_options
1932 
1933 - The overhead field precedes the duration field in case of
1934   reached duration thresholds.
1935 
1936         hide: echo nofuncgraph-overhead > trace_options
1937         show: echo funcgraph-overhead > trace_options
1938         depends on: funcgraph-duration
1939 
1940   ie:
1941 
1942   0)               |    up_write() {
1943   0)   0.646 us    |      _spin_lock_irqsave();
1944   0)   0.684 us    |      _spin_unlock_irqrestore();
1945   0)   3.123 us    |    }
1946   0)   0.548 us    |    fput();
1947   0) + 58.628 us   |  }
1948 
1949   [...]
1950 
1951   0)               |      putname() {
1952   0)               |        kmem_cache_free() {
1953   0)   0.518 us    |          __phys_addr();
1954   0)   1.757 us    |        }
1955   0)   2.861 us    |      }
1956   0) ! 115.305 us  |    }
1957   0) ! 116.402 us  |  }
1958 
1959   + means that the function exceeded 10 usecs.
1960   ! means that the function exceeded 100 usecs.
1961   # means that the function exceeded 1000 usecs.
1962   $ means that the function exceeded 1 sec.
1963 
1964 
1965 - The task/pid field displays the thread cmdline and pid which
1966   executed the function. It is default disabled.
1967 
1968         hide: echo nofuncgraph-proc > trace_options
1969         show: echo funcgraph-proc > trace_options
1970 
1971   ie:
1972 
1973   # tracer: function_graph
1974   #
1975   # CPU  TASK/PID        DURATION                  FUNCTION CALLS
1976   # |    |    |           |   |                     |   |   |   |
1977   0)    sh-4802     |               |                  d_free() {
1978   0)    sh-4802     |               |                    call_rcu() {
1979   0)    sh-4802     |               |                      __call_rcu() {
1980   0)    sh-4802     |   0.616 us    |                        rcu_process_gp_end();
1981   0)    sh-4802     |   0.586 us    |                        check_for_new_grace_period();
1982   0)    sh-4802     |   2.899 us    |                      }
1983   0)    sh-4802     |   4.040 us    |                    }
1984   0)    sh-4802     |   5.151 us    |                  }
1985   0)    sh-4802     | + 49.370 us   |                }
1986 
1987 
1988 - The absolute time field is an absolute timestamp given by the
1989   system clock since it started. A snapshot of this time is
1990   given on each entry/exit of functions
1991 
1992         hide: echo nofuncgraph-abstime > trace_options
1993         show: echo funcgraph-abstime > trace_options
1994 
1995   ie:
1996 
1997   #
1998   #      TIME       CPU  DURATION                  FUNCTION CALLS
1999   #       |         |     |   |                     |   |   |   |
2000   360.774522 |   1)   0.541 us    |                                          }
2001   360.774522 |   1)   4.663 us    |                                        }
2002   360.774523 |   1)   0.541 us    |                                        __wake_up_bit();
2003   360.774524 |   1)   6.796 us    |                                      }
2004   360.774524 |   1)   7.952 us    |                                    }
2005   360.774525 |   1)   9.063 us    |                                  }
2006   360.774525 |   1)   0.615 us    |                                  journal_mark_dirty();
2007   360.774527 |   1)   0.578 us    |                                  __brelse();
2008   360.774528 |   1)               |                                  reiserfs_prepare_for_journal() {
2009   360.774528 |   1)               |                                    unlock_buffer() {
2010   360.774529 |   1)               |                                      wake_up_bit() {
2011   360.774529 |   1)               |                                        bit_waitqueue() {
2012   360.774530 |   1)   0.594 us    |                                          __phys_addr();
2013 
2014 
2015 The function name is always displayed after the closing bracket
2016 for a function if the start of that function is not in the
2017 trace buffer.
2018 
2019 Display of the function name after the closing bracket may be
2020 enabled for functions whose start is in the trace buffer,
2021 allowing easier searching with grep for function durations.
