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Linux/Documentation/trace/ftrace.txt

  1                 ftrace - Function Tracer
  2                 ========================
  3 
  4 Copyright 2008 Red Hat Inc.
  5    Author:   Steven Rostedt <srostedt@redhat.com>
  6   License:   The GNU Free Documentation License, Version 1.2
  7                (dual licensed under the GPL v2)
  8 Reviewers:   Elias Oltmanns, Randy Dunlap, Andrew Morton,
  9              John Kacur, and David Teigland.
 10 Written for: 2.6.28-rc2
 11 Updated for: 3.10
 12 
 13 Introduction
 14 ------------
 15 
 16 Ftrace is an internal tracer designed to help out developers and
 17 designers of systems to find what is going on inside the kernel.
 18 It can be used for debugging or analyzing latencies and
 19 performance issues that take place outside of user-space.
 20 
 21 Although ftrace is typically considered the function tracer, it
 22 is really a frame work of several assorted tracing utilities.
 23 There's latency tracing to examine what occurs between interrupts
 24 disabled and enabled, as well as for preemption and from a time
 25 a task is woken to the task is actually scheduled in.
 26 
 27 One of the most common uses of ftrace is the event tracing.
 28 Through out the kernel is hundreds of static event points that
 29 can be enabled via the debugfs file system to see what is
 30 going on in certain parts of the kernel.
 31 
 32 
 33 Implementation Details
 34 ----------------------
 35 
 36 See ftrace-design.txt for details for arch porters and such.
 37 
 38 
 39 The File System
 40 ---------------
 41 
 42 Ftrace uses the debugfs file system to hold the control files as
 43 well as the files to display output.
 44 
 45 When debugfs is configured into the kernel (which selecting any ftrace
 46 option will do) the directory /sys/kernel/debug will be created. To mount
 47 this directory, you can add to your /etc/fstab file:
 48 
 49  debugfs       /sys/kernel/debug          debugfs defaults        0       0
 50 
 51 Or you can mount it at run time with:
 52 
 53  mount -t debugfs nodev /sys/kernel/debug
 54 
 55 For quicker access to that directory you may want to make a soft link to
 56 it:
 57 
 58  ln -s /sys/kernel/debug /debug
 59 
 60 Any selected ftrace option will also create a directory called tracing
 61 within the debugfs. The rest of the document will assume that you are in
 62 the ftrace directory (cd /sys/kernel/debug/tracing) and will only concentrate
 63 on the files within that directory and not distract from the content with
 64 the extended "/sys/kernel/debug/tracing" path name.
 65 
 66 That's it! (assuming that you have ftrace configured into your kernel)
 67 
 68 After mounting debugfs, you can see a directory called
 69 "tracing".  This directory contains the control and output files
 70 of ftrace. Here is a list of some of the key files:
 71 
 72 
 73  Note: all time values are in microseconds.
 74 
 75   current_tracer:
 76 
 77         This is used to set or display the current tracer
 78         that is configured.
 79 
 80   available_tracers:
 81 
 82         This holds the different types of tracers that
 83         have been compiled into the kernel. The
 84         tracers listed here can be configured by
 85         echoing their name into current_tracer.
 86 
 87   tracing_on:
 88 
 89         This sets or displays whether writing to the trace
 90         ring buffer is enabled. Echo 0 into this file to disable
 91         the tracer or 1 to enable it. Note, this only disables
 92         writing to the ring buffer, the tracing overhead may
 93         still be occurring.
 94 
 95   trace:
 96 
 97         This file holds the output of the trace in a human
 98         readable format (described below).
 99 
100   trace_pipe:
101 
102         The output is the same as the "trace" file but this
103         file is meant to be streamed with live tracing.
104         Reads from this file will block until new data is
105         retrieved.  Unlike the "trace" file, this file is a
106         consumer. This means reading from this file causes
107         sequential reads to display more current data. Once
108         data is read from this file, it is consumed, and
109         will not be read again with a sequential read. The
110         "trace" file is static, and if the tracer is not
111         adding more data, it will display the same
112         information every time it is read.
