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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   function_profile_enabled:
238 
239         When set it will enable all functions with either the function
240         tracer, or if enabled, the function graph tracer. It will
241         keep a histogram of the number of functions that were called
242         and if run with the function graph tracer, it will also keep
243         track of the time spent in those functions. The histogram
244         content can be displayed in the files:
245 
246         trace_stats/function<cpu> ( function0, function1, etc).
247 
248   trace_stats:
249 
250         A directory that holds different tracing stats.
251 
252   kprobe_events:
253  
254         Enable dynamic trace points. See kprobetrace.txt.
255 
256   kprobe_profile:
257 
258         Dynamic trace points stats. See kprobetrace.txt.
259 
260   max_graph_depth:
261 
262         Used with the function graph tracer. This is the max depth
263         it will trace into a function. Setting this to a value of
264         one will show only the first kernel function that is called
265         from user space.
266 
267   printk_formats:
268 
269         This is for tools that read the raw format files. If an event in
270         the ring buffer references a string (currently only trace_printk()
271         does this), only a pointer to the string is recorded into the buffer
272         and not the string itself. This prevents tools from knowing what
273         that string was. This file displays the string and address for
274         the string allowing tools to map the pointers to what the
275         strings were.
276 
277   saved_cmdlines:
278 
279         Only the pid of the task is recorded in a trace event unless
280         the event specifically saves the task comm as well. Ftrace
281         makes a cache of pid mappings to comms to try to display
282         comms for events. If a pid for a comm is not listed, then
283         "<...>" is displayed in the output.
284 
285   snapshot:
286 
287         This displays the "snapshot" buffer and also lets the user
288         take a snapshot of the current running trace.
289         See the "Snapshot" section below for more details.
290 
291   stack_max_size:
292 
293         When the stack tracer is activated, this will display the
294         maximum stack size it has encountered.
295         See the "Stack Trace" section below.
296 
297   stack_trace:
298 
299         This displays the stack back trace of the largest stack
300         that was encountered when the stack tracer is activated.
301         See the "Stack Trace" section below.
302 
303   stack_trace_filter:
304 
305         This is similar to "set_ftrace_filter" but it limits what
306         functions the stack tracer will check.
307 
308   trace_clock:
309 
310         Whenever an event is recorded into the ring buffer, a
311         "timestamp" is added. This stamp comes from a specified
312         clock. By default, ftrace uses the "local" clock. This
313         clock is very fast and strictly per cpu, but on some
314         systems it may not be monotonic with respect to other
315         CPUs. In other words, the local clocks may not be in sync
316         with local clocks on other CPUs.
317 
318         Usual clocks for tracing:
319 
320           # cat trace_clock
321           [local] global counter x86-tsc
322 
323           local: Default clock, but may not be in sync across CPUs
324 
325           global: This clock is in sync with all CPUs but may
326                   be a bit slower than the local clock.
327 
328           counter: This is not a clock at all, but literally an atomic
329                    counter. It counts up one by one, but is in sync
330                    with all CPUs. This is useful when you need to
331                    know exactly the order events occurred with respect to
332                    each other on different CPUs.
333 
334           uptime: This uses the jiffies counter and the time stamp
335                   is relative to the time since boot up.
336 
337           perf: This makes ftrace use the same clock that perf uses.
338                 Eventually perf will be able to read ftrace buffers
339                 and this will help out in interleaving the data.
340 
341           x86-tsc: Architectures may define their own clocks. For
342                    example, x86 uses its own TSC cycle clock here.
343 
344         To set a clock, simply echo the clock name into this file.
345 
346           echo global > trace_clock
347 
348   trace_marker:
349 
350         This is a very useful file for synchronizing user space
351         with events happening in the kernel. Writing strings into
352         this file will be written into the ftrace buffer.
353 
354         It is useful in applications to open this file at the start
355         of the application and just reference the file descriptor
356         for the file.
357 
358         void trace_write(const char *fmt, ...)
359         {
360                 va_list ap;
361                 char buf[256];
362                 int n;
363 
364                 if (trace_fd < 0)
365                         return;
366 
367                 va_start(ap, fmt);
368                 n = vsnprintf(buf, 256, fmt, ap);
369                 va_end(ap);
370 
371                 write(trace_fd, buf, n);
372         }
373 
374         start:
375 
376                 trace_fd = open("trace_marker", WR_ONLY);
377 
378   uprobe_events:
379  
380         Add dynamic tracepoints in programs.
381         See uprobetracer.txt
382 
383   uprobe_profile:
384 
385         Uprobe statistics. See uprobetrace.txt
386 
387   instances:
388 
389         This is a way to make multiple trace buffers where different
390         events can be recorded in different buffers.
391         See "Instances" section below.
392 
393   events:
394 
395         This is the trace event directory. It holds event tracepoints
396         (also known as static tracepoints) that have been compiled
397         into the kernel. It shows what event tracepoints exist
398         and how they are grouped by system. There are "enable"
399         files at various levels that can enable the tracepoints
400         when a "1" is written to them.
401 
402         See events.txt for more information.
403 
404   per_cpu:
405 
406         This is a directory that contains the trace per_cpu information.
407 
408   per_cpu/cpu0/buffer_size_kb:
409 
410         The ftrace buffer is defined per_cpu. That is, there's a separate
411         buffer for each CPU to allow writes to be done atomically,
412         and free from cache bouncing. These buffers may have different
413         size buffers. This file is similar to the buffer_size_kb
414         file, but it only displays or sets the buffer size for the
415         specific CPU. (here cpu0).
416 
417   per_cpu/cpu0/trace:
418 
419         This is similar to the "trace" file, but it will only display
420         the data specific for the CPU. If written to, it only clears
421         the specific CPU buffer.
422 
423   per_cpu/cpu0/trace_pipe
424 
425         This is similar to the "trace_pipe" file, and is a consuming
426         read, but it will only display (and consume) the data specific
427         for the CPU.
428 
429   per_cpu/cpu0/trace_pipe_raw
430 
431         For tools that can parse the ftrace ring buffer binary format,
432         the trace_pipe_raw file can be used to extract the data
433         from the ring buffer directly. With the use of the splice()
434         system call, the buffer data can be quickly transferred to
435         a file or to the network where a server is collecting the
436         data.
437 
438         Like trace_pipe, this is a consuming reader, where multiple
439         reads will always produce different data.
440 
441   per_cpu/cpu0/snapshot:
442 
443         This is similar to the main "snapshot" file, but will only
444         snapshot the current CPU (if supported). It only displays
445         the content of the snapshot for a given CPU, and if
446         written to, only clears this CPU buffer.
447 
448   per_cpu/cpu0/snapshot_raw:
449 
450         Similar to the trace_pipe_raw, but will read the binary format
451         from the snapshot buffer for the given CPU.
452 
453   per_cpu/cpu0/stats:
454 
455         This displays certain stats about the ring buffer:
456 
457          entries: The number of events that are still in the buffer.
458 
459          overrun: The number of lost events due to overwriting when
460                   the buffer was full.
461 
462          commit overrun: Should always be zero.
463                 This gets set if so many events happened within a nested
464                 event (ring buffer is re-entrant), that it fills the
465                 buffer and starts dropping events.
466 
467          bytes: Bytes actually read (not overwritten).
468 
469          oldest event ts: The oldest timestamp in the buffer
470 
471          now ts: The current timestamp
472 
473          dropped events: Events lost due to overwrite option being off.
474 
475          read events: The number of events read.
476 
477 The Tracers
478 -----------
479 
480 Here is the list of current tracers that may be configured.
481 
482   "function"
483 
484         Function call tracer to trace all kernel functions.
485 
486   "function_graph"
487 
488         Similar to the function tracer except that the
489         function tracer probes the functions on their entry
490         whereas the function graph tracer traces on both entry
491         and exit of the functions. It then provides the ability
492         to draw a graph of function calls similar to C code
493         source.
494 
495   "irqsoff"
496 
497         Traces the areas that disable interrupts and saves
498         the trace with the longest max latency.
499         See tracing_max_latency. When a new max is recorded,
500         it replaces the old trace. It is best to view this
501         trace with the latency-format option enabled.
502 
503   "preemptoff"
504 
505         Similar to irqsoff but traces and records the amount of
506         time for which preemption is disabled.
507 
508   "preemptirqsoff"
509 
510         Similar to irqsoff and preemptoff, but traces and
511         records the largest time for which irqs and/or preemption
512         is disabled.
513 
514   "wakeup"
515 
516         Traces and records the max latency that it takes for
517         the highest priority task to get scheduled after
518         it has been woken up.
519         Traces all tasks as an average developer would expect.
520 
521   "wakeup_rt"
522 
523         Traces and records the max latency that it takes for just
524         RT tasks (as the current "wakeup" does). This is useful
525         for those interested in wake up timings of RT tasks.
526 
527   "nop"
528 
529         This is the "trace nothing" tracer. To remove all
530         tracers from tracing simply echo "nop" into
531         current_tracer.
532 
533 
534 Examples of using the tracer
535 ----------------------------
536 
537 Here are typical examples of using the tracers when controlling
538 them only with the debugfs interface (without using any
539 user-land utilities).
540 
541 Output format:
542 --------------
543 
544 Here is an example of the output format of the file "trace"
545 
546                              --------
547 # tracer: function
548 #
549 # entries-in-buffer/entries-written: 140080/250280   #P:4
550 #
551 #                              _-----=> irqs-off
552 #                             / _----=> need-resched
553 #                            | / _---=> hardirq/softirq
554 #                            || / _--=> preempt-depth
555 #                            ||| /     delay
556 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
557 #              | |       |   ||||       |         |
558             bash-1977  [000] .... 17284.993652: sys_close <-system_call_fastpath
559             bash-1977  [000] .... 17284.993653: __close_fd <-sys_close
560             bash-1977  [000] .... 17284.993653: _raw_spin_lock <-__close_fd
561             sshd-1974  [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
562             bash-1977  [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
563             bash-1977  [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
564             bash-1977  [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
565             bash-1977  [000] .... 17284.993657: filp_close <-__close_fd
566             bash-1977  [000] .... 17284.993657: dnotify_flush <-filp_close
567             sshd-1974  [003] .... 17284.993658: sys_select <-system_call_fastpath
568                              --------
569 
570 A header is printed with the tracer name that is represented by
571 the trace. In this case the tracer is "function". Then it shows the
572 number of events in the buffer as well as the total number of entries
573 that were written. The difference is the number of entries that were
574 lost due to the buffer filling up (250280 - 140080 = 110200 events
575 lost).
576 
577 The header explains the content of the events. Task name "bash", the task
578 PID "1977", the CPU that it was running on "000", the latency format
579 (explained below), the timestamp in <secs>.<usecs> format, the
580 function name that was traced "sys_close" and the parent function that
581 called this function "system_call_fastpath". The timestamp is the time
582 at which the function was entered.
583 
584 Latency trace format
585 --------------------
586 
587 When the latency-format option is enabled or when one of the latency
588 tracers is set, the trace file gives somewhat more information to see
589 why a latency happened. Here is a typical trace.
590 
591 # tracer: irqsoff
592 #
593 # irqsoff latency trace v1.1.5 on 3.8.0-test+
594 # --------------------------------------------------------------------
595 # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
596 #    -----------------
597 #    | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
598 #    -----------------
599 #  => started at: __lock_task_sighand
600 #  => ended at:   _raw_spin_unlock_irqrestore
601 #
602 #
603 #                  _------=> CPU#            
604 #                 / _-----=> irqs-off        
605 #                | / _----=> need-resched    
606 #                || / _---=> hardirq/softirq 
607 #                ||| / _--=> preempt-depth   
608 #                |||| /     delay             
609 #  cmd     pid   ||||| time  |   caller      
610 #     \   /      |||||  \    |   /           
611       ps-6143    2d...    0us!: trace_hardirqs_off <-__lock_task_sighand
612       ps-6143    2d..1  259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
613       ps-6143    2d..1  263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
614       ps-6143    2d..1  306us : <stack trace>
615  => trace_hardirqs_on_caller
616  => trace_hardirqs_on
617  => _raw_spin_unlock_irqrestore
618  => do_task_stat
619  => proc_tgid_stat
620  => proc_single_show
621  => seq_read
622  => vfs_read
623  => sys_read
624  => system_call_fastpath
625 
626 
627 This shows that the current tracer is "irqsoff" tracing the time
628 for which interrupts were disabled. It gives the trace version (which
629 never changes) and the version of the kernel upon which this was executed on
630 (3.10). Then it displays the max latency in microseconds (259 us). The number
631 of trace entries displayed and the total number (both are four: #4/4).
632 VP, KP, SP, and HP are always zero and are reserved for later use.
633 #P is the number of online CPUs (#P:4).