2022 It is default disabled.
2023 
2024         hide: echo nofuncgraph-tail > trace_options
2025         show: echo funcgraph-tail > trace_options
2026 
2027   Example with nofuncgraph-tail (default):
2028   0)               |      putname() {
2029   0)               |        kmem_cache_free() {
2030   0)   0.518 us    |          __phys_addr();
2031   0)   1.757 us    |        }
2032   0)   2.861 us    |      }
2033 
2034   Example with funcgraph-tail:
2035   0)               |      putname() {
2036   0)               |        kmem_cache_free() {
2037   0)   0.518 us    |          __phys_addr();
2038   0)   1.757 us    |        } /* kmem_cache_free() */
2039   0)   2.861 us    |      } /* putname() */
2040 
2041 You can put some comments on specific functions by using
2042 trace_printk() For example, if you want to put a comment inside
2043 the __might_sleep() function, you just have to include
2044 <linux/ftrace.h> and call trace_printk() inside __might_sleep()
2045 
2046 trace_printk("I'm a comment!\n")
2047 
2048 will produce:
2049 
2050  1)               |             __might_sleep() {
2051  1)               |                /* I'm a comment! */
2052  1)   1.449 us    |             }
2053 
2054 
2055 You might find other useful features for this tracer in the
2056 following "dynamic ftrace" section such as tracing only specific
2057 functions or tasks.
2058 
2059 dynamic ftrace
2060 --------------
2061 
2062 If CONFIG_DYNAMIC_FTRACE is set, the system will run with
2063 virtually no overhead when function tracing is disabled. The way
2064 this works is the mcount function call (placed at the start of
2065 every kernel function, produced by the -pg switch in gcc),
2066 starts of pointing to a simple return. (Enabling FTRACE will
2067 include the -pg switch in the compiling of the kernel.)
2068 
2069 At compile time every C file object is run through the
2070 recordmcount program (located in the scripts directory). This
2071 program will parse the ELF headers in the C object to find all
2072 the locations in the .text section that call mcount. (Note, only
2073 white listed .text sections are processed, since processing other
2074 sections like .init.text may cause races due to those sections
2075 being freed unexpectedly).
2076 
2077 A new section called "__mcount_loc" is created that holds
2078 references to all the mcount call sites in the .text section.
2079 The recordmcount program re-links this section back into the
2080 original object. The final linking stage of the kernel will add all these
2081 references into a single table.
2082 
2083 On boot up, before SMP is initialized, the dynamic ftrace code
2084 scans this table and updates all the locations into nops. It
2085 also records the locations, which are added to the
2086 available_filter_functions list.  Modules are processed as they
2087 are loaded and before they are executed.  When a module is
2088 unloaded, it also removes its functions from the ftrace function
2089 list. This is automatic in the module unload code, and the
2090 module author does not need to worry about it.
2091 
2092 When tracing is enabled, the process of modifying the function
2093 tracepoints is dependent on architecture. The old method is to use
2094 kstop_machine to prevent races with the CPUs executing code being
2095 modified (which can cause the CPU to do undesirable things, especially
2096 if the modified code crosses cache (or page) boundaries), and the nops are
2097 patched back to calls. But this time, they do not call mcount
2098 (which is just a function stub). They now call into the ftrace
2099 infrastructure.
2100 
2101 The new method of modifying the function tracepoints is to place
2102 a breakpoint at the location to be modified, sync all CPUs, modify
2103 the rest of the instruction not covered by the breakpoint. Sync
2104 all CPUs again, and then remove the breakpoint with the finished
2105 version to the ftrace call site.
2106 
2107 Some archs do not even need to monkey around with the synchronization,
2108 and can just slap the new code on top of the old without any
2109 problems with other CPUs executing it at the same time.
2110 
2111 One special side-effect to the recording of the functions being
2112 traced is that we can now selectively choose which functions we
2113 wish to trace and which ones we want the mcount calls to remain
2114 as nops.
2115 
2116 Two files are used, one for enabling and one for disabling the
2117 tracing of specified functions. They are:
2118 
2119   set_ftrace_filter
2120 
2121 and
2122 
2123   set_ftrace_notrace
2124 
2125 A list of available functions that you can add to these files is
2126 listed in:
2127 
2128    available_filter_functions
2129 
2130  # cat available_filter_functions
2131 put_prev_task_idle
2132 kmem_cache_create
2133 pick_next_task_rt
2134 get_online_cpus
2135 pick_next_task_fair
2136 mutex_lock
2137 [...]