113 
114   trace_options:
115 
116         This file lets the user control the amount of data
117         that is displayed in one of the above output
118         files. Options also exist to modify how a tracer
119         or events work (stack traces, timestamps, etc).
120 
121   options:
122 
123         This is a directory that has a file for every available
124         trace option (also in trace_options). Options may also be set
125         or cleared by writing a "1" or "0" respectively into the
126         corresponding file with the option name.
127 
128   tracing_max_latency:
129 
130         Some of the tracers record the max latency.
131         For example, the time interrupts are disabled.
132         This time is saved in this file. The max trace
133         will also be stored, and displayed by "trace".
134         A new max trace will only be recorded if the
135         latency is greater than the value in this
136         file. (in microseconds)
137 
138   tracing_thresh:
139 
140         Some latency tracers will record a trace whenever the
141         latency is greater than the number in this file.
142         Only active when the file contains a number greater than 0.
143         (in microseconds)
144 
145   buffer_size_kb:
146 
147         This sets or displays the number of kilobytes each CPU
148         buffer holds. By default, the trace buffers are the same size
149         for each CPU. The displayed number is the size of the
150         CPU buffer and not total size of all buffers. The
151         trace buffers are allocated in pages (blocks of memory
152         that the kernel uses for allocation, usually 4 KB in size).
153         If the last page allocated has room for more bytes
154         than requested, the rest of the page will be used,
155         making the actual allocation bigger than requested.
156         ( Note, the size may not be a multiple of the page size
157           due to buffer management meta-data. )
158 
159   buffer_total_size_kb:
160 
161         This displays the total combined size of all the trace buffers.
162 
163   free_buffer:
164 
165         If a process is performing the tracing, and the ring buffer
166         should be shrunk "freed" when the process is finished, even
167         if it were to be killed by a signal, this file can be used
168         for that purpose. On close of this file, the ring buffer will
169         be resized to its minimum size. Having a process that is tracing
170         also open this file, when the process exits its file descriptor
171         for this file will be closed, and in doing so, the ring buffer
172         will be "freed".
173 
174         It may also stop tracing if disable_on_free option is set.
175 
176   tracing_cpumask:
177 
178         This is a mask that lets the user only trace
179         on specified CPUs. The format is a hex string
180         representing the CPUs.
181 
182   set_ftrace_filter:
183 
184         When dynamic ftrace is configured in (see the
185         section below "dynamic ftrace"), the code is dynamically
186         modified (code text rewrite) to disable calling of the
187         function profiler (mcount). This lets tracing be configured
188         in with practically no overhead in performance.  This also
189         has a side effect of enabling or disabling specific functions
190         to be traced. Echoing names of functions into this file
191         will limit the trace to only those functions.
192 
193         This interface also allows for commands to be used. See the
194         "Filter commands" section for more details.
195 
196   set_ftrace_notrace:
197 
198         This has an effect opposite to that of
199         set_ftrace_filter. Any function that is added here will not
200         be traced. If a function exists in both set_ftrace_filter
201         and set_ftrace_notrace, the function will _not_ be traced.
202 
203   set_ftrace_pid:
204 
205         Have the function tracer only trace a single thread.
206 
207   set_event_pid:
208 
209         Have the events only trace a task with a PID listed in this file.
210         Note, sched_switch and sched_wake_up will also trace events
211         listed in this file.
212 
213         To have the PIDs of children of tasks with their PID in this file
214         added on fork, enable the "event-fork" option. That option will also
215         cause the PIDs of tasks to be removed from this file when the task
216         exits.
217 
218   set_graph_function:
219 
220         Set a "trigger" function where tracing should start
221         with the function graph tracer (See the section
222         "dynamic ftrace" for more details).
223 
224   available_filter_functions:
225 
226         This lists the functions that ftrace
227         has processed and can trace. These are the function
228         names that you can pass to "set_ftrace_filter" or
229         "set_ftrace_notrace". (See the section "dynamic ftrace"
230         below for more details.)
231 
232   enabled_functions:
233 
234         This file is more for debugging ftrace, but can also be useful
235         in seeing if any function has a callback attached to it.