634 
635 The task is the process that was running when the latency
636 occurred. (ps pid: 6143).
637 
638 The start and stop (the functions in which the interrupts were
639 disabled and enabled respectively) that caused the latencies:
640 
641  __lock_task_sighand is where the interrupts were disabled.
642  _raw_spin_unlock_irqrestore is where they were enabled again.
643 
644 The next lines after the header are the trace itself. The header
645 explains which is which.
646 
647   cmd: The name of the process in the trace.
648 
649   pid: The PID of that process.
650 
651   CPU#: The CPU which the process was running on.
652 
653   irqs-off: 'd' interrupts are disabled. '.' otherwise.
654             Note: If the architecture does not support a way to
655                   read the irq flags variable, an 'X' will always
656                   be printed here.
657 
658   need-resched:
659         'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
660         'n' only TIF_NEED_RESCHED is set,
661         'p' only PREEMPT_NEED_RESCHED is set,
662         '.' otherwise.
663 
664   hardirq/softirq:
665         'H' - hard irq occurred inside a softirq.
666         'h' - hard irq is running
667         's' - soft irq is running
668         '.' - normal context.
669 
670   preempt-depth: The level of preempt_disabled
671 
672 The above is mostly meaningful for kernel developers.
673 
674   time: When the latency-format option is enabled, the trace file
675         output includes a timestamp relative to the start of the
676         trace. This differs from the output when latency-format
677         is disabled, which includes an absolute timestamp.
678 
679   delay: This is just to help catch your eye a bit better. And
680          needs to be fixed to be only relative to the same CPU.
681          The marks are determined by the difference between this
682          current trace and the next trace.
683           '!' - greater than preempt_mark_thresh (default 100)
684           '+' - greater than 1 microsecond
685           ' ' - less than or equal to 1 microsecond.
686 
687   The rest is the same as the 'trace' file.
688 
689   Note, the latency tracers will usually end with a back trace
690   to easily find where the latency occurred.
691 
692 trace_options
693 -------------
694 
695 The trace_options file (or the options directory) is used to control
696 what gets printed in the trace output, or manipulate the tracers.
697 To see what is available, simply cat the file:
698 
699   cat trace_options
700 print-parent
701 nosym-offset
702 nosym-addr
703 noverbose
704 noraw
705 nohex
706 nobin
707 noblock
708 nostacktrace
709 trace_printk
710 noftrace_preempt
711 nobranch
712 annotate
713 nouserstacktrace
714 nosym-userobj
715 noprintk-msg-only
716 context-info
717 latency-format
718 sleep-time
719 graph-time
720 record-cmd
721 overwrite
722 nodisable_on_free
723 irq-info
724 markers
725 function-trace
726 
727 To disable one of the options, echo in the option prepended with
728 "no".
729 
730   echo noprint-parent > trace_options
731 
732 To enable an option, leave off the "no".
733 
734   echo sym-offset > trace_options
735 
736 Here are the available options:
737 
738   print-parent - On function traces, display the calling (parent)
739                  function as well as the function being traced.
740 
741   print-parent:
742    bash-4000  [01]  1477.606694: simple_strtoul <-kstrtoul
743 
744   noprint-parent:
745    bash-4000  [01]  1477.606694: simple_strtoul
746 
747 
748   sym-offset - Display not only the function name, but also the
749                offset in the function. For example, instead of
750                seeing just "ktime_get", you will see
751                "ktime_get+0xb/0x20".
752 
753   sym-offset:
754    bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0
755 
756   sym-addr - this will also display the function address as well
757              as the function name.
758 
759   sym-addr:
760    bash-4000  [01]  1477.606694: simple_strtoul <c0339346>
761 
762   verbose - This deals with the trace file when the
763             latency-format option is enabled.
764 
765     bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
766     (+0.000ms): simple_strtoul (kstrtoul)
767 
768   raw - This will display raw numbers. This option is best for
769         use with user applications that can translate the raw
770         numbers better than having it done in the kernel.
771 
772   hex - Similar to raw, but the numbers will be in a hexadecimal
773         format.
774 
775   bin - This will print out the formats in raw binary.
776 
777   block - When set, reading trace_pipe will not block when polled.
778 
779   stacktrace - This is one of the options that changes the trace
780                itself. When a trace is recorded, so is the stack
781                of functions. This allows for back traces of
782                trace sites.
783 
784   trace_printk - Can disable trace_printk() from writing into the buffer.
785 
786   branch - Enable branch tracing with the tracer.
787 
788   annotate - It is sometimes confusing when the CPU buffers are full
789              and one CPU buffer had a lot of events recently, thus
790              a shorter time frame, were another CPU may have only had
791              a few events, which lets it have older events. When
792              the trace is reported, it shows the oldest events first,
793              and it may look like only one CPU ran (the one with the
794              oldest events). When the annotate option is set, it will
795              display when a new CPU buffer started:
796 
797           <idle>-0     [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
798           <idle>-0     [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
799           <idle>-0     [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
800 ##### CPU 2 buffer started ####
801           <idle>-0     [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
802           <idle>-0     [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
803           <idle>-0     [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
804 
805   userstacktrace - This option changes the trace. It records a
806                    stacktrace of the current userspace thread.
807 
808   sym-userobj - when user stacktrace are enabled, look up which
809                 object the address belongs to, and print a
810                 relative address. This is especially useful when
811                 ASLR is on, otherwise you don't get a chance to
812                 resolve the address to object/file/line after
813                 the app is no longer running
814 
815                 The lookup is performed when you read
816                 trace,trace_pipe. Example:
817 
818                 a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
819 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
820 
821 
822   printk-msg-only - When set, trace_printk()s will only show the format
823                     and not their parameters (if trace_bprintk() or
824                     trace_bputs() was used to save the trace_printk()).
825 
826   context-info - Show only the event data. Hides the comm, PID,
827                  timestamp, CPU, and other useful data.
828 
829   latency-format - This option changes the trace. When
830                    it is enabled, the trace displays
831                    additional information about the
832                    latencies, as described in "Latency
833                    trace format".
834 
835   sleep-time - When running function graph tracer, to include
836                the time a task schedules out in its function.
837                When enabled, it will account time the task has been
838                scheduled out as part of the function call.
839 
840   graph-time - When running function graph tracer, to include the
841                time to call nested functions. When this is not set,
842                the time reported for the function will only include
843                the time the function itself executed for, not the time
844                for functions that it called.
845 
846   record-cmd - When any event or tracer is enabled, a hook is enabled
847                in the sched_switch trace point to fill comm cache
848                with mapped pids and comms. But this may cause some
849                overhead, and if you only care about pids, and not the
850                name of the task, disabling this option can lower the
851                impact of tracing.
852 
853   overwrite - This controls what happens when the trace buffer is
854               full. If "1" (default), the oldest events are
855               discarded and overwritten. If "0", then the newest
856               events are discarded.
857                 (see per_cpu/cpu0/stats for overrun and dropped)
858 
859   disable_on_free - When the free_buffer is closed, tracing will
860                     stop (tracing_on set to 0).
861 
862   irq-info - Shows the interrupt, preempt count, need resched data.
863              When disabled, the trace looks like:
864 
865 # tracer: function
866 #
867 # entries-in-buffer/entries-written: 144405/9452052   #P:4
868 #
869 #           TASK-PID   CPU#      TIMESTAMP  FUNCTION
870 #              | |       |          |         |
871           <idle>-0     [002]  23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
872           <idle>-0     [002]  23636.756054: activate_task <-ttwu_do_activate.constprop.89
873           <idle>-0     [002]  23636.756055: enqueue_task <-activate_task
874 
875 
876   markers - When set, the trace_marker is writable (only by root).
877             When disabled, the trace_marker will error with EINVAL
878             on write.
879 
880 
881   function-trace - The latency tracers will enable function tracing
882             if this option is enabled (default it is). When
883             it is disabled, the latency tracers do not trace
884             functions. This keeps the overhead of the tracer down
885             when performing latency tests.
886 
887  Note: Some tracers have their own options. They only appear
888        when the tracer is active.
889 
890 
891 
892 irqsoff
893 -------
894 
895 When interrupts are disabled, the CPU can not react to any other
896 external event (besides NMIs and SMIs). This prevents the timer
897 interrupt from triggering or the mouse interrupt from letting
898 the kernel know of a new mouse event. The result is a latency
899 with the reaction time.
900 
901 The irqsoff tracer tracks the time for which interrupts are
902 disabled. When a new maximum latency is hit, the tracer saves
903 the trace leading up to that latency point so that every time a
904 new maximum is reached, the old saved trace is discarded and the
905 new trace is saved.
906 
907 To reset the maximum, echo 0 into tracing_max_latency. Here is
908 an example:
909 
910  # echo 0 > options/function-trace
911  # echo irqsoff > current_tracer
912  # echo 1 > tracing_on
913  # echo 0 > tracing_max_latency
914  # ls -ltr
915  [...]
916  # echo 0 > tracing_on
917  # cat trace
918 # tracer: irqsoff
919 #
920 # irqsoff latency trace v1.1.5 on 3.8.0-test+
921 # --------------------------------------------------------------------
922 # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
923 #    -----------------
924 #    | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
925 #    -----------------
926 #  => started at: run_timer_softirq
927 #  => ended at:   run_timer_softirq
928 #
929 #
930 #                  _------=> CPU#            
931 #                 / _-----=> irqs-off        
932 #                | / _----=> need-resched    
933 #                || / _---=> hardirq/softirq 
934 #                ||| / _--=> preempt-depth   
935 #                |||| /     delay             
936 #  cmd     pid   ||||| time  |   caller      
937 #     \   /      |||||  \    |   /           
938   <idle>-0       0d.s2    0us+: _raw_spin_lock_irq <-run_timer_softirq
939   <idle>-0       0dNs3   17us : _raw_spin_unlock_irq <-run_timer_softirq
940   <idle>-0       0dNs3   17us+: trace_hardirqs_on <-run_timer_softirq
941   <idle>-0       0dNs3   25us : <stack trace>
942  => _raw_spin_unlock_irq
943  => run_timer_softirq
944  => __do_softirq
945  => call_softirq
946  => do_softirq
947  => irq_exit
948  => smp_apic_timer_interrupt
949  => apic_timer_interrupt
950  => rcu_idle_exit
951  => cpu_idle
952  => rest_init
953  => start_kernel
954  => x86_64_start_reservations
955  => x86_64_start_kernel
956 
957 Here we see that that we had a latency of 16 microseconds (which is
958 very good). The _raw_spin_lock_irq in run_timer_softirq disabled
959 interrupts. The difference between the 16 and the displayed
960 timestamp 25us occurred because the clock was incremented
961 between the time of recording the max latency and the time of
962 recording the function that had that latency.
963 
964 Note the above example had function-trace not set. If we set
965 function-trace, we get a much larger output:
966 
967  with echo 1 > options/function-trace
968 
969 # tracer: irqsoff
970 #
971 # irqsoff latency trace v1.1.5 on 3.8.0-test+
972 # --------------------------------------------------------------------
973 # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
974 #    -----------------
975 #    | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
976 #    -----------------
977 #  => started at: ata_scsi_queuecmd
978 #  => ended at:   ata_scsi_queuecmd
979 #
980 #
981 #                  _------=> CPU#            
982 #                 / _-----=> irqs-off        
983 #                | / _----=> need-resched    
984 #                || / _---=> hardirq/softirq 
985 #                ||| / _--=> preempt-depth   
986 #                |||| /     delay             
987 #  cmd     pid   ||||| time  |   caller      
988 #     \   /      |||||  \    |   /           
989     bash-2042    3d...    0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
990     bash-2042    3d...    0us : add_preempt_count <-_raw_spin_lock_irqsave
991     bash-2042    3d..1    1us : ata_scsi_find_dev <-ata_scsi_queuecmd
992     bash-2042    3d..1    1us : __ata_scsi_find_dev <-ata_scsi_find_dev
993     bash-2042    3d..1    2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
994     bash-2042    3d..1    2us : ata_qc_new_init <-__ata_scsi_queuecmd
995     bash-2042    3d..1    3us : ata_sg_init <-__ata_scsi_queuecmd
996     bash-2042    3d..1    4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
997     bash-2042    3d..1    4us : ata_build_rw_tf <-ata_scsi_rw_xlat
998 [...]