2138 
2139 If I am only interested in sys_nanosleep and hrtimer_interrupt:
2140 
2141  # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
2142  # echo function > current_tracer
2143  # echo 1 > tracing_on
2144  # usleep 1
2145  # echo 0 > tracing_on
2146  # cat trace
2147 # tracer: function
2148 #
2149 # entries-in-buffer/entries-written: 5/5   #P:4
2150 #
2151 #                              _-----=> irqs-off
2152 #                             / _----=> need-resched
2153 #                            | / _---=> hardirq/softirq
2154 #                            || / _--=> preempt-depth
2155 #                            ||| /     delay
2156 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2157 #              | |       |   ||||       |         |
2158           usleep-2665  [001] ....  4186.475355: sys_nanosleep <-system_call_fastpath
2159           <idle>-0     [001] d.h1  4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
2160           usleep-2665  [001] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2161           <idle>-0     [003] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2162           <idle>-0     [002] d.h1  4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
2163 
2164 To see which functions are being traced, you can cat the file:
2165 
2166  # cat set_ftrace_filter
2167 hrtimer_interrupt
2168 sys_nanosleep
2169 
2170 
2171 Perhaps this is not enough. The filters also allow simple wild
2172 cards. Only the following are currently available
2173 
2174   <match>*  - will match functions that begin with <match>
2175   *<match>  - will match functions that end with <match>
2176   *<match>* - will match functions that have <match> in it
2177 
2178 These are the only wild cards which are supported.
2179 
2180   <match>*<match> will not work.
2181 
2182 Note: It is better to use quotes to enclose the wild cards,
2183       otherwise the shell may expand the parameters into names
2184       of files in the local directory.
2185 
2186  # echo 'hrtimer_*' > set_ftrace_filter
2187 
2188 Produces:
2189 
2190 # tracer: function
2191 #
2192 # entries-in-buffer/entries-written: 897/897   #P:4
2193 #
2194 #                              _-----=> irqs-off
2195 #                             / _----=> need-resched
2196 #                            | / _---=> hardirq/softirq
2197 #                            || / _--=> preempt-depth
2198 #                            ||| /     delay
2199 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2200 #              | |       |   ||||       |         |
2201           <idle>-0     [003] dN.1  4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
2202           <idle>-0     [003] dN.1  4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
2203           <idle>-0     [003] dN.2  4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
2204           <idle>-0     [003] dN.1  4228.547805: hrtimer_forward <-tick_nohz_idle_exit
2205           <idle>-0     [003] dN.1  4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
2206           <idle>-0     [003] d..1  4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
2207           <idle>-0     [003] d..1  4228.547859: hrtimer_start <-__tick_nohz_idle_enter
2208           <idle>-0     [003] d..2  4228.547860: hrtimer_force_reprogram <-__rem
2209 
2210 Notice that we lost the sys_nanosleep.
2211 
2212  # cat set_ftrace_filter
2213 hrtimer_run_queues
2214 hrtimer_run_pending
2215 hrtimer_init
2216 hrtimer_cancel
2217 hrtimer_try_to_cancel
2218 hrtimer_forward
2219 hrtimer_start
2220 hrtimer_reprogram
2221 hrtimer_force_reprogram
2222 hrtimer_get_next_event
2223 hrtimer_interrupt
2224 hrtimer_nanosleep
2225 hrtimer_wakeup
2226 hrtimer_get_remaining
2227 hrtimer_get_res
2228 hrtimer_init_sleeper
2229 
2230 
2231 This is because the '>' and '>>' act just like they do in bash.
2232 To rewrite the filters, use '>'
2233 To append to the filters, use '>>'
2234 
2235 To clear out a filter so that all functions will be recorded
2236 again:
2237 
2238  # echo > set_ftrace_filter
2239  # cat set_ftrace_filter
2240  #
2241 
2242 Again, now we want to append.
2243 
2244  # echo sys_nanosleep > set_ftrace_filter
2245  # cat set_ftrace_filter
2246 sys_nanosleep
2247  # echo 'hrtimer_*' >> set_ftrace_filter
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 sys_nanosleep
2261 hrtimer_nanosleep
2262 hrtimer_wakeup
2263 hrtimer_get_remaining
2264 hrtimer_get_res
2265 hrtimer_init_sleeper
2266 
2267 
2268 The set_ftrace_notrace prevents those functions from being
2269 traced.