236         Not only does the trace infrastructure use ftrace function
237         trace utility, but other subsystems might too. This file
238         displays all functions that have a callback attached to them
239         as well as the number of callbacks that have been attached.
240         Note, a callback may also call multiple functions which will
241         not be listed in this count.
242 
243         If the callback registered to be traced by a function with
244         the "save regs" attribute (thus even more overhead), a 'R'
245         will be displayed on the same line as the function that
246         is returning registers.
247 
248         If the callback registered to be traced by a function with
249         the "ip modify" attribute (thus the regs->ip can be changed),
250         an 'I' will be displayed on the same line as the function that
251         can be overridden.
252 
253   function_profile_enabled:
254 
255         When set it will enable all functions with either the function
256         tracer, or if enabled, the function graph tracer. It will
257         keep a histogram of the number of functions that were called
258         and if run with the function graph tracer, it will also keep
259         track of the time spent in those functions. The histogram
260         content can be displayed in the files:
261 
262         trace_stats/function<cpu> ( function0, function1, etc).
263 
264   trace_stats:
265 
266         A directory that holds different tracing stats.
267 
268   kprobe_events:
269  
270         Enable dynamic trace points. See kprobetrace.txt.
271 
272   kprobe_profile:
273 
274         Dynamic trace points stats. See kprobetrace.txt.
275 
276   max_graph_depth:
277 
278         Used with the function graph tracer. This is the max depth
279         it will trace into a function. Setting this to a value of
280         one will show only the first kernel function that is called
281         from user space.
282 
283   printk_formats:
284 
285         This is for tools that read the raw format files. If an event in
286         the ring buffer references a string (currently only trace_printk()
287         does this), only a pointer to the string is recorded into the buffer
288         and not the string itself. This prevents tools from knowing what
289         that string was. This file displays the string and address for
290         the string allowing tools to map the pointers to what the
291         strings were.
292 
293   saved_cmdlines:
294 
295         Only the pid of the task is recorded in a trace event unless
296         the event specifically saves the task comm as well. Ftrace
297         makes a cache of pid mappings to comms to try to display
298         comms for events. If a pid for a comm is not listed, then
299         "<...>" is displayed in the output.
300 
301   snapshot:
302 
303         This displays the "snapshot" buffer and also lets the user
304         take a snapshot of the current running trace.
305         See the "Snapshot" section below for more details.
306 
307   stack_max_size:
308 
309         When the stack tracer is activated, this will display the
310         maximum stack size it has encountered.
311         See the "Stack Trace" section below.
312 
313   stack_trace:
314 
315         This displays the stack back trace of the largest stack
316         that was encountered when the stack tracer is activated.
317         See the "Stack Trace" section below.
318 
319   stack_trace_filter:
320 
321         This is similar to "set_ftrace_filter" but it limits what
322         functions the stack tracer will check.
323 
324   trace_clock:
325 
326         Whenever an event is recorded into the ring buffer, a
327         "timestamp" is added. This stamp comes from a specified
328         clock. By default, ftrace uses the "local" clock. This
329         clock is very fast and strictly per cpu, but on some
330         systems it may not be monotonic with respect to other
331         CPUs. In other words, the local clocks may not be in sync
332         with local clocks on other CPUs.
333 
334         Usual clocks for tracing:
335 
336           # cat trace_clock
337           [local] global counter x86-tsc
338 
339           local: Default clock, but may not be in sync across CPUs
340 
341           global: This clock is in sync with all CPUs but may
342                   be a bit slower than the local clock.
343 
344           counter: This is not a clock at all, but literally an atomic
345                    counter. It counts up one by one, but is in sync
346                    with all CPUs. This is useful when you need to
347                    know exactly the order events occurred with respect to
348                    each other on different CPUs.
349 
350           uptime: This uses the jiffies counter and the time stamp
351                   is relative to the time since boot up.
352 
353           perf: This makes ftrace use the same clock that perf uses.
354                 Eventually perf will be able to read ftrace buffers
355                 and this will help out in interleaving the data.