999     bash-2042    3d..1   67us : delay_tsc <-__delay
1000     bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
1001     bash-2042    3d..2   67us : sub_preempt_count <-delay_tsc
1002     bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
1003     bash-2042    3d..2   68us : sub_preempt_count <-delay_tsc
1004     bash-2042    3d..1   68us+: ata_bmdma_start <-ata_bmdma_qc_issue
1005     bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1006     bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1007     bash-2042    3d..1   72us+: trace_hardirqs_on <-ata_scsi_queuecmd
1008     bash-2042    3d..1  120us : <stack trace>
1009  => _raw_spin_unlock_irqrestore
1010  => ata_scsi_queuecmd
1011  => scsi_dispatch_cmd
1012  => scsi_request_fn
1013  => __blk_run_queue_uncond
1014  => __blk_run_queue
1015  => blk_queue_bio
1016  => generic_make_request
1017  => submit_bio
1018  => submit_bh
1019  => __ext3_get_inode_loc
1020  => ext3_iget
1021  => ext3_lookup
1022  => lookup_real
1023  => __lookup_hash
1024  => walk_component
1025  => lookup_last
1026  => path_lookupat
1027  => filename_lookup
1028  => user_path_at_empty
1029  => user_path_at
1030  => vfs_fstatat
1031  => vfs_stat
1032  => sys_newstat
1033  => system_call_fastpath
1034 
1035 
1036 Here we traced a 71 microsecond latency. But we also see all the
1037 functions that were called during that time. Note that by
1038 enabling function tracing, we incur an added overhead. This
1039 overhead may extend the latency times. But nevertheless, this
1040 trace has provided some very helpful debugging information.
1041 
1042 
1043 preemptoff
1044 ----------
1045 
1046 When preemption is disabled, we may be able to receive
1047 interrupts but the task cannot be preempted and a higher
1048 priority task must wait for preemption to be enabled again
1049 before it can preempt a lower priority task.
1050 
1051 The preemptoff tracer traces the places that disable preemption.
1052 Like the irqsoff tracer, it records the maximum latency for
1053 which preemption was disabled. The control of preemptoff tracer
1054 is much like the irqsoff tracer.
1055 
1056  # echo 0 > options/function-trace
1057  # echo preemptoff > current_tracer
1058  # echo 1 > tracing_on
1059  # echo 0 > tracing_max_latency
1060  # ls -ltr
1061  [...]
1062  # echo 0 > tracing_on
1063  # cat trace
1064 # tracer: preemptoff
1065 #
1066 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1067 # --------------------------------------------------------------------
1068 # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1069 #    -----------------
1070 #    | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
1071 #    -----------------
1072 #  => started at: do_IRQ
1073 #  => ended at:   do_IRQ
1074 #
1075 #
1076 #                  _------=> CPU#            
1077 #                 / _-----=> irqs-off        
1078 #                | / _----=> need-resched    
1079 #                || / _---=> hardirq/softirq 
1080 #                ||| / _--=> preempt-depth   
1081 #                |||| /     delay             
1082 #  cmd     pid   ||||| time  |   caller      
1083 #     \   /      |||||  \    |   /           
1084     sshd-1991    1d.h.    0us+: irq_enter <-do_IRQ
1085     sshd-1991    1d..1   46us : irq_exit <-do_IRQ
1086     sshd-1991    1d..1   47us+: trace_preempt_on <-do_IRQ
1087     sshd-1991    1d..1   52us : <stack trace>
1088  => sub_preempt_count
1089  => irq_exit
1090  => do_IRQ
1091  => ret_from_intr
1092 
1093 
1094 This has some more changes. Preemption was disabled when an
1095 interrupt came in (notice the 'h'), and was enabled on exit.
1096 But we also see that interrupts have been disabled when entering
1097 the preempt off section and leaving it (the 'd'). We do not know if
1098 interrupts were enabled in the mean time or shortly after this
1099 was over.
1100 
1101 # tracer: preemptoff
1102 #
1103 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1104 # --------------------------------------------------------------------
1105 # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1106 #    -----------------
1107 #    | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
1108 #    -----------------
1109 #  => started at: wake_up_new_task
1110 #  => ended at:   task_rq_unlock
1111 #
1112 #
1113 #                  _------=> CPU#            
1114 #                 / _-----=> irqs-off        
1115 #                | / _----=> need-resched    
1116 #                || / _---=> hardirq/softirq 
1117 #                ||| / _--=> preempt-depth   
1118 #                |||| /     delay             
1119 #  cmd     pid   ||||| time  |   caller      
1120 #     \   /      |||||  \    |   /           
1121     bash-1994    1d..1    0us : _raw_spin_lock_irqsave <-wake_up_new_task
1122     bash-1994    1d..1    0us : select_task_rq_fair <-select_task_rq
1123     bash-1994    1d..1    1us : __rcu_read_lock <-select_task_rq_fair
1124     bash-1994    1d..1    1us : source_load <-select_task_rq_fair
1125     bash-1994    1d..1    1us : source_load <-select_task_rq_fair
1126 [...]
1127     bash-1994    1d..1   12us : irq_enter <-smp_apic_timer_interrupt
1128     bash-1994    1d..1   12us : rcu_irq_enter <-irq_enter
1129     bash-1994    1d..1   13us : add_preempt_count <-irq_enter
1130     bash-1994    1d.h1   13us : exit_idle <-smp_apic_timer_interrupt
1131     bash-1994    1d.h1   13us : hrtimer_interrupt <-smp_apic_timer_interrupt
1132     bash-1994    1d.h1   13us : _raw_spin_lock <-hrtimer_interrupt
1133     bash-1994    1d.h1   14us : add_preempt_count <-_raw_spin_lock
1134     bash-1994    1d.h2   14us : ktime_get_update_offsets <-hrtimer_interrupt
1135 [...]
1136     bash-1994    1d.h1   35us : lapic_next_event <-clockevents_program_event
1137     bash-1994    1d.h1   35us : irq_exit <-smp_apic_timer_interrupt
1138     bash-1994    1d.h1   36us : sub_preempt_count <-irq_exit
1139     bash-1994    1d..2   36us : do_softirq <-irq_exit
1140     bash-1994    1d..2   36us : __do_softirq <-call_softirq
1141     bash-1994    1d..2   36us : __local_bh_disable <-__do_softirq
1142     bash-1994    1d.s2   37us : add_preempt_count <-_raw_spin_lock_irq
1143     bash-1994    1d.s3   38us : _raw_spin_unlock <-run_timer_softirq
1144     bash-1994    1d.s3   39us : sub_preempt_count <-_raw_spin_unlock
1145     bash-1994    1d.s2   39us : call_timer_fn <-run_timer_softirq
1146 [...]
1147     bash-1994    1dNs2   81us : cpu_needs_another_gp <-rcu_process_callbacks
1148     bash-1994    1dNs2   82us : __local_bh_enable <-__do_softirq
1149     bash-1994    1dNs2   82us : sub_preempt_count <-__local_bh_enable
1150     bash-1994    1dN.2   82us : idle_cpu <-irq_exit
1151     bash-1994    1dN.2   83us : rcu_irq_exit <-irq_exit
1152     bash-1994    1dN.2   83us : sub_preempt_count <-irq_exit
1153     bash-1994    1.N.1   84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
1154     bash-1994    1.N.1   84us+: trace_preempt_on <-task_rq_unlock
1155     bash-1994    1.N.1  104us : <stack trace>
1156  => sub_preempt_count
1157  => _raw_spin_unlock_irqrestore
1158  => task_rq_unlock
1159  => wake_up_new_task
1160  => do_fork
1161  => sys_clone
1162  => stub_clone
1163 
1164 
1165 The above is an example of the preemptoff trace with
1166 function-trace set. Here we see that interrupts were not disabled
1167 the entire time. The irq_enter code lets us know that we entered
1168 an interrupt 'h'. Before that, the functions being traced still
1169 show that it is not in an interrupt, but we can see from the
1170 functions themselves that this is not the case.
1171 
1172 preemptirqsoff
1173 --------------
1174 
1175 Knowing the locations that have interrupts disabled or
1176 preemption disabled for the longest times is helpful. But
1177 sometimes we would like to know when either preemption and/or
1178 interrupts are disabled.
1179 
1180 Consider the following code:
1181 
1182     local_irq_disable();
1183     call_function_with_irqs_off();
1184     preempt_disable();
1185     call_function_with_irqs_and_preemption_off();
1186     local_irq_enable();
1187     call_function_with_preemption_off();
1188     preempt_enable();
1189 
1190 The irqsoff tracer will record the total length of
1191 call_function_with_irqs_off() and
1192 call_function_with_irqs_and_preemption_off().
1193 
1194 The preemptoff tracer will record the total length of
1195 call_function_with_irqs_and_preemption_off() and
1196 call_function_with_preemption_off().
1197 
1198 But neither will trace the time that interrupts and/or
1199 preemption is disabled. This total time is the time that we can
1200 not schedule. To record this time, use the preemptirqsoff
1201 tracer.
1202 
1203 Again, using this trace is much like the irqsoff and preemptoff
1204 tracers.
1205 
1206  # echo 0 > options/function-trace
1207  # echo preemptirqsoff > current_tracer
1208  # echo 1 > tracing_on
1209  # echo 0 > tracing_max_latency
1210  # ls -ltr
1211  [...]
1212  # echo 0 > tracing_on
1213  # cat trace
1214 # tracer: preemptirqsoff
1215 #
1216 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1217 # --------------------------------------------------------------------
1218 # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1219 #    -----------------
1220 #    | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
1221 #    -----------------
1222 #  => started at: ata_scsi_queuecmd
1223 #  => ended at:   ata_scsi_queuecmd
1224 #
1225 #
1226 #                  _------=> CPU#            
1227 #                 / _-----=> irqs-off        
1228 #                | / _----=> need-resched    
1229 #                || / _---=> hardirq/softirq 
1230 #                ||| / _--=> preempt-depth   
1231 #                |||| /     delay             
1232 #  cmd     pid   ||||| time  |   caller      
1233 #     \   /      |||||  \    |   /           
1234       ls-2230    3d...    0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1235       ls-2230    3...1  100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1236       ls-2230    3...1  101us+: trace_preempt_on <-ata_scsi_queuecmd
1237       ls-2230    3...1  111us : <stack trace>
1238  => sub_preempt_count
1239  => _raw_spin_unlock_irqrestore
1240  => ata_scsi_queuecmd
1241  => scsi_dispatch_cmd
1242  => scsi_request_fn
1243  => __blk_run_queue_uncond
1244  => __blk_run_queue
1245  => blk_queue_bio
1246  => generic_make_request
1247  => submit_bio
1248  => submit_bh
1249  => ext3_bread
1250  => ext3_dir_bread
1251  => htree_dirblock_to_tree
1252  => ext3_htree_fill_tree
1253  => ext3_readdir
1254  => vfs_readdir
1255  => sys_getdents
1256  => system_call_fastpath
1257 
1258 
1259 The trace_hardirqs_off_thunk is called from assembly on x86 when
1260 interrupts are disabled in the assembly code. Without the
1261 function tracing, we do not know if interrupts were enabled
1262 within the preemption points. We do see that it started with
1263 preemption enabled.
1264 
1265 Here is a trace with function-trace set:
1266 
1267 # tracer: preemptirqsoff
1268 #
1269 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1270 # --------------------------------------------------------------------
1271 # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1272 #    -----------------
1273 #    | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
1274 #    -----------------
1275 #  => started at: schedule
1276 #  => ended at:   mutex_unlock
1277 #
1278 #
1279 #                  _------=> CPU#            
1280 #                 / _-----=> irqs-off        
1281 #                | / _----=> need-resched    
1282 #                || / _---=> hardirq/softirq 
1283 #                ||| / _--=> preempt-depth   
1284 #                |||| /     delay             
1285 #  cmd     pid   ||||| time  |   caller      
1286 #     \   /      |||||  \    |   /           
1287 kworker/-59      3...1    0us : __schedule <-schedule
1288 kworker/-59      3d..1    0us : rcu_preempt_qs <-rcu_note_context_switch
1289 kworker/-59      3d..1    1us : add_preempt_count <-_raw_spin_lock_irq
1290 kworker/-59      3d..2    1us : deactivate_task <-__schedule
1291 kworker/-59      3d..2    1us : dequeue_task <-deactivate_task
1292 kworker/-59      3d..2    2us : update_rq_clock <-dequeue_task
1293 kworker/-59      3d..2    2us : dequeue_task_fair <-dequeue_task
1294 kworker/-59      3d..2    2us : update_curr <-dequeue_task_fair
1295 kworker/-59      3d..2    2us : update_min_vruntime <-update_curr
1296 kworker/-59      3d..2    3us : cpuacct_charge <-update_curr
1297 kworker/-59      3d..2    3us : __rcu_read_lock <-cpuacct_charge
1298 kworker/-59      3d..2    3us : __rcu_read_unlock <-cpuacct_charge
1299 kworker/-59      3d..2    3us : update_cfs_rq_blocked_load <-dequeue_task_fair
1300 kworker/-59      3d..2    4us : clear_buddies <-dequeue_task_fair
1301 kworker/-59      3d..2    4us : account_entity_dequeue <-dequeue_task_fair
1302 kworker/-59      3d..2    4us : update_min_vruntime <-dequeue_task_fair
1303 kworker/-59      3d..2    4us : update_cfs_shares <-dequeue_task_fair
1304 kworker/-59      3d..2    5us : hrtick_update <-dequeue_task_fair
1305 kworker/-59      3d..2    5us : wq_worker_sleeping <-__schedule
1306 kworker/-59      3d..2    5us : kthread_data <-wq_worker_sleeping
1307 kworker/-59      3d..2    5us : put_prev_task_fair <-__schedule
1308 kworker/-59      3d..2    6us : pick_next_task_fair <-pick_next_task
1309 kworker/-59      3d..2    6us : clear_buddies <-pick_next_task_fair
1310 kworker/-59      3d..2    6us : set_next_entity <-pick_next_task_fair
1311 kworker/-59      3d..2    6us : update_stats_wait_end <-set_next_entity
1312       ls-2269    3d..2    7us : finish_task_switch <-__schedule
1313       ls-2269    3d..2    7us : _raw_spin_unlock_irq <-finish_task_switch
1314       ls-2269    3d..2    8us : do_IRQ <-ret_from_intr
1315       ls-2269    3d..2    8us : irq_enter <-do_IRQ
1316       ls-2269    3d..2    8us : rcu_irq_enter <-irq_enter
1317       ls-2269    3d..2    9us : add_preempt_count <-irq_enter
1318       ls-2269    3d.h2    9us : exit_idle <-do_IRQ
1319 [...]