2270 
2271  # echo '*preempt*' '*lock*' > set_ftrace_notrace
2272 
2273 Produces:
2274 
2275 # tracer: function
2276 #
2277 # entries-in-buffer/entries-written: 39608/39608   #P:4
2278 #
2279 #                              _-----=> irqs-off
2280 #                             / _----=> need-resched
2281 #                            | / _---=> hardirq/softirq
2282 #                            || / _--=> preempt-depth
2283 #                            ||| /     delay
2284 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2285 #              | |       |   ||||       |         |
2286             bash-1994  [000] ....  4342.324896: file_ra_state_init <-do_dentry_open
2287             bash-1994  [000] ....  4342.324897: open_check_o_direct <-do_last
2288             bash-1994  [000] ....  4342.324897: ima_file_check <-do_last
2289             bash-1994  [000] ....  4342.324898: process_measurement <-ima_file_check
2290             bash-1994  [000] ....  4342.324898: ima_get_action <-process_measurement
2291             bash-1994  [000] ....  4342.324898: ima_match_policy <-ima_get_action
2292             bash-1994  [000] ....  4342.324899: do_truncate <-do_last
2293             bash-1994  [000] ....  4342.324899: should_remove_suid <-do_truncate
2294             bash-1994  [000] ....  4342.324899: notify_change <-do_truncate
2295             bash-1994  [000] ....  4342.324900: current_fs_time <-notify_change
2296             bash-1994  [000] ....  4342.324900: current_kernel_time <-current_fs_time
2297             bash-1994  [000] ....  4342.324900: timespec_trunc <-current_fs_time
2298 
2299 We can see that there's no more lock or preempt tracing.
2300 
2301 
2302 Dynamic ftrace with the function graph tracer
2303 ---------------------------------------------
2304 
2305 Although what has been explained above concerns both the
2306 function tracer and the function-graph-tracer, there are some
2307 special features only available in the function-graph tracer.
2308 
2309 If you want to trace only one function and all of its children,
2310 you just have to echo its name into set_graph_function:
2311 
2312  echo __do_fault > set_graph_function
2313 
2314 will produce the following "expanded" trace of the __do_fault()
2315 function:
2316 
2317  0)               |  __do_fault() {
2318  0)               |    filemap_fault() {
2319  0)               |      find_lock_page() {
2320  0)   0.804 us    |        find_get_page();
2321  0)               |        __might_sleep() {
2322  0)   1.329 us    |        }
2323  0)   3.904 us    |      }
2324  0)   4.979 us    |    }
2325  0)   0.653 us    |    _spin_lock();
2326  0)   0.578 us    |    page_add_file_rmap();
2327  0)   0.525 us    |    native_set_pte_at();
2328  0)   0.585 us    |    _spin_unlock();
2329  0)               |    unlock_page() {
2330  0)   0.541 us    |      page_waitqueue();
2331  0)   0.639 us    |      __wake_up_bit();
2332  0)   2.786 us    |    }
2333  0) + 14.237 us   |  }
2334  0)               |  __do_fault() {
2335  0)               |    filemap_fault() {
2336  0)               |      find_lock_page() {
2337  0)   0.698 us    |        find_get_page();
2338  0)               |        __might_sleep() {
2339  0)   1.412 us    |        }
2340  0)   3.950 us    |      }
2341  0)   5.098 us    |    }
2342  0)   0.631 us    |    _spin_lock();
2343  0)   0.571 us    |    page_add_file_rmap();
2344  0)   0.526 us    |    native_set_pte_at();
2345  0)   0.586 us    |    _spin_unlock();
2346  0)               |    unlock_page() {
2347  0)   0.533 us    |      page_waitqueue();
2348  0)   0.638 us    |      __wake_up_bit();
2349  0)   2.793 us    |    }
2350  0) + 14.012 us   |  }
2351 
2352 You can also expand several functions at once:
2353 
2354  echo sys_open > set_graph_function
2355  echo sys_close >> set_graph_function
2356 
2357 Now if you want to go back to trace all functions you can clear
2358 this special filter via:
2359 
2360  echo > set_graph_function
2361 
2362 
2363 ftrace_enabled
2364 --------------
2365 
2366 Note, the proc sysctl ftrace_enable is a big on/off switch for the
2367 function tracer. By default it is enabled (when function tracing is
2368 enabled in the kernel). If it is disabled, all function tracing is
2369 disabled. This includes not only the function tracers for ftrace, but
2370 also for any other uses (perf, kprobes, stack tracing, profiling, etc).