356 
357           x86-tsc: Architectures may define their own clocks. For
358                    example, x86 uses its own TSC cycle clock here.
359 
360           ppc-tb: This uses the powerpc timebase register value.
361                   This is in sync across CPUs and can also be used
362                   to correlate events across hypervisor/guest if
363                   tb_offset is known.
364 
365           mono: This uses the fast monotonic clock (CLOCK_MONOTONIC)
366                 which is monotonic and is subject to NTP rate adjustments.
367 
368           mono_raw:
369                 This is the raw monotonic clock (CLOCK_MONOTONIC_RAW)
370                 which is montonic but is not subject to any rate adjustments
371                 and ticks at the same rate as the hardware clocksource.
372 
373           boot: This is the boot clock (CLOCK_BOOTTIME) and is based on the
374                 fast monotonic clock, but also accounts for time spent in
375                 suspend. Since the clock access is designed for use in
376                 tracing in the suspend path, some side effects are possible
377                 if clock is accessed after the suspend time is accounted before
378                 the fast mono clock is updated. In this case, the clock update
379                 appears to happen slightly sooner than it normally would have.
380                 Also on 32-bit systems, it's possible that the 64-bit boot offset
381                 sees a partial update. These effects are rare and post
382                 processing should be able to handle them. See comments in the
383                 ktime_get_boot_fast_ns() function for more information.
384 
385         To set a clock, simply echo the clock name into this file.
386 
387           echo global > trace_clock
388 
389   trace_marker:
390 
391         This is a very useful file for synchronizing user space
392         with events happening in the kernel. Writing strings into
393         this file will be written into the ftrace buffer.
394 
395         It is useful in applications to open this file at the start
396         of the application and just reference the file descriptor
397         for the file.
398 
399         void trace_write(const char *fmt, ...)
400         {
401                 va_list ap;
402                 char buf[256];
403                 int n;
404 
405                 if (trace_fd < 0)
406                         return;
407 
408                 va_start(ap, fmt);
409                 n = vsnprintf(buf, 256, fmt, ap);
410                 va_end(ap);
411 
412                 write(trace_fd, buf, n);
413         }
414 
415         start:
416 
417                 trace_fd = open("trace_marker", WR_ONLY);
418 
419   trace_marker_raw:
420 
421         This is similar to trace_marker above, but is meant for for binary data
422         to be written to it, where a tool can be used to parse the data
423         from trace_pipe_raw.
424 
425   uprobe_events:
426  
427         Add dynamic tracepoints in programs.
428         See uprobetracer.txt
429 
430   uprobe_profile:
431 
432         Uprobe statistics. See uprobetrace.txt
433 
434   instances:
435 
436         This is a way to make multiple trace buffers where different
437         events can be recorded in different buffers.
438         See "Instances" section below.
439 
440   events:
441 
442         This is the trace event directory. It holds event tracepoints
443         (also known as static tracepoints) that have been compiled
444         into the kernel. It shows what event tracepoints exist
445         and how they are grouped by system. There are "enable"
446         files at various levels that can enable the tracepoints
447         when a "1" is written to them.
448 
449         See events.txt for more information.
450 
451   per_cpu:
452 
453         This is a directory that contains the trace per_cpu information.
454 
455   per_cpu/cpu0/buffer_size_kb:
456 
457         The ftrace buffer is defined per_cpu. That is, there's a separate
458         buffer for each CPU to allow writes to be done atomically,
459         and free from cache bouncing. These buffers may have different
460         size buffers. This file is similar to the buffer_size_kb
461         file, but it only displays or sets the buffer size for the
462         specific CPU. (here cpu0).
463 
464   per_cpu/cpu0/trace:
465 
466         This is similar to the "trace" file, but it will only display
467         the data specific for the CPU. If written to, it only clears
468         the specific CPU buffer.
469 
470   per_cpu/cpu0/trace_pipe
471 
472         This is similar to the "trace_pipe" file, and is a consuming
473         read, but it will only display (and consume) the data specific
474         for the CPU.