1320       ls-2269    3d.h3   20us : sub_preempt_count <-_raw_spin_unlock
1321       ls-2269    3d.h2   20us : irq_exit <-do_IRQ
1322       ls-2269    3d.h2   21us : sub_preempt_count <-irq_exit
1323       ls-2269    3d..3   21us : do_softirq <-irq_exit
1324       ls-2269    3d..3   21us : __do_softirq <-call_softirq
1325       ls-2269    3d..3   21us+: __local_bh_disable <-__do_softirq
1326       ls-2269    3d.s4   29us : sub_preempt_count <-_local_bh_enable_ip
1327       ls-2269    3d.s5   29us : sub_preempt_count <-_local_bh_enable_ip
1328       ls-2269    3d.s5   31us : do_IRQ <-ret_from_intr
1329       ls-2269    3d.s5   31us : irq_enter <-do_IRQ
1330       ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
1331 [...]
1332       ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
1333       ls-2269    3d.s5   32us : add_preempt_count <-irq_enter
1334       ls-2269    3d.H5   32us : exit_idle <-do_IRQ
1335       ls-2269    3d.H5   32us : handle_irq <-do_IRQ
1336       ls-2269    3d.H5   32us : irq_to_desc <-handle_irq
1337       ls-2269    3d.H5   33us : handle_fasteoi_irq <-handle_irq
1338 [...]
1339       ls-2269    3d.s5  158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
1340       ls-2269    3d.s3  158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
1341       ls-2269    3d.s3  159us : __local_bh_enable <-__do_softirq
1342       ls-2269    3d.s3  159us : sub_preempt_count <-__local_bh_enable
1343       ls-2269    3d..3  159us : idle_cpu <-irq_exit
1344       ls-2269    3d..3  159us : rcu_irq_exit <-irq_exit
1345       ls-2269    3d..3  160us : sub_preempt_count <-irq_exit
1346       ls-2269    3d...  161us : __mutex_unlock_slowpath <-mutex_unlock
1347       ls-2269    3d...  162us+: trace_hardirqs_on <-mutex_unlock
1348       ls-2269    3d...  186us : <stack trace>
1349  => __mutex_unlock_slowpath
1350  => mutex_unlock
1351  => process_output
1352  => n_tty_write
1353  => tty_write
1354  => vfs_write
1355  => sys_write
1356  => system_call_fastpath
1357 
1358 This is an interesting trace. It started with kworker running and
1359 scheduling out and ls taking over. But as soon as ls released the
1360 rq lock and enabled interrupts (but not preemption) an interrupt
1361 triggered. When the interrupt finished, it started running softirqs.
1362 But while the softirq was running, another interrupt triggered.
1363 When an interrupt is running inside a softirq, the annotation is 'H'.
1364 
1365 
1366 wakeup
1367 ------
1368 
1369 One common case that people are interested in tracing is the
1370 time it takes for a task that is woken to actually wake up.
1371 Now for non Real-Time tasks, this can be arbitrary. But tracing
1372 it none the less can be interesting. 
1373 
1374 Without function tracing:
1375 
1376  # echo 0 > options/function-trace
1377  # echo wakeup > current_tracer
1378  # echo 1 > tracing_on
1379  # echo 0 > tracing_max_latency
1380  # chrt -f 5 sleep 1
1381  # echo 0 > tracing_on
1382  # cat trace
1383 # tracer: wakeup
1384 #
1385 # wakeup latency trace v1.1.5 on 3.8.0-test+
1386 # --------------------------------------------------------------------
1387 # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1388 #    -----------------
1389 #    | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
1390 #    -----------------
1391 #
1392 #                  _------=> CPU#            
1393 #                 / _-----=> irqs-off        
1394 #                | / _----=> need-resched    
1395 #                || / _---=> hardirq/softirq 
1396 #                ||| / _--=> preempt-depth   
1397 #                |||| /     delay             
1398 #  cmd     pid   ||||| time  |   caller      
1399 #     \   /      |||||  \    |   /           
1400   <idle>-0       3dNs7    0us :      0:120:R   + [003]   312:100:R kworker/3:1H
1401   <idle>-0       3dNs7    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1402   <idle>-0       3d..3   15us : __schedule <-schedule
1403   <idle>-0       3d..3   15us :      0:120:R ==> [003]   312:100:R kworker/3:1H
1404 
1405 The tracer only traces the highest priority task in the system
1406 to avoid tracing the normal circumstances. Here we see that
1407 the kworker with a nice priority of -20 (not very nice), took
1408 just 15 microseconds from the time it woke up, to the time it
1409 ran.
1410 
1411 Non Real-Time tasks are not that interesting. A more interesting
1412 trace is to concentrate only on Real-Time tasks.
1413 
1414 wakeup_rt
1415 ---------
1416 
1417 In a Real-Time environment it is very important to know the
1418 wakeup time it takes for the highest priority task that is woken
1419 up to the time that it executes. This is also known as "schedule
1420 latency". I stress the point that this is about RT tasks. It is
1421 also important to know the scheduling latency of non-RT tasks,
1422 but the average schedule latency is better for non-RT tasks.
1423 Tools like LatencyTop are more appropriate for such
1424 measurements.
1425 
1426 Real-Time environments are interested in the worst case latency.
1427 That is the longest latency it takes for something to happen,
1428 and not the average. We can have a very fast scheduler that may
1429 only have a large latency once in a while, but that would not
1430 work well with Real-Time tasks.  The wakeup_rt tracer was designed
1431 to record the worst case wakeups of RT tasks. Non-RT tasks are
1432 not recorded because the tracer only records one worst case and
1433 tracing non-RT tasks that are unpredictable will overwrite the
1434 worst case latency of RT tasks (just run the normal wakeup
1435 tracer for a while to see that effect).
1436 
1437 Since this tracer only deals with RT tasks, we will run this
1438 slightly differently than we did with the previous tracers.
1439 Instead of performing an 'ls', we will run 'sleep 1' under
1440 'chrt' which changes the priority of the task.
1441 
1442  # echo 0 > options/function-trace
1443  # echo wakeup_rt > current_tracer
1444  # echo 1 > tracing_on
1445  # echo 0 > tracing_max_latency
1446  # chrt -f 5 sleep 1
1447  # echo 0 > tracing_on
1448  # cat trace
1449 # tracer: wakeup
1450 #
1451 # tracer: wakeup_rt
1452 #
1453 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1454 # --------------------------------------------------------------------
1455 # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1456 #    -----------------
1457 #    | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
1458 #    -----------------
1459 #
1460 #                  _------=> CPU#            
1461 #                 / _-----=> irqs-off        
1462 #                | / _----=> need-resched    
1463 #                || / _---=> hardirq/softirq 
1464 #                ||| / _--=> preempt-depth   
1465 #                |||| /     delay             
1466 #  cmd     pid   ||||| time  |   caller      
1467 #     \   /      |||||  \    |   /           
1468   <idle>-0       3d.h4    0us :      0:120:R   + [003]  2389: 94:R sleep
1469   <idle>-0       3d.h4    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1470   <idle>-0       3d..3    5us : __schedule <-schedule
1471   <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
1472 
1473 
1474 Running this on an idle system, we see that it only took 5 microseconds
1475 to perform the task switch.  Note, since the trace point in the schedule
1476 is before the actual "switch", we stop the tracing when the recorded task
1477 is about to schedule in. This may change if we add a new marker at the
1478 end of the scheduler.
1479 
1480 Notice that the recorded task is 'sleep' with the PID of 2389
1481 and it has an rt_prio of 5. This priority is user-space priority
1482 and not the internal kernel priority. The policy is 1 for
1483 SCHED_FIFO and 2 for SCHED_RR.
1484 
1485 Note, that the trace data shows the internal priority (99 - rtprio).
1486 
1487   <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
1488 
1489 The 0:120:R means idle was running with a nice priority of 0 (120 - 20)
1490 and in the running state 'R'. The sleep task was scheduled in with
1491 2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
1492 and it too is in the running state.