2371 
2372 Please disable this with care.
2373 
2374 This can be disable (and enabled) with:
2375 
2376   sysctl kernel.ftrace_enabled=0
2377   sysctl kernel.ftrace_enabled=1
2378 
2379  or
2380 
2381   echo 0 > /proc/sys/kernel/ftrace_enabled
2382   echo 1 > /proc/sys/kernel/ftrace_enabled
2383 
2384 
2385 Filter commands
2386 ---------------
2387 
2388 A few commands are supported by the set_ftrace_filter interface.
2389 Trace commands have the following format:
2390 
2391 <function>:<command>:<parameter>
2392 
2393 The following commands are supported:
2394 
2395 - mod
2396   This command enables function filtering per module. The
2397   parameter defines the module. For example, if only the write*
2398   functions in the ext3 module are desired, run:
2399 
2400    echo 'write*:mod:ext3' > set_ftrace_filter
2401 
2402   This command interacts with the filter in the same way as
2403   filtering based on function names. Thus, adding more functions
2404   in a different module is accomplished by appending (>>) to the
2405   filter file. Remove specific module functions by prepending
2406   '!':
2407 
2408    echo '!writeback*:mod:ext3' >> set_ftrace_filter
2409 
2410 - traceon/traceoff
2411   These commands turn tracing on and off when the specified
2412   functions are hit. The parameter determines how many times the
2413   tracing system is turned on and off. If unspecified, there is
2414   no limit. For example, to disable tracing when a schedule bug
2415   is hit the first 5 times, run:
2416 
2417    echo '__schedule_bug:traceoff:5' > set_ftrace_filter
2418 
2419   To always disable tracing when __schedule_bug is hit:
2420 
2421    echo '__schedule_bug:traceoff' > set_ftrace_filter
2422 
2423   These commands are cumulative whether or not they are appended
2424   to set_ftrace_filter. To remove a command, prepend it by '!'
2425   and drop the parameter:
2426 
2427    echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
2428 
2429     The above removes the traceoff command for __schedule_bug
2430     that have a counter. To remove commands without counters:
2431 
2432    echo '!__schedule_bug:traceoff' > set_ftrace_filter
2433 
2434 - snapshot
2435   Will cause a snapshot to be triggered when the function is hit.
2436 
2437    echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
2438 
2439   To only snapshot once:
2440 
2441    echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
2442 
2443   To remove the above commands:
2444 
2445    echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
2446    echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
2447 
2448 - enable_event/disable_event
2449   These commands can enable or disable a trace event. Note, because
2450   function tracing callbacks are very sensitive, when these commands
2451   are registered, the trace point is activated, but disabled in
2452   a "soft" mode. That is, the tracepoint will be called, but
2453   just will not be traced. The event tracepoint stays in this mode
2454   as long as there's a command that triggers it.
2455 
2456    echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
2457          set_ftrace_filter
2458 
2459   The format is:
2460 
2461     <function>:enable_event:<system>:<event>[:count]
2462     <function>:disable_event:<system>:<event>[:count]
2463 
2464   To remove the events commands:
2465 
2466 
2467    echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
2468          set_ftrace_filter
2469    echo '!schedule:disable_event:sched:sched_switch' > \
2470          set_ftrace_filter
2471 
2472 - dump
2473   When the function is hit, it will dump the contents of the ftrace
2474   ring buffer to the console. This is useful if you need to debug
2475   something, and want to dump the trace when a certain function
2476   is hit. Perhaps its a function that is called before a tripple
2477   fault happens and does not allow you to get a regular dump.
2478 
2479 - cpudump
2480   When the function is hit, it will dump the contents of the ftrace
2481   ring buffer for the current CPU to the console. Unlike the "dump"
2482   command, it only prints out the contents of the ring buffer for the
2483   CPU that executed the function that triggered the dump.
2484 
2485 trace_pipe
2486 ----------
2487 
2488 The trace_pipe outputs the same content as the trace file, but
2489 the effect on the tracing is different. Every read from
2490 trace_pipe is consumed. This means that subsequent reads will be
2491 different. The trace is live.
2492 
2493  # echo function > current_tracer
2494  # cat trace_pipe > /tmp/trace.out &
2495 [1] 4153
2496  # echo 1 > tracing_on
2497  # usleep 1
2498  # echo 0 > tracing_on
2499  # cat trace
2500 # tracer: function
2501 #
2502 # entries-in-buffer/entries-written: 0/0   #P:4
2503 #
2504 #                              _-----=> irqs-off
2505 #                             / _----=> need-resched
2506 #                            | / _---=> hardirq/softirq
2507 #                            || / _--=> preempt-depth
2508 #                            ||| /     delay
2509 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2510 #              | |       |   ||||       |         |
2511 
2512  #
2513  # cat /tmp/trace.out
2514             bash-1994  [000] ....  5281.568961: mutex_unlock <-rb_simple_write
2515             bash-1994  [000] ....  5281.568963: __mutex_unlock_slowpath <-mutex_unlock
2516             bash-1994  [000] ....  5281.568963: __fsnotify_parent <-fsnotify_modify
2517             bash-1994  [000] ....  5281.568964: fsnotify <-fsnotify_modify
2518             bash-1994  [000] ....  5281.568964: __srcu_read_lock <-fsnotify
2519             bash-1994  [000] ....  5281.568964: add_preempt_count <-__srcu_read_lock
2520             bash-1994  [000] ...1  5281.568965: sub_preempt_count <-__srcu_read_lock
2521             bash-1994  [000] ....  5281.568965: __srcu_read_unlock <-fsnotify
2522             bash-1994  [000] ....  5281.568967: sys_dup2 <-system_call_fastpath
2523 
2524 
2525 Note, reading the trace_pipe file will block until more input is
2526 added.