475 
476   per_cpu/cpu0/trace_pipe_raw
477 
478         For tools that can parse the ftrace ring buffer binary format,
479         the trace_pipe_raw file can be used to extract the data
480         from the ring buffer directly. With the use of the splice()
481         system call, the buffer data can be quickly transferred to
482         a file or to the network where a server is collecting the
483         data.
484 
485         Like trace_pipe, this is a consuming reader, where multiple
486         reads will always produce different data.
487 
488   per_cpu/cpu0/snapshot:
489 
490         This is similar to the main "snapshot" file, but will only
491         snapshot the current CPU (if supported). It only displays
492         the content of the snapshot for a given CPU, and if
493         written to, only clears this CPU buffer.
494 
495   per_cpu/cpu0/snapshot_raw:
496 
497         Similar to the trace_pipe_raw, but will read the binary format
498         from the snapshot buffer for the given CPU.
499 
500   per_cpu/cpu0/stats:
501 
502         This displays certain stats about the ring buffer:
503 
504          entries: The number of events that are still in the buffer.
505 
506          overrun: The number of lost events due to overwriting when
507                   the buffer was full.
508 
509          commit overrun: Should always be zero.
510                 This gets set if so many events happened within a nested
511                 event (ring buffer is re-entrant), that it fills the
512                 buffer and starts dropping events.
513 
514          bytes: Bytes actually read (not overwritten).
515 
516          oldest event ts: The oldest timestamp in the buffer
517 
518          now ts: The current timestamp
519 
520          dropped events: Events lost due to overwrite option being off.
521 
522          read events: The number of events read.
523 
524 The Tracers
525 -----------
526 
527 Here is the list of current tracers that may be configured.
528 
529   "function"
530 
531         Function call tracer to trace all kernel functions.
532 
533   "function_graph"
534 
535         Similar to the function tracer except that the
536         function tracer probes the functions on their entry
537         whereas the function graph tracer traces on both entry
538         and exit of the functions. It then provides the ability
539         to draw a graph of function calls similar to C code
540         source.
541 
542   "irqsoff"
543 
544         Traces the areas that disable interrupts and saves
545         the trace with the longest max latency.
546         See tracing_max_latency. When a new max is recorded,
547         it replaces the old trace. It is best to view this
548         trace with the latency-format option enabled.
549 
550   "preemptoff"
551 
552         Similar to irqsoff but traces and records the amount of
553         time for which preemption is disabled.
554 
555   "preemptirqsoff"
556 
557         Similar to irqsoff and preemptoff, but traces and
558         records the largest time for which irqs and/or preemption
559         is disabled.
560 
561   "wakeup"
562 
563         Traces and records the max latency that it takes for
564         the highest priority task to get scheduled after
565         it has been woken up.
566         Traces all tasks as an average developer would expect.
567 
568   "wakeup_rt"
569 
570         Traces and records the max latency that it takes for just
571         RT tasks (as the current "wakeup" does). This is useful
572         for those interested in wake up timings of RT tasks.
573 
574   "nop"
575 
576         This is the "trace nothing" tracer. To remove all
577         tracers from tracing simply echo "nop" into
578         current_tracer.
579 
580 
581 Examples of using the tracer
582 ----------------------------
583 
584 Here are typical examples of using the tracers when controlling
585 them only with the debugfs interface (without using any
586 user-land utilities).
587 
588 Output format:
589 --------------
590 
591 Here is an example of the output format of the file "trace"
592 
593                              --------
594 # tracer: function
595 #
596 # entries-in-buffer/entries-written: 140080/250280   #P:4
597 #
598 #                              _-----=> irqs-off
599 #                             / _----=> need-resched
600 #                            | / _---=> hardirq/softirq
601 #                            || / _--=> preempt-depth
602 #                            ||| /     delay
603 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
604 #              | |       |   ||||       |         |
605             bash-1977  [000] .... 17284.993652: sys_close <-system_call_fastpath
606             bash-1977  [000] .... 17284.993653: __close_fd <-sys_close
607             bash-1977  [000] .... 17284.993653: _raw_spin_lock <-__close_fd
608             sshd-1974  [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
609             bash-1977  [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
610             bash-1977  [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
611             bash-1977  [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
612             bash-1977  [000] .... 17284.993657: filp_close <-__close_fd
613             bash-1977  [000] .... 17284.993657: dnotify_flush <-filp_close
614             sshd-1974  [003] .... 17284.993658: sys_select <-system_call_fastpath
615                              --------
616 
617 A header is printed with the tracer name that is represented by
618 the trace. In this case the tracer is "function". Then it shows the
619 number of events in the buffer as well as the total number of entries
620 that were written. The difference is the number of entries that were
621 lost due to the buffer filling up (250280 - 140080 = 110200 events
622 lost).