1493 
1494 Doing the same with chrt -r 5 and function-trace set.
1495 
1496   echo 1 > options/function-trace
1497 
1498 # tracer: wakeup_rt
1499 #
1500 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1501 # --------------------------------------------------------------------
1502 # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1503 #    -----------------
1504 #    | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
1505 #    -----------------
1506 #
1507 #                  _------=> CPU#            
1508 #                 / _-----=> irqs-off        
1509 #                | / _----=> need-resched    
1510 #                || / _---=> hardirq/softirq 
1511 #                ||| / _--=> preempt-depth   
1512 #                |||| /     delay             
1513 #  cmd     pid   ||||| time  |   caller      
1514 #     \   /      |||||  \    |   /           
1515   <idle>-0       3d.h4    1us+:      0:120:R   + [003]  2448: 94:R sleep
1516   <idle>-0       3d.h4    2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
1517   <idle>-0       3d.h3    3us : check_preempt_curr <-ttwu_do_wakeup
1518   <idle>-0       3d.h3    3us : resched_task <-check_preempt_curr
1519   <idle>-0       3dNh3    4us : task_woken_rt <-ttwu_do_wakeup
1520   <idle>-0       3dNh3    4us : _raw_spin_unlock <-try_to_wake_up
1521   <idle>-0       3dNh3    4us : sub_preempt_count <-_raw_spin_unlock
1522   <idle>-0       3dNh2    5us : ttwu_stat <-try_to_wake_up
1523   <idle>-0       3dNh2    5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
1524   <idle>-0       3dNh2    6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1525   <idle>-0       3dNh1    6us : _raw_spin_lock <-__run_hrtimer
1526   <idle>-0       3dNh1    6us : add_preempt_count <-_raw_spin_lock
1527   <idle>-0       3dNh2    7us : _raw_spin_unlock <-hrtimer_interrupt
1528   <idle>-0       3dNh2    7us : sub_preempt_count <-_raw_spin_unlock
1529   <idle>-0       3dNh1    7us : tick_program_event <-hrtimer_interrupt
1530   <idle>-0       3dNh1    7us : clockevents_program_event <-tick_program_event
1531   <idle>-0       3dNh1    8us : ktime_get <-clockevents_program_event
1532   <idle>-0       3dNh1    8us : lapic_next_event <-clockevents_program_event
1533   <idle>-0       3dNh1    8us : irq_exit <-smp_apic_timer_interrupt
1534   <idle>-0       3dNh1    9us : sub_preempt_count <-irq_exit
1535   <idle>-0       3dN.2    9us : idle_cpu <-irq_exit
1536   <idle>-0       3dN.2    9us : rcu_irq_exit <-irq_exit
1537   <idle>-0       3dN.2   10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
1538   <idle>-0       3dN.2   10us : sub_preempt_count <-irq_exit
1539   <idle>-0       3.N.1   11us : rcu_idle_exit <-cpu_idle
1540   <idle>-0       3dN.1   11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
1541   <idle>-0       3.N.1   11us : tick_nohz_idle_exit <-cpu_idle
1542   <idle>-0       3dN.1   12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
1543   <idle>-0       3dN.1   12us : ktime_get <-tick_nohz_idle_exit
1544   <idle>-0       3dN.1   12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
1545   <idle>-0       3dN.1   13us : update_cpu_load_nohz <-tick_nohz_idle_exit
1546   <idle>-0       3dN.1   13us : _raw_spin_lock <-update_cpu_load_nohz
1547   <idle>-0       3dN.1   13us : add_preempt_count <-_raw_spin_lock
1548   <idle>-0       3dN.2   13us : __update_cpu_load <-update_cpu_load_nohz
1549   <idle>-0       3dN.2   14us : sched_avg_update <-__update_cpu_load
1550   <idle>-0       3dN.2   14us : _raw_spin_unlock <-update_cpu_load_nohz
1551   <idle>-0       3dN.2   14us : sub_preempt_count <-_raw_spin_unlock
1552   <idle>-0       3dN.1   15us : calc_load_exit_idle <-tick_nohz_idle_exit
1553   <idle>-0       3dN.1   15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
1554   <idle>-0       3dN.1   15us : hrtimer_cancel <-tick_nohz_idle_exit
1555   <idle>-0       3dN.1   15us : hrtimer_try_to_cancel <-hrtimer_cancel
1556   <idle>-0       3dN.1   16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
1557   <idle>-0       3dN.1   16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
1558   <idle>-0       3dN.1   16us : add_preempt_count <-_raw_spin_lock_irqsave
1559   <idle>-0       3dN.2   17us : __remove_hrtimer <-remove_hrtimer.part.16
1560   <idle>-0       3dN.2   17us : hrtimer_force_reprogram <-__remove_hrtimer
1561   <idle>-0       3dN.2   17us : tick_program_event <-hrtimer_force_reprogram
1562   <idle>-0       3dN.2   18us : clockevents_program_event <-tick_program_event
1563   <idle>-0       3dN.2   18us : ktime_get <-clockevents_program_event
1564   <idle>-0       3dN.2   18us : lapic_next_event <-clockevents_program_event
1565   <idle>-0       3dN.2   19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
1566   <idle>-0       3dN.2   19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1567   <idle>-0       3dN.1   19us : hrtimer_forward <-tick_nohz_idle_exit
1568   <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
1569   <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
1570   <idle>-0       3dN.1   20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
1571   <idle>-0       3dN.1   20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
1572   <idle>-0       3dN.1   21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
1573   <idle>-0       3dN.1   21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
1574   <idle>-0       3dN.1   21us : add_preempt_count <-_raw_spin_lock_irqsave
1575   <idle>-0       3dN.2   22us : ktime_add_safe <-__hrtimer_start_range_ns
1576   <idle>-0       3dN.2   22us : enqueue_hrtimer <-__hrtimer_start_range_ns
1577   <idle>-0       3dN.2   22us : tick_program_event <-__hrtimer_start_range_ns
1578   <idle>-0       3dN.2   23us : clockevents_program_event <-tick_program_event
1579   <idle>-0       3dN.2   23us : ktime_get <-clockevents_program_event
1580   <idle>-0       3dN.2   23us : lapic_next_event <-clockevents_program_event
1581   <idle>-0       3dN.2   24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
1582   <idle>-0       3dN.2   24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1583   <idle>-0       3dN.1   24us : account_idle_ticks <-tick_nohz_idle_exit
1584   <idle>-0       3dN.1   24us : account_idle_time <-account_idle_ticks
1585   <idle>-0       3.N.1   25us : sub_preempt_count <-cpu_idle
1586   <idle>-0       3.N..   25us : schedule <-cpu_idle
1587   <idle>-0       3.N..   25us : __schedule <-preempt_schedule
1588   <idle>-0       3.N..   26us : add_preempt_count <-__schedule
1589   <idle>-0       3.N.1   26us : rcu_note_context_switch <-__schedule
1590   <idle>-0       3.N.1   26us : rcu_sched_qs <-rcu_note_context_switch
1591   <idle>-0       3dN.1   27us : rcu_preempt_qs <-rcu_note_context_switch
1592   <idle>-0       3.N.1   27us : _raw_spin_lock_irq <-__schedule
1593   <idle>-0       3dN.1   27us : add_preempt_count <-_raw_spin_lock_irq
1594   <idle>-0       3dN.2   28us : put_prev_task_idle <-__schedule
1595   <idle>-0       3dN.2   28us : pick_next_task_stop <-pick_next_task
1596   <idle>-0       3dN.2   28us : pick_next_task_rt <-pick_next_task
1597   <idle>-0       3dN.2   29us : dequeue_pushable_task <-pick_next_task_rt
1598   <idle>-0       3d..3   29us : __schedule <-preempt_schedule
1599   <idle>-0       3d..3   30us :      0:120:R ==> [003]  2448: 94:R sleep
1600 
1601 This isn't that big of a trace, even with function tracing enabled,
1602 so I included the entire trace.
1603 
1604 The interrupt went off while when the system was idle. Somewhere
1605 before task_woken_rt() was called, the NEED_RESCHED flag was set,
1606 this is indicated by the first occurrence of the 'N' flag.
1607 
1608 Latency tracing and events
1609 --------------------------
1610 As function tracing can induce a much larger latency, but without
1611 seeing what happens within the latency it is hard to know what
1612 caused it. There is a middle ground, and that is with enabling
1613 events.
1614 
1615  # echo 0 > options/function-trace
1616  # echo wakeup_rt > current_tracer
1617  # echo 1 > events/enable
1618  # echo 1 > tracing_on
1619  # echo 0 > tracing_max_latency
1620  # chrt -f 5 sleep 1
1621  # echo 0 > tracing_on
1622  # cat trace
1623 # tracer: wakeup_rt
1624 #
1625 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1626 # --------------------------------------------------------------------
1627 # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1628 #    -----------------
1629 #    | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
1630 #    -----------------
1631 #
1632 #                  _------=> CPU#            
1633 #                 / _-----=> irqs-off        
1634 #                | / _----=> need-resched    
1635 #                || / _---=> hardirq/softirq 
1636 #                ||| / _--=> preempt-depth   
1637 #                |||| /     delay             
1638 #  cmd     pid   ||||| time  |   caller      
1639 #     \   /      |||||  \    |   /           
1640   <idle>-0       2d.h4    0us :      0:120:R   + [002]  5882: 94:R sleep
1641   <idle>-0       2d.h4    0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
1642   <idle>-0       2d.h4    1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
1643   <idle>-0       2dNh2    1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
1644   <idle>-0       2.N.2    2us : power_end: cpu_id=2
1645   <idle>-0       2.N.2    3us : cpu_idle: state=4294967295 cpu_id=2
1646   <idle>-0       2dN.3    4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
1647   <idle>-0       2dN.3    4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
1648   <idle>-0       2.N.2    5us : rcu_utilization: Start context switch
1649   <idle>-0       2.N.2    5us : rcu_utilization: End context switch
1650   <idle>-0       2d..3    6us : __schedule <-schedule
1651   <idle>-0       2d..3    6us :      0:120:R ==> [002]  5882: 94:R sleep
1652 
1653 
1654 function
1655 --------
1656 
1657 This tracer is the function tracer. Enabling the function tracer
1658 can be done from the debug file system. Make sure the
1659 ftrace_enabled is set; otherwise this tracer is a nop.
1660 See the "ftrace_enabled" section below.
1661 
1662  # sysctl kernel.ftrace_enabled=1
1663  # echo function > current_tracer
1664  # echo 1 > tracing_on
1665  # usleep 1
1666  # echo 0 > tracing_on
1667  # cat trace
1668 # tracer: function
1669 #
1670 # entries-in-buffer/entries-written: 24799/24799   #P:4
1671 #
1672 #                              _-----=> irqs-off
1673 #                             / _----=> need-resched
1674 #                            | / _---=> hardirq/softirq
1675 #                            || / _--=> preempt-depth
1676 #                            ||| /     delay
1677 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
1678 #              | |       |   ||||       |         |
1679             bash-1994  [002] ....  3082.063030: mutex_unlock <-rb_simple_write
1680             bash-1994  [002] ....  3082.063031: __mutex_unlock_slowpath <-mutex_unlock
1681             bash-1994  [002] ....  3082.063031: __fsnotify_parent <-fsnotify_modify
1682             bash-1994  [002] ....  3082.063032: fsnotify <-fsnotify_modify
1683             bash-1994  [002] ....  3082.063032: __srcu_read_lock <-fsnotify
1684             bash-1994  [002] ....  3082.063032: add_preempt_count <-__srcu_read_lock
1685             bash-1994  [002] ...1  3082.063032: sub_preempt_count <-__srcu_read_lock
1686             bash-1994  [002] ....  3082.063033: __srcu_read_unlock <-fsnotify
1687 [...]
1688 
1689 
1690 Note: function tracer uses ring buffers to store the above
1691 entries. The newest data may overwrite the oldest data.
1692 Sometimes using echo to stop the trace is not sufficient because
1693 the tracing could have overwritten the data that you wanted to
1694 record. For this reason, it is sometimes better to disable
1695 tracing directly from a program. This allows you to stop the
1696 tracing at the point that you hit the part that you are
1697 interested in. To disable the tracing directly from a C program,
1698 something like following code snippet can be used:
1699 
1700 int trace_fd;
1701 [...]
1702 int main(int argc, char *argv[]) {
1703         [...]
1704         trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
1705         [...]
1706         if (condition_hit()) {
1707                 write(trace_fd, "0", 1);
1708         }
1709         [...]
1710 }
1711 
1712 
1713 Single thread tracing
1714 ---------------------
1715 
1716 By writing into set_ftrace_pid you can trace a
1717 single thread. For example:
1718 
1719 # cat set_ftrace_pid
1720 no pid
1721 # echo 3111 > set_ftrace_pid
1722 # cat set_ftrace_pid
1723 3111
1724 # echo function > current_tracer
1725 # cat trace | head
1726  # tracer: function
1727  #
1728  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
1729  #              | |       |          |         |
1730      yum-updatesd-3111  [003]  1637.254676: finish_task_switch <-thread_return
1731      yum-updatesd-3111  [003]  1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
1732      yum-updatesd-3111  [003]  1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
1733      yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
1734      yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
1735      yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll
1736 # echo -1 > set_ftrace_pid
1737 # cat trace |head
1738  # tracer: function
1739  #
1740  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
1741  #              | |       |          |         |
1742  ##### CPU 3 buffer started ####
1743      yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
1744      yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
1745      yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
1746      yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
1747      yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit
1748 
1749 If you want to trace a function when executing, you could use
1750 something like this simple program:
1751 
1752 #include <stdio.h>
1753 #include <stdlib.h>
1754 #include <sys/types.h>
1755 #include <sys/stat.h>
1756 #include <fcntl.h>
1757 #include <unistd.h>
1758 #include <string.h>
1759 
1760 #define _STR(x) #x
1761 #define STR(x) _STR(x)
1762 #define MAX_PATH 256
1763 
1764 const char *find_debugfs(void)
1765 {
1766        static char debugfs[MAX_PATH+1];
1767        static int debugfs_found;
1768        char type[100];
1769        FILE *fp;
1770 
1771        if (debugfs_found)
1772                return debugfs;
1773 
1774        if ((fp = fopen("/proc/mounts","r")) == NULL) {
1775                perror("/proc/mounts");
1776                return NULL;
1777        }
1778 
1779        while (fscanf(fp, "%*s %"
1780                      STR(MAX_PATH)
1781                      "s %99s %*s %*d %*d\n",
1782                      debugfs, type) == 2) {
1783                if (strcmp(type, "debugfs") == 0)
1784                        break;
1785        }
1786        fclose(fp);
1787 
1788        if (strcmp(type, "debugfs") != 0) {
1789                fprintf(stderr, "debugfs not mounted");
1790                return NULL;
1791        }
1792 
1793        strcat(debugfs, "/tracing/");
1794        debugfs_found = 1;
1795 
1796        return debugfs;
1797 }
1798 
1799 const char *tracing_file(const char *file_name)
1800 {
1801        static char trace_file[MAX_PATH+1];
1802        snprintf(trace_file, MAX_PATH, "%s/%s", find_debugfs(), file_name);
1803        return trace_file;
1804 }
1805 
1806 int main (int argc, char **argv)
1807 {
1808         if (argc < 1)
1809                 exit(-1);
1810 
1811         if (fork() > 0) {
1812                 int fd, ffd;
1813                 char line[64];
1814                 int s;
1815 
1816                 ffd = open(tracing_file("current_tracer"), O_WRONLY);
1817                 if (ffd < 0)
1818                         exit(-1);
1819                 write(ffd, "nop", 3);
1820 
1821                 fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
1822                 s = sprintf(line, "%d\n", getpid());
1823                 write(fd, line, s);
1824 
1825                 write(ffd, "function", 8);
1826 
1827                 close(fd);
1828                 close(ffd);
1829 
1830                 execvp(argv[1], argv+1);
1831         }
1832 
1833         return 0;
1834 }
1835 
1836 Or this simple script!
1837 
1838 ------
1839 #!/bin/bash
1840 
1841 debugfs=`sed -ne 's/^debugfs \(.*\) debugfs.*/\1/p' /proc/mounts`
1842 echo nop > $debugfs/tracing/current_tracer
1843 echo 0 > $debugfs/tracing/tracing_on
1844 echo $$ > $debugfs/tracing/set_ftrace_pid
1845 echo function > $debugfs/tracing/current_tracer
1846 echo 1 > $debugfs/tracing/tracing_on
1847 exec "$@"
1848 ------
1849 
1850 
1851 function graph tracer
1852 ---------------------------
1853 
1854 This tracer is similar to the function tracer except that it
1855 probes a function on its entry and its exit. This is done by
1856 using a dynamically allocated stack of return addresses in each
1857 task_struct. On function entry the tracer overwrites the return
1858 address of each function traced to set a custom probe. Thus the
1859 original return address is stored on the stack of return address
1860 in the task_struct.