2527 
2528 trace entries
2529 -------------
2530 
2531 Having too much or not enough data can be troublesome in
2532 diagnosing an issue in the kernel. The file buffer_size_kb is
2533 used to modify the size of the internal trace buffers. The
2534 number listed is the number of entries that can be recorded per
2535 CPU. To know the full size, multiply the number of possible CPUs
2536 with the number of entries.
2537 
2538  # cat buffer_size_kb
2539 1408 (units kilobytes)
2540 
2541 Or simply read buffer_total_size_kb
2542 
2543  # cat buffer_total_size_kb 
2544 5632
2545 
2546 To modify the buffer, simple echo in a number (in 1024 byte segments).
2547 
2548  # echo 10000 > buffer_size_kb
2549  # cat buffer_size_kb
2550 10000 (units kilobytes)
2551 
2552 It will try to allocate as much as possible. If you allocate too
2553 much, it can cause Out-Of-Memory to trigger.
2554 
2555  # echo 1000000000000 > buffer_size_kb
2556 -bash: echo: write error: Cannot allocate memory
2557  # cat buffer_size_kb
2558 85
2559 
2560 The per_cpu buffers can be changed individually as well:
2561 
2562  # echo 10000 > per_cpu/cpu0/buffer_size_kb
2563  # echo 100 > per_cpu/cpu1/buffer_size_kb
2564 
2565 When the per_cpu buffers are not the same, the buffer_size_kb
2566 at the top level will just show an X
2567 
2568  # cat buffer_size_kb
2569 X
2570 
2571 This is where the buffer_total_size_kb is useful:
2572 
2573  # cat buffer_total_size_kb 
2574 12916
2575 
2576 Writing to the top level buffer_size_kb will reset all the buffers
2577 to be the same again.
2578 
2579 Snapshot
2580 --------
2581 CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
2582 available to all non latency tracers. (Latency tracers which
2583 record max latency, such as "irqsoff" or "wakeup", can't use
2584 this feature, since those are already using the snapshot
2585 mechanism internally.)
2586 
2587 Snapshot preserves a current trace buffer at a particular point
2588 in time without stopping tracing. Ftrace swaps the current
2589 buffer with a spare buffer, and tracing continues in the new
2590 current (=previous spare) buffer.
2591 
2592 The following debugfs files in "tracing" are related to this
2593 feature:
2594 
2595   snapshot:
2596 
2597         This is used to take a snapshot and to read the output
2598         of the snapshot. Echo 1 into this file to allocate a
2599         spare buffer and to take a snapshot (swap), then read
2600         the snapshot from this file in the same format as
2601         "trace" (described above in the section "The File
2602         System"). Both reads snapshot and tracing are executable
2603         in parallel. When the spare buffer is allocated, echoing
2604         0 frees it, and echoing else (positive) values clear the
2605         snapshot contents.
2606         More details are shown in the table below.
2607 
2608         status\input  |     0      |     1      |    else    |
2609         --------------+------------+------------+------------+
2610         not allocated |(do nothing)| alloc+swap |(do nothing)|
2611         --------------+------------+------------+------------+
2612         allocated     |    free    |    swap    |   clear    |
2613         --------------+------------+------------+------------+
2614 
2615 Here is an example of using the snapshot feature.
2616 
2617  # echo 1 > events/sched/enable
2618  # echo 1 > snapshot
2619  # cat snapshot
2620 # tracer: nop
2621 #
2622 # entries-in-buffer/entries-written: 71/71   #P:8
2623 #
2624 #                              _-----=> irqs-off
2625 #                             / _----=> need-resched
2626 #                            | / _---=> hardirq/softirq
2627 #                            || / _--=> preempt-depth
2628 #                            ||| /     delay
2629 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2630 #              | |       |   ||||       |         |
2631           <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
2632            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
2633 [...]
2634           <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
2635 
2636  # cat trace
2637 # tracer: nop
2638 #
2639 # entries-in-buffer/entries-written: 77/77   #P:8
2640 #
2641 #                              _-----=> irqs-off
2642 #                             / _----=> need-resched
2643 #                            | / _---=> hardirq/softirq
2644 #                            || / _--=> preempt-depth
2645 #                            ||| /     delay
2646 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2647 #              | |       |   ||||       |         |
2648           <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
2649  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
2650 [...]