1861 
1862 Probing on both ends of a function leads to special features
1863 such as:
1864 
1865 - measure of a function's time execution
1866 - having a reliable call stack to draw function calls graph
1867 
1868 This tracer is useful in several situations:
1869 
1870 - you want to find the reason of a strange kernel behavior and
1871   need to see what happens in detail on any areas (or specific
1872   ones).
1873 
1874 - you are experiencing weird latencies but it's difficult to
1875   find its origin.
1876 
1877 - you want to find quickly which path is taken by a specific
1878   function
1879 
1880 - you just want to peek inside a working kernel and want to see
1881   what happens there.
1882 
1883 # tracer: function_graph
1884 #
1885 # CPU  DURATION                  FUNCTION CALLS
1886 # |     |   |                     |   |   |   |
1887 
1888  0)               |  sys_open() {
1889  0)               |    do_sys_open() {
1890  0)               |      getname() {
1891  0)               |        kmem_cache_alloc() {
1892  0)   1.382 us    |          __might_sleep();
1893  0)   2.478 us    |        }
1894  0)               |        strncpy_from_user() {
1895  0)               |          might_fault() {
1896  0)   1.389 us    |            __might_sleep();
1897  0)   2.553 us    |          }
1898  0)   3.807 us    |        }
1899  0)   7.876 us    |      }
1900  0)               |      alloc_fd() {
1901  0)   0.668 us    |        _spin_lock();
1902  0)   0.570 us    |        expand_files();
1903  0)   0.586 us    |        _spin_unlock();
1904 
1905 
1906 There are several columns that can be dynamically
1907 enabled/disabled. You can use every combination of options you
1908 want, depending on your needs.
1909 
1910 - The cpu number on which the function executed is default
1911   enabled.  It is sometimes better to only trace one cpu (see
1912   tracing_cpu_mask file) or you might sometimes see unordered
1913   function calls while cpu tracing switch.
1914 
1915         hide: echo nofuncgraph-cpu > trace_options
1916         show: echo funcgraph-cpu > trace_options
1917 
1918 - The duration (function's time of execution) is displayed on
1919   the closing bracket line of a function or on the same line
1920   than the current function in case of a leaf one. It is default
1921   enabled.
1922 
1923         hide: echo nofuncgraph-duration > trace_options
1924         show: echo funcgraph-duration > trace_options
1925 
1926 - The overhead field precedes the duration field in case of
1927   reached duration thresholds.
1928 
1929         hide: echo nofuncgraph-overhead > trace_options
1930         show: echo funcgraph-overhead > trace_options
1931         depends on: funcgraph-duration
1932 
1933   ie:
1934 
1935   0)               |    up_write() {
1936   0)   0.646 us    |      _spin_lock_irqsave();
1937   0)   0.684 us    |      _spin_unlock_irqrestore();
1938   0)   3.123 us    |    }
1939   0)   0.548 us    |    fput();
1940   0) + 58.628 us   |  }
1941 
1942   [...]
1943 
1944   0)               |      putname() {
1945   0)               |        kmem_cache_free() {
1946   0)   0.518 us    |          __phys_addr();
1947   0)   1.757 us    |        }
1948   0)   2.861 us    |      }
1949   0) ! 115.305 us  |    }
1950   0) ! 116.402 us  |  }
1951 
1952   + means that the function exceeded 10 usecs.
1953   ! means that the function exceeded 100 usecs.
1954 
1955 
1956 - The task/pid field displays the thread cmdline and pid which
1957   executed the function. It is default disabled.
1958 
1959         hide: echo nofuncgraph-proc > trace_options
1960         show: echo funcgraph-proc > trace_options
1961 
1962   ie:
1963 
1964   # tracer: function_graph
1965   #
1966   # CPU  TASK/PID        DURATION                  FUNCTION CALLS
1967   # |    |    |           |   |                     |   |   |   |
1968   0)    sh-4802     |               |                  d_free() {
1969   0)    sh-4802     |               |                    call_rcu() {
1970   0)    sh-4802     |               |                      __call_rcu() {
1971   0)    sh-4802     |   0.616 us    |                        rcu_process_gp_end();
1972   0)    sh-4802     |   0.586 us    |                        check_for_new_grace_period();
1973   0)    sh-4802     |   2.899 us    |                      }
1974   0)    sh-4802     |   4.040 us    |                    }
1975   0)    sh-4802     |   5.151 us    |                  }
1976   0)    sh-4802     | + 49.370 us   |                }
1977 
1978 
1979 - The absolute time field is an absolute timestamp given by the
1980   system clock since it started. A snapshot of this time is
1981   given on each entry/exit of functions
1982 
1983         hide: echo nofuncgraph-abstime > trace_options
1984         show: echo funcgraph-abstime > trace_options
1985 
1986   ie:
1987 
1988   #
1989   #      TIME       CPU  DURATION                  FUNCTION CALLS
1990   #       |         |     |   |                     |   |   |   |
1991   360.774522 |   1)   0.541 us    |                                          }
1992   360.774522 |   1)   4.663 us    |                                        }
1993   360.774523 |   1)   0.541 us    |                                        __wake_up_bit();
1994   360.774524 |   1)   6.796 us    |                                      }
1995   360.774524 |   1)   7.952 us    |                                    }
1996   360.774525 |   1)   9.063 us    |                                  }
1997   360.774525 |   1)   0.615 us    |                                  journal_mark_dirty();
1998   360.774527 |   1)   0.578 us    |                                  __brelse();
1999   360.774528 |   1)               |                                  reiserfs_prepare_for_journal() {
2000   360.774528 |   1)               |                                    unlock_buffer() {
2001   360.774529 |   1)               |                                      wake_up_bit() {
2002   360.774529 |   1)               |                                        bit_waitqueue() {
2003   360.774530 |   1)   0.594 us    |                                          __phys_addr();
2004 
2005 
2006 You can put some comments on specific functions by using
2007 trace_printk() For example, if you want to put a comment inside
2008 the __might_sleep() function, you just have to include
2009 <linux/ftrace.h> and call trace_printk() inside __might_sleep()
2010 
2011 trace_printk("I'm a comment!\n")
2012 
2013 will produce:
2014 
2015  1)               |             __might_sleep() {
2016  1)               |                /* I'm a comment! */
2017  1)   1.449 us    |             }
2018 
2019 
2020 You might find other useful features for this tracer in the
2021 following "dynamic ftrace" section such as tracing only specific
2022 functions or tasks.
2023 
2024 dynamic ftrace
2025 --------------
2026 
2027 If CONFIG_DYNAMIC_FTRACE is set, the system will run with
2028 virtually no overhead when function tracing is disabled. The way
2029 this works is the mcount function call (placed at the start of
2030 every kernel function, produced by the -pg switch in gcc),
2031 starts of pointing to a simple return. (Enabling FTRACE will
2032 include the -pg switch in the compiling of the kernel.)
2033 
2034 At compile time every C file object is run through the
2035 recordmcount program (located in the scripts directory). This
2036 program will parse the ELF headers in the C object to find all
2037 the locations in the .text section that call mcount. (Note, only
2038 white listed .text sections are processed, since processing other
2039 sections like .init.text may cause races due to those sections
2040 being freed unexpectedly).
2041 
2042 A new section called "__mcount_loc" is created that holds
2043 references to all the mcount call sites in the .text section.
2044 The recordmcount program re-links this section back into the
2045 original object. The final linking stage of the kernel will add all these
2046 references into a single table.
2047 
2048 On boot up, before SMP is initialized, the dynamic ftrace code
2049 scans this table and updates all the locations into nops. It
2050 also records the locations, which are added to the
2051 available_filter_functions list.  Modules are processed as they
2052 are loaded and before they are executed.  When a module is
2053 unloaded, it also removes its functions from the ftrace function
2054 list. This is automatic in the module unload code, and the
2055 module author does not need to worry about it.
2056 
2057 When tracing is enabled, the process of modifying the function
2058 tracepoints is dependent on architecture. The old method is to use
2059 kstop_machine to prevent races with the CPUs executing code being
2060 modified (which can cause the CPU to do undesirable things, especially
2061 if the modified code crosses cache (or page) boundaries), and the nops are
2062 patched back to calls. But this time, they do not call mcount
2063 (which is just a function stub). They now call into the ftrace
2064 infrastructure.
2065 
2066 The new method of modifying the function tracepoints is to place
2067 a breakpoint at the location to be modified, sync all CPUs, modify
2068 the rest of the instruction not covered by the breakpoint. Sync
2069 all CPUs again, and then remove the breakpoint with the finished
2070 version to the ftrace call site.
2071 
2072 Some archs do not even need to monkey around with the synchronization,
2073 and can just slap the new code on top of the old without any
2074 problems with other CPUs executing it at the same time.
2075 
2076 One special side-effect to the recording of the functions being
2077 traced is that we can now selectively choose which functions we
2078 wish to trace and which ones we want the mcount calls to remain
2079 as nops.
2080 
2081 Two files are used, one for enabling and one for disabling the
2082 tracing of specified functions. They are:
2083 
2084   set_ftrace_filter
2085 
2086 and
2087 
2088   set_ftrace_notrace
2089 
2090 A list of available functions that you can add to these files is
2091 listed in:
2092 
2093    available_filter_functions
2094 
2095  # cat available_filter_functions
2096 put_prev_task_idle
2097 kmem_cache_create
2098 pick_next_task_rt
2099 get_online_cpus
2100 pick_next_task_fair
2101 mutex_lock
2102 [...]
2103 
2104 If I am only interested in sys_nanosleep and hrtimer_interrupt:
2105 
2106  # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
2107  # echo function > current_tracer
2108  # echo 1 > tracing_on
2109  # usleep 1
2110  # echo 0 > tracing_on
2111  # cat trace
2112 # tracer: function
2113 #
2114 # entries-in-buffer/entries-written: 5/5   #P:4
2115 #
2116 #                              _-----=> irqs-off
2117 #                             / _----=> need-resched
2118 #                            | / _---=> hardirq/softirq
2119 #                            || / _--=> preempt-depth
2120 #                            ||| /     delay
2121 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2122 #              | |       |   ||||       |         |
2123           usleep-2665  [001] ....  4186.475355: sys_nanosleep <-system_call_fastpath
2124           <idle>-0     [001] d.h1  4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
2125           usleep-2665  [001] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2126           <idle>-0     [003] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2127           <idle>-0     [002] d.h1  4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
2128 
2129 To see which functions are being traced, you can cat the file:
2130 
2131  # cat set_ftrace_filter
2132 hrtimer_interrupt
2133 sys_nanosleep
2134 
2135 
2136 Perhaps this is not enough. The filters also allow simple wild
2137 cards. Only the following are currently available
2138 
2139   <match>*  - will match functions that begin with <match>
2140   *<match>  - will match functions that end with <match>
2141   *<match>* - will match functions that have <match> in it
2142 
2143 These are the only wild cards which are supported.
2144 
2145   <match>*<match> will not work.
2146 
2147 Note: It is better to use quotes to enclose the wild cards,
2148       otherwise the shell may expand the parameters into names
2149       of files in the local directory.
2150 
2151  # echo 'hrtimer_*' > set_ftrace_filter
2152 
2153 Produces:
2154 
2155 # tracer: function
2156 #
2157 # entries-in-buffer/entries-written: 897/897   #P:4
2158 #
2159 #                              _-----=> irqs-off
2160 #                             / _----=> need-resched
2161 #                            | / _---=> hardirq/softirq
2162 #                            || / _--=> preempt-depth
2163 #                            ||| /     delay
2164 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2165 #              | |       |   ||||       |         |
2166           <idle>-0     [003] dN.1  4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
2167           <idle>-0     [003] dN.1  4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
2168           <idle>-0     [003] dN.2  4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
2169           <idle>-0     [003] dN.1  4228.547805: hrtimer_forward <-tick_nohz_idle_exit
2170           <idle>-0     [003] dN.1  4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
2171           <idle>-0     [003] d..1  4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
2172           <idle>-0     [003] d..1  4228.547859: hrtimer_start <-__tick_nohz_idle_enter
2173           <idle>-0     [003] d..2  4228.547860: hrtimer_force_reprogram <-__rem
2174 
2175 Notice that we lost the sys_nanosleep.
2176 
2177  # cat set_ftrace_filter
2178 hrtimer_run_queues
2179 hrtimer_run_pending
2180 hrtimer_init
2181 hrtimer_cancel
2182 hrtimer_try_to_cancel
2183 hrtimer_forward
2184 hrtimer_start
2185 hrtimer_reprogram
2186 hrtimer_force_reprogram
2187 hrtimer_get_next_event
2188 hrtimer_interrupt
2189 hrtimer_nanosleep
2190 hrtimer_wakeup
2191 hrtimer_get_remaining
2192 hrtimer_get_res
2193 hrtimer_init_sleeper
2194 
2195 
2196 This is because the '>' and '>>' act just like they do in bash.
2197 To rewrite the filters, use '>'
2198 To append to the filters, use '>>'
2199 
2200 To clear out a filter so that all functions will be recorded
2201 again:
2202 
2203  # echo > set_ftrace_filter
2204  # cat set_ftrace_filter
2205  #
2206 
2207 Again, now we want to append.
2208 
2209  # echo sys_nanosleep > set_ftrace_filter
2210  # cat set_ftrace_filter
2211 sys_nanosleep
2212  # echo 'hrtimer_*' >> set_ftrace_filter
2213  # cat set_ftrace_filter
2214 hrtimer_run_queues
2215 hrtimer_run_pending
2216 hrtimer_init
2217 hrtimer_cancel
2218 hrtimer_try_to_cancel
2219 hrtimer_forward
2220 hrtimer_start
2221 hrtimer_reprogram
2222 hrtimer_force_reprogram
2223 hrtimer_get_next_event
2224 hrtimer_interrupt
2225 sys_nanosleep
2226 hrtimer_nanosleep
2227 hrtimer_wakeup
2228 hrtimer_get_remaining
2229 hrtimer_get_res
2230 hrtimer_init_sleeper
2231 
2232 
2233 The set_ftrace_notrace prevents those functions from being
2234 traced.
2235 
2236  # echo '*preempt*' '*lock*' > set_ftrace_notrace
2237 
2238 Produces:
2239 
2240 # tracer: function
2241 #
2242 # entries-in-buffer/entries-written: 39608/39608   #P:4
2243 #
2244 #                              _-----=> irqs-off
2245 #                             / _----=> need-resched
2246 #                            | / _---=> hardirq/softirq
2247 #                            || / _--=> preempt-depth
2248 #                            ||| /     delay
2249 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2250 #              | |       |   ||||       |         |
2251             bash-1994  [000] ....  4342.324896: file_ra_state_init <-do_dentry_open
2252             bash-1994  [000] ....  4342.324897: open_check_o_direct <-do_last
2253             bash-1994  [000] ....  4342.324897: ima_file_check <-do_last
2254             bash-1994  [000] ....  4342.324898: process_measurement <-ima_file_check
2255             bash-1994  [000] ....  4342.324898: ima_get_action <-process_measurement
2256             bash-1994  [000] ....  4342.324898: ima_match_policy <-ima_get_action
2257             bash-1994  [000] ....  4342.324899: do_truncate <-do_last
2258             bash-1994  [000] ....  4342.324899: should_remove_suid <-do_truncate
2259             bash-1994  [000] ....  4342.324899: notify_change <-do_truncate
2260             bash-1994  [000] ....  4342.324900: current_fs_time <-notify_change
2261             bash-1994  [000] ....  4342.324900: current_kernel_time <-current_fs_time
2262             bash-1994  [000] ....  4342.324900: timespec_trunc <-current_fs_time
2263 
2264 We can see that there's no more lock or preempt tracing.
2265 
2266 
2267 Dynamic ftrace with the function graph tracer
2268 ---------------------------------------------
2269 
2270 Although what has been explained above concerns both the
2271 function tracer and the function-graph-tracer, there are some
2272 special features only available in the function-graph tracer.
2273 
2274 If you want to trace only one function and all of its children,
2275 you just have to echo its name into set_graph_function:
2276 
2277  echo __do_fault > set_graph_function
2278 
2279 will produce the following "expanded" trace of the __do_fault()
2280 function:
2281 
2282  0)               |  __do_fault() {
2283  0)               |    filemap_fault() {
2284  0)               |      find_lock_page() {
2285  0)   0.804 us    |        find_get_page();
2286  0)               |        __might_sleep() {
2287  0)   1.329 us    |        }
2288  0)   3.904 us    |      }
2289  0)   4.979 us    |    }
2290  0)   0.653 us    |    _spin_lock();
2291  0)   0.578 us    |    page_add_file_rmap();
2292  0)   0.525 us    |    native_set_pte_at();
2293  0)   0.585 us    |    _spin_unlock();
2294  0)               |    unlock_page() {
2295  0)   0.541 us    |      page_waitqueue();
2296  0)   0.639 us    |      __wake_up_bit();
2297  0)   2.786 us    |    }
2298  0) + 14.237 us   |  }
2299  0)               |  __do_fault() {
2300  0)               |    filemap_fault() {
2301  0)               |      find_lock_page() {
2302  0)   0.698 us    |        find_get_page();
2303  0)               |        __might_sleep() {
2304  0)   1.412 us    |        }
2305  0)   3.950 us    |      }
2306  0)   5.098 us    |    }
2307  0)   0.631 us    |    _spin_lock();
2308  0)   0.571 us    |    page_add_file_rmap();
2309  0)   0.526 us    |    native_set_pte_at();
2310  0)   0.586 us    |    _spin_unlock();
2311  0)               |    unlock_page() {
2312  0)   0.533 us    |      page_waitqueue();
2313  0)   0.638 us    |      __wake_up_bit();
2314  0)   2.793 us    |    }
2315  0) + 14.012 us   |  }
2316 
2317 You can also expand several functions at once:
2318 
2319  echo sys_open > set_graph_function
2320  echo sys_close >> set_graph_function
2321 
2322 Now if you want to go back to trace all functions you can clear
2323 this special filter via:
2324 
2325  echo > set_graph_function
2326 
2327 
2328 ftrace_enabled
2329 --------------
2330 
2331 Note, the proc sysctl ftrace_enable is a big on/off switch for the
2332 function tracer. By default it is enabled (when function tracing is
2333 enabled in the kernel). If it is disabled, all function tracing is
2334 disabled. This includes not only the function tracers for ftrace, but
2335 also for any other uses (perf, kprobes, stack tracing, profiling, etc).
2336 
2337 Please disable this with care.
2338 
2339 This can be disable (and enabled) with:
2340 
2341   sysctl kernel.ftrace_enabled=0
2342   sysctl kernel.ftrace_enabled=1
2343 
2344  or
2345 
2346   echo 0 > /proc/sys/kernel/ftrace_enabled
2347   echo 1 > /proc/sys/kernel/ftrace_enabled
2348 
2349 
2350 Filter commands
2351 ---------------
2352 
2353 A few commands are supported by the set_ftrace_filter interface.
2354 Trace commands have the following format:
2355 
2356 <function>:<command>:<parameter>
2357 
2358 The following commands are supported:
2359 
2360 - mod
2361   This command enables function filtering per module. The
2362   parameter defines the module. For example, if only the write*
2363   functions in the ext3 module are desired, run:
2364 
2365    echo 'write*:mod:ext3' > set_ftrace_filter
2366 
2367   This command interacts with the filter in the same way as
2368   filtering based on function names. Thus, adding more functions
2369   in a different module is accomplished by appending (>>) to the
2370   filter file. Remove specific module functions by prepending
2371   '!':
2372 
2373    echo '!writeback*:mod:ext3' >> set_ftrace_filter
2374 
2375 - traceon/traceoff
2376   These commands turn tracing on and off when the specified
2377   functions are hit. The parameter determines how many times the
2378   tracing system is turned on and off. If unspecified, there is
2379   no limit. For example, to disable tracing when a schedule bug
2380   is hit the first 5 times, run:
2381 
2382    echo '__schedule_bug:traceoff:5' > set_ftrace_filter
2383 
2384   To always disable tracing when __schedule_bug is hit:
2385 
2386    echo '__schedule_bug:traceoff' > set_ftrace_filter
2387 
2388   These commands are cumulative whether or not they are appended
2389   to set_ftrace_filter. To remove a command, prepend it by '!'
2390   and drop the parameter:
2391 
2392    echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
2393 
2394     The above removes the traceoff command for __schedule_bug
2395     that have a counter. To remove commands without counters:
2396 
2397    echo '!__schedule_bug:traceoff' > set_ftrace_filter
2398 
2399 - snapshot
2400   Will cause a snapshot to be triggered when the function is hit.
2401 
2402    echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
2403 
2404   To only snapshot once:
2405 
2406    echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
2407 
2408   To remove the above commands:
2409 
2410    echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
2411    echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
2412 
2413 - enable_event/disable_event
2414   These commands can enable or disable a trace event. Note, because
2415   function tracing callbacks are very sensitive, when these commands
2416   are registered, the trace point is activated, but disabled in
2417   a "soft" mode. That is, the tracepoint will be called, but
2418   just will not be traced. The event tracepoint stays in this mode
2419   as long as there's a command that triggers it.
2420 
2421    echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
2422          set_ftrace_filter
2423 
2424   The format is:
2425 
2426     <function>:enable_event:<system>:<event>[:count]
2427     <function>:disable_event:<system>:<event>[:count]
2428 
2429   To remove the events commands:
2430 
2431 
2432    echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
2433          set_ftrace_filter
2434    echo '!schedule:disable_event:sched:sched_switch' > \
2435          set_ftrace_filter
2436 
2437 - dump
2438   When the function is hit, it will dump the contents of the ftrace
2439   ring buffer to the console. This is useful if you need to debug
2440   something, and want to dump the trace when a certain function
2441   is hit. Perhaps its a function that is called before a tripple
2442   fault happens and does not allow you to get a regular dump.
2443 
2444 - cpudump
2445   When the function is hit, it will dump the contents of the ftrace
2446   ring buffer for the current CPU to the console. Unlike the "dump"
2447   command, it only prints out the contents of the ring buffer for the
2448   CPU that executed the function that triggered the dump.
2449 
2450 trace_pipe
2451 ----------
2452 
2453 The trace_pipe outputs the same content as the trace file, but
2454 the effect on the tracing is different. Every read from
2455 trace_pipe is consumed. This means that subsequent reads will be
2456 different. The trace is live.
2457 
2458  # echo function > current_tracer
2459  # cat trace_pipe > /tmp/trace.out &
2460 [1] 4153
2461  # echo 1 > tracing_on
2462  # usleep 1
2463  # echo 0 > tracing_on
2464  # cat trace
2465 # tracer: function
2466 #
2467 # entries-in-buffer/entries-written: 0/0   #P:4
2468 #
2469 #                              _-----=> irqs-off
2470 #                             / _----=> need-resched
2471 #                            | / _---=> hardirq/softirq
2472 #                            || / _--=> preempt-depth
2473 #                            ||| /     delay
2474 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2475 #              | |       |   ||||       |         |
2476 
2477  #
2478  # cat /tmp/trace.out
2479             bash-1994  [000] ....  5281.568961: mutex_unlock <-rb_simple_write
2480             bash-1994  [000] ....  5281.568963: __mutex_unlock_slowpath <-mutex_unlock
2481             bash-1994  [000] ....  5281.568963: __fsnotify_parent <-fsnotify_modify
2482             bash-1994  [000] ....  5281.568964: fsnotify <-fsnotify_modify
2483             bash-1994  [000] ....  5281.568964: __srcu_read_lock <-fsnotify
2484             bash-1994  [000] ....  5281.568964: add_preempt_count <-__srcu_read_lock
2485             bash-1994  [000] ...1  5281.568965: sub_preempt_count <-__srcu_read_lock
2486             bash-1994  [000] ....  5281.568965: __srcu_read_unlock <-fsnotify
2487             bash-1994  [000] ....  5281.568967: sys_dup2 <-system_call_fastpath
2488 
2489 
2490 Note, reading the trace_pipe file will block until more input is
2491 added.
2492 
2493 trace entries
2494 -------------
2495 
2496 Having too much or not enough data can be troublesome in
2497 diagnosing an issue in the kernel. The file buffer_size_kb is
2498 used to modify the size of the internal trace buffers. The
2499 number listed is the number of entries that can be recorded per
2500 CPU. To know the full size, multiply the number of possible CPUs
2501 with the number of entries.