2651 
2652 
2653 If you try to use this snapshot feature when current tracer is
2654 one of the latency tracers, you will get the following results.
2655 
2656  # echo wakeup > current_tracer
2657  # echo 1 > snapshot
2658 bash: echo: write error: Device or resource busy
2659  # cat snapshot
2660 cat: snapshot: Device or resource busy
2661 
2662 
2663 Instances
2664 ---------
2665 In the debugfs tracing directory is a directory called "instances".
2666 This directory can have new directories created inside of it using
2667 mkdir, and removing directories with rmdir. The directory created
2668 with mkdir in this directory will already contain files and other
2669 directories after it is created.
2670 
2671  # mkdir instances/foo
2672  # ls instances/foo
2673 buffer_size_kb  buffer_total_size_kb  events  free_buffer  per_cpu
2674 set_event  snapshot  trace  trace_clock  trace_marker  trace_options
2675 trace_pipe  tracing_on
2676 
2677 As you can see, the new directory looks similar to the tracing directory
2678 itself. In fact, it is very similar, except that the buffer and
2679 events are agnostic from the main director, or from any other
2680 instances that are created.
2681 
2682 The files in the new directory work just like the files with the
2683 same name in the tracing directory except the buffer that is used
2684 is a separate and new buffer. The files affect that buffer but do not
2685 affect the main buffer with the exception of trace_options. Currently,
2686 the trace_options affect all instances and the top level buffer
2687 the same, but this may change in future releases. That is, options
2688 may become specific to the instance they reside in.
2689 
2690 Notice that none of the function tracer files are there, nor is
2691 current_tracer and available_tracers. This is because the buffers
2692 can currently only have events enabled for them.
2693 
2694  # mkdir instances/foo
2695  # mkdir instances/bar
2696  # mkdir instances/zoot
2697  # echo 100000 > buffer_size_kb
2698  # echo 1000 > instances/foo/buffer_size_kb
2699  # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
2700  # echo function > current_trace
2701  # echo 1 > instances/foo/events/sched/sched_wakeup/enable
2702  # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
2703  # echo 1 > instances/foo/events/sched/sched_switch/enable
2704  # echo 1 > instances/bar/events/irq/enable
2705  # echo 1 > instances/zoot/events/syscalls/enable
2706  # cat trace_pipe
2707 CPU:2 [LOST 11745 EVENTS]
2708             bash-2044  [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
2709             bash-2044  [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
2710             bash-2044  [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
2711             bash-2044  [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
2712             bash-2044  [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
2713             bash-2044  [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
2714             bash-2044  [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
2715             bash-2044  [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
2716             bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
2717             bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
2718             bash-2044  [002] .... 10594.481035: arch_dup_task_struct <-copy_process
2719 [...]
2720 
2721  # cat instances/foo/trace_pipe
2722             bash-1998  [000] d..4   136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
2723             bash-1998  [000] dN.4   136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
2724           <idle>-0     [003] d.h3   136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
2725           <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
2726      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
2727             bash-1998  [000] d..4   136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
2728             bash-1998  [000] dN.4   136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
2729             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
2730      kworker/0:1-59    [000] d..4   136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
2731      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
2732 [...]
2733 
2734  # cat instances/bar/trace_pipe
2735      migration/1-14    [001] d.h3   138.732674: softirq_raise: vec=3 [action=NET_RX]
2736           <idle>-0     [001] dNh3   138.732725: softirq_raise: vec=3 [action=NET_RX]
2737             bash-1998  [000] d.h1   138.733101: softirq_raise: vec=1 [action=TIMER]
2738             bash-1998  [000] d.h1   138.733102: softirq_raise: vec=9 [action=RCU]
2739             bash-1998  [000] ..s2   138.733105: softirq_entry: vec=1 [action=TIMER]
2740             bash-1998  [000] ..s2   138.733106: softirq_exit: vec=1 [action=TIMER]
2741             bash-1998  [000] ..s2   138.733106: softirq_entry: vec=9 [action=RCU]
2742             bash-1998  [000] ..s2   138.733109: softirq_exit: vec=9 [action=RCU]
2743             sshd-1995  [001] d.h1   138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
2744             sshd-1995  [001] d.h1   138.733280: irq_handler_exit: irq=21 ret=unhandled
2745             sshd-1995  [001] d.h1   138.733281: irq_handler_entry: irq=21 name=eth0
2746             sshd-1995  [001] d.h1   138.733283: irq_handler_exit: irq=21 ret=handled
2747 [...]
2748 
2749  # cat instances/zoot/trace
2750 # tracer: nop
2751 #
2752 # entries-in-buffer/entries-written: 18996/18996   #P:4
2753 #
2754 #                              _-----=> irqs-off
2755 #                             / _----=> need-resched
2756 #                            | / _---=> hardirq/softirq
2757 #                            || / _--=> preempt-depth
2758 #                            ||| /     delay
2759 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2760 #              | |       |   ||||       |         |
2761             bash-1998  [000] d...   140.733501: sys_write -> 0x2
2762             bash-1998  [000] d...   140.733504: sys_dup2(oldfd: a, newfd: 1)
2763             bash-1998  [000] d...   140.733506: sys_dup2 -> 0x1
2764             bash-1998  [000] d...   140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
2765             bash-1998  [000] d...   140.733509: sys_fcntl -> 0x1
2766             bash-1998  [000] d...   140.733510: sys_close(fd: a)
2767             bash-1998  [000] d...   140.733510: sys_close -> 0x0
2768             bash-1998  [000] d...   140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
2769             bash-1998  [000] d...   140.733515: sys_rt_sigprocmask -> 0x0
2770             bash-1998  [000] d...   140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
2771             bash-1998  [000] d...   140.733516: sys_rt_sigaction -> 0x0
2772 
2773 You can see that the trace of the top most trace buffer shows only
2774 the function tracing. The foo instance displays wakeups and task
2775 switches.
2776 
2777 To remove the instances, simply delete their directories:
2778 
2779  # rmdir instances/foo
2780  # rmdir instances/bar
2781  # rmdir instances/zoot
2782 
2783 Note, if a process has a trace file open in one of the instance
2784 directories, the rmdir will fail with EBUSY.
2785 
2786 
2787 Stack trace
2788 -----------
2789 Since the kernel has a fixed sized stack, it is important not to
2790 waste it in functions. A kernel developer must be conscience of
2791 what they allocate on the stack. If they add too much, the system
2792 can be in danger of a stack overflow, and corruption will occur,
2793 usually leading to a system panic.
2794 
2795 There are some tools that check this, usually with interrupts
2796 periodically checking usage. But if you can perform a check
2797 at every function call that will become very useful. As ftrace provides
2798 a function tracer, it makes it convenient to check the stack size
2799 at every function call. This is enabled via the stack tracer.
2800 
2801 CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
2802 To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
2803 
2804  # echo 1 > /proc/sys/kernel/stack_tracer_enabled
2805 
2806 You can also enable it from the kernel command line to trace
2807 the stack size of the kernel during boot up, by adding "stacktrace"
2808 to the kernel command line parameter.
2809 
2810 After running it for a few minutes, the output looks like:
2811 
2812  # cat stack_max_size
2813 2928
2814 
2815  # cat stack_trace
2816         Depth    Size   Location    (18 entries)
2817         -----    ----   --------
2818   0)     2928     224   update_sd_lb_stats+0xbc/0x4ac
2819   1)     2704     160   find_busiest_group+0x31/0x1f1
2820   2)     2544     256   load_balance+0xd9/0x662
2821   3)     2288      80   idle_balance+0xbb/0x130
2822   4)     2208     128   __schedule+0x26e/0x5b9
2823   5)     2080      16   schedule+0x64/0x66
2824   6)     2064     128   schedule_timeout+0x34/0xe0
2825   7)     1936     112   wait_for_common+0x97/0xf1
2826   8)     1824      16   wait_for_completion+0x1d/0x1f
2827   9)     1808     128   flush_work+0xfe/0x119
2828  10)     1680      16   tty_flush_to_ldisc+0x1e/0x20
2829  11)     1664      48   input_available_p+0x1d/0x5c
2830  12)     1616      48   n_tty_poll+0x6d/0x134
2831  13)     1568      64   tty_poll+0x64/0x7f
2832  14)     1504     880   do_select+0x31e/0x511
2833  15)      624     400   core_sys_select+0x177/0x216
2834  16)      224      96   sys_select+0x91/0xb9
2835  17)      128     128   system_call_fastpath+0x16/0x1b
2836 
2837 Note, if -mfentry is being used by gcc, functions get traced before
2838 they set up the stack frame. This means that leaf level functions
2839 are not tested by the stack tracer when -mfentry is used.
2840 
2841 Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
2842 
2843 ---------
2844 
2845 More details can be found in the source code, in the
2846 kernel/trace/*.c files.

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