2502 
2503  # cat buffer_size_kb
2504 1408 (units kilobytes)
2505 
2506 Or simply read buffer_total_size_kb
2507 
2508  # cat buffer_total_size_kb 
2509 5632
2510 
2511 To modify the buffer, simple echo in a number (in 1024 byte segments).
2512 
2513  # echo 10000 > buffer_size_kb
2514  # cat buffer_size_kb
2515 10000 (units kilobytes)
2516 
2517 It will try to allocate as much as possible. If you allocate too
2518 much, it can cause Out-Of-Memory to trigger.
2519 
2520  # echo 1000000000000 > buffer_size_kb
2521 -bash: echo: write error: Cannot allocate memory
2522  # cat buffer_size_kb
2523 85
2524 
2525 The per_cpu buffers can be changed individually as well:
2526 
2527  # echo 10000 > per_cpu/cpu0/buffer_size_kb
2528  # echo 100 > per_cpu/cpu1/buffer_size_kb
2529 
2530 When the per_cpu buffers are not the same, the buffer_size_kb
2531 at the top level will just show an X
2532 
2533  # cat buffer_size_kb
2534 X
2535 
2536 This is where the buffer_total_size_kb is useful:
2537 
2538  # cat buffer_total_size_kb 
2539 12916
2540 
2541 Writing to the top level buffer_size_kb will reset all the buffers
2542 to be the same again.
2543 
2544 Snapshot
2545 --------
2546 CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
2547 available to all non latency tracers. (Latency tracers which
2548 record max latency, such as "irqsoff" or "wakeup", can't use
2549 this feature, since those are already using the snapshot
2550 mechanism internally.)
2551 
2552 Snapshot preserves a current trace buffer at a particular point
2553 in time without stopping tracing. Ftrace swaps the current
2554 buffer with a spare buffer, and tracing continues in the new
2555 current (=previous spare) buffer.
2556 
2557 The following debugfs files in "tracing" are related to this
2558 feature:
2559 
2560   snapshot:
2561 
2562         This is used to take a snapshot and to read the output
2563         of the snapshot. Echo 1 into this file to allocate a
2564         spare buffer and to take a snapshot (swap), then read
2565         the snapshot from this file in the same format as
2566         "trace" (described above in the section "The File
2567         System"). Both reads snapshot and tracing are executable
2568         in parallel. When the spare buffer is allocated, echoing
2569         0 frees it, and echoing else (positive) values clear the
2570         snapshot contents.
2571         More details are shown in the table below.
2572 
2573         status\input  |     0      |     1      |    else    |
2574         --------------+------------+------------+------------+
2575         not allocated |(do nothing)| alloc+swap |(do nothing)|
2576         --------------+------------+------------+------------+
2577         allocated     |    free    |    swap    |   clear    |
2578         --------------+------------+------------+------------+
2579 
2580 Here is an example of using the snapshot feature.
2581 
2582  # echo 1 > events/sched/enable
2583  # echo 1 > snapshot
2584  # cat snapshot
2585 # tracer: nop
2586 #
2587 # entries-in-buffer/entries-written: 71/71   #P:8
2588 #
2589 #                              _-----=> irqs-off
2590 #                             / _----=> need-resched
2591 #                            | / _---=> hardirq/softirq
2592 #                            || / _--=> preempt-depth
2593 #                            ||| /     delay
2594 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2595 #              | |       |   ||||       |         |
2596           <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
2597            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
2598 [...]
2599           <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
2600 
2601  # cat trace
2602 # tracer: nop
2603 #
2604 # entries-in-buffer/entries-written: 77/77   #P:8
2605 #
2606 #                              _-----=> irqs-off
2607 #                             / _----=> need-resched
2608 #                            | / _---=> hardirq/softirq
2609 #                            || / _--=> preempt-depth
2610 #                            ||| /     delay
2611 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2612 #              | |       |   ||||       |         |
2613           <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
2614  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
2615 [...]
2616 
2617 
2618 If you try to use this snapshot feature when current tracer is
2619 one of the latency tracers, you will get the following results.
2620 
2621  # echo wakeup > current_tracer
2622  # echo 1 > snapshot
2623 bash: echo: write error: Device or resource busy
2624  # cat snapshot
2625 cat: snapshot: Device or resource busy
2626 
2627 
2628 Instances
2629 ---------
2630 In the debugfs tracing directory is a directory called "instances".
2631 This directory can have new directories created inside of it using
2632 mkdir, and removing directories with rmdir. The directory created
2633 with mkdir in this directory will already contain files and other
2634 directories after it is created.
2635 
2636  # mkdir instances/foo
2637  # ls instances/foo
2638 buffer_size_kb  buffer_total_size_kb  events  free_buffer  per_cpu
2639 set_event  snapshot  trace  trace_clock  trace_marker  trace_options
2640 trace_pipe  tracing_on
2641 
2642 As you can see, the new directory looks similar to the tracing directory
2643 itself. In fact, it is very similar, except that the buffer and
2644 events are agnostic from the main director, or from any other
2645 instances that are created.
2646 
2647 The files in the new directory work just like the files with the
2648 same name in the tracing directory except the buffer that is used
2649 is a separate and new buffer. The files affect that buffer but do not
2650 affect the main buffer with the exception of trace_options. Currently,
2651 the trace_options affect all instances and the top level buffer
2652 the same, but this may change in future releases. That is, options
2653 may become specific to the instance they reside in.
2654 
2655 Notice that none of the function tracer files are there, nor is
2656 current_tracer and available_tracers. This is because the buffers
2657 can currently only have events enabled for them.
2658 
2659  # mkdir instances/foo
2660  # mkdir instances/bar
2661  # mkdir instances/zoot
2662  # echo 100000 > buffer_size_kb
2663  # echo 1000 > instances/foo/buffer_size_kb
2664  # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
2665  # echo function > current_trace
2666  # echo 1 > instances/foo/events/sched/sched_wakeup/enable
2667  # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
2668  # echo 1 > instances/foo/events/sched/sched_switch/enable
2669  # echo 1 > instances/bar/events/irq/enable
2670  # echo 1 > instances/zoot/events/syscalls/enable
2671  # cat trace_pipe
2672 CPU:2 [LOST 11745 EVENTS]
2673             bash-2044  [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
2674             bash-2044  [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
2675             bash-2044  [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
2676             bash-2044  [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
2677             bash-2044  [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
2678             bash-2044  [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
2679             bash-2044  [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
2680             bash-2044  [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
2681             bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
2682             bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
2683             bash-2044  [002] .... 10594.481035: arch_dup_task_struct <-copy_process
2684 [...]
2685 
2686  # cat instances/foo/trace_pipe
2687             bash-1998  [000] d..4   136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
2688             bash-1998  [000] dN.4   136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
2689           <idle>-0     [003] d.h3   136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
2690           <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
2691      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
2692             bash-1998  [000] d..4   136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
2693             bash-1998  [000] dN.4   136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
2694             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
2695      kworker/0:1-59    [000] d..4   136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
2696      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
2697 [...]
2698 
2699  # cat instances/bar/trace_pipe
2700      migration/1-14    [001] d.h3   138.732674: softirq_raise: vec=3 [action=NET_RX]
2701           <idle>-0     [001] dNh3   138.732725: softirq_raise: vec=3 [action=NET_RX]
2702             bash-1998  [000] d.h1   138.733101: softirq_raise: vec=1 [action=TIMER]
2703             bash-1998  [000] d.h1   138.733102: softirq_raise: vec=9 [action=RCU]
2704             bash-1998  [000] ..s2   138.733105: softirq_entry: vec=1 [action=TIMER]
2705             bash-1998  [000] ..s2   138.733106: softirq_exit: vec=1 [action=TIMER]
2706             bash-1998  [000] ..s2   138.733106: softirq_entry: vec=9 [action=RCU]
2707             bash-1998  [000] ..s2   138.733109: softirq_exit: vec=9 [action=RCU]
2708             sshd-1995  [001] d.h1   138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
2709             sshd-1995  [001] d.h1   138.733280: irq_handler_exit: irq=21 ret=unhandled
2710             sshd-1995  [001] d.h1   138.733281: irq_handler_entry: irq=21 name=eth0
2711             sshd-1995  [001] d.h1   138.733283: irq_handler_exit: irq=21 ret=handled
2712 [...]
2713 
2714  # cat instances/zoot/trace
2715 # tracer: nop
2716 #
2717 # entries-in-buffer/entries-written: 18996/18996   #P:4
2718 #
2719 #                              _-----=> irqs-off
2720 #                             / _----=> need-resched
2721 #                            | / _---=> hardirq/softirq
2722 #                            || / _--=> preempt-depth
2723 #                            ||| /     delay
2724 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2725 #              | |       |   ||||       |         |
2726             bash-1998  [000] d...   140.733501: sys_write -> 0x2
2727             bash-1998  [000] d...   140.733504: sys_dup2(oldfd: a, newfd: 1)
2728             bash-1998  [000] d...   140.733506: sys_dup2 -> 0x1
2729             bash-1998  [000] d...   140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
2730             bash-1998  [000] d...   140.733509: sys_fcntl -> 0x1
2731             bash-1998  [000] d...   140.733510: sys_close(fd: a)
2732             bash-1998  [000] d...   140.733510: sys_close -> 0x0
2733             bash-1998  [000] d...   140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
2734             bash-1998  [000] d...   140.733515: sys_rt_sigprocmask -> 0x0
2735             bash-1998  [000] d...   140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
2736             bash-1998  [000] d...   140.733516: sys_rt_sigaction -> 0x0
2737 
2738 You can see that the trace of the top most trace buffer shows only
2739 the function tracing. The foo instance displays wakeups and task
2740 switches.
2741 
2742 To remove the instances, simply delete their directories:
2743 
2744  # rmdir instances/foo
2745  # rmdir instances/bar
2746  # rmdir instances/zoot
2747 
2748 Note, if a process has a trace file open in one of the instance
2749 directories, the rmdir will fail with EBUSY.
2750 
2751 
2752 Stack trace
2753 -----------
2754 Since the kernel has a fixed sized stack, it is important not to
2755 waste it in functions. A kernel developer must be conscience of
2756 what they allocate on the stack. If they add too much, the system
2757 can be in danger of a stack overflow, and corruption will occur,
2758 usually leading to a system panic.
2759 
2760 There are some tools that check this, usually with interrupts
2761 periodically checking usage. But if you can perform a check
2762 at every function call that will become very useful. As ftrace provides
2763 a function tracer, it makes it convenient to check the stack size
2764 at every function call. This is enabled via the stack tracer.
2765 
2766 CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
2767 To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
2768 
2769  # echo 1 > /proc/sys/kernel/stack_tracer_enabled
2770 
2771 You can also enable it from the kernel command line to trace
2772 the stack size of the kernel during boot up, by adding "stacktrace"
2773 to the kernel command line parameter.
2774 
2775 After running it for a few minutes, the output looks like:
2776 
2777  # cat stack_max_size
2778 2928
2779 
2780  # cat stack_trace
2781         Depth    Size   Location    (18 entries)
2782         -----    ----   --------
2783   0)     2928     224   update_sd_lb_stats+0xbc/0x4ac
2784   1)     2704     160   find_busiest_group+0x31/0x1f1
2785   2)     2544     256   load_balance+0xd9/0x662
2786   3)     2288      80   idle_balance+0xbb/0x130
2787   4)     2208     128   __schedule+0x26e/0x5b9
2788   5)     2080      16   schedule+0x64/0x66
2789   6)     2064     128   schedule_timeout+0x34/0xe0
2790   7)     1936     112   wait_for_common+0x97/0xf1
2791   8)     1824      16   wait_for_completion+0x1d/0x1f
2792   9)     1808     128   flush_work+0xfe/0x119
2793  10)     1680      16   tty_flush_to_ldisc+0x1e/0x20
2794  11)     1664      48   input_available_p+0x1d/0x5c
2795  12)     1616      48   n_tty_poll+0x6d/0x134
2796  13)     1568      64   tty_poll+0x64/0x7f
2797  14)     1504     880   do_select+0x31e/0x511
2798  15)      624     400   core_sys_select+0x177/0x216
2799  16)      224      96   sys_select+0x91/0xb9
2800  17)      128     128   system_call_fastpath+0x16/0x1b
2801 
2802 Note, if -mfentry is being used by gcc, functions get traced before
2803 they set up the stack frame. This means that leaf level functions
2804 are not tested by the stack tracer when -mfentry is used.
2805 
2806 Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
2807 
2808 ---------
2809 
2810 More details can be found in the source code, in the
2811 kernel/trace/*.c files.

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