623 
624 The header explains the content of the events. Task name "bash", the task
625 PID "1977", the CPU that it was running on "000", the latency format
626 (explained below), the timestamp in <secs>.<usecs> format, the
627 function name that was traced "sys_close" and the parent function that
628 called this function "system_call_fastpath". The timestamp is the time
629 at which the function was entered.
630 
631 Latency trace format
632 --------------------
633 
634 When the latency-format option is enabled or when one of the latency
635 tracers is set, the trace file gives somewhat more information to see
636 why a latency happened. Here is a typical trace.
637 
638 # tracer: irqsoff
639 #
640 # irqsoff latency trace v1.1.5 on 3.8.0-test+
641 # --------------------------------------------------------------------
642 # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
643 #    -----------------
644 #    | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
645 #    -----------------
646 #  => started at: __lock_task_sighand
647 #  => ended at:   _raw_spin_unlock_irqrestore
648 #
649 #
650 #                  _------=> CPU#            
651 #                 / _-----=> irqs-off        
652 #                | / _----=> need-resched    
653 #                || / _---=> hardirq/softirq 
654 #                ||| / _--=> preempt-depth   
655 #                |||| /     delay             
656 #  cmd     pid   ||||| time  |   caller      
657 #     \   /      |||||  \    |   /           
658       ps-6143    2d...    0us!: trace_hardirqs_off <-__lock_task_sighand
659       ps-6143    2d..1  259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
660       ps-6143    2d..1  263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
661       ps-6143    2d..1  306us : <stack trace>
662  => trace_hardirqs_on_caller
663  => trace_hardirqs_on
664  => _raw_spin_unlock_irqrestore
665  => do_task_stat
666  => proc_tgid_stat
667  => proc_single_show
668  => seq_read
669  => vfs_read
670  => sys_read
671  => system_call_fastpath
672 
673 
674 This shows that the current tracer is "irqsoff" tracing the time
675 for which interrupts were disabled. It gives the trace version (which
676 never changes) and the version of the kernel upon which this was executed on
677 (3.10). Then it displays the max latency in microseconds (259 us). The number
678 of trace entries displayed and the total number (both are four: #4/4).
679 VP, KP, SP, and HP are always zero and are reserved for later use.
680 #P is the number of online CPUs (#P:4).
681 
682 The task is the process that was running when the latency
683 occurred. (ps pid: 6143).
684 
685 The start and stop (the functions in which the interrupts were
686 disabled and enabled respectively) that caused the latencies:
687 
688  __lock_task_sighand is where the interrupts were disabled.
689  _raw_spin_unlock_irqrestore is where they were enabled again.
690 
691 The next lines after the header are the trace itself. The header
692 explains which is which.
693 
694   cmd: The name of the process in the trace.
695 
696   pid: The PID of that process.
697 
698   CPU#: The CPU which the process was running on.
699 
700   irqs-off: 'd' interrupts are disabled. '.' otherwise.
701             Note: If the architecture does not support a way to
702                   read the irq flags variable, an 'X' will always
703                   be printed here.
704 
705   need-resched:
706         'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
707         'n' only TIF_NEED_RESCHED is set,
708         'p' only PREEMPT_NEED_RESCHED is set,
709         '.' otherwise.
710 
711   hardirq/softirq:
712         'H' - hard irq occurred inside a softirq.
713         'h' - hard irq is running
714         's' - soft irq is running
715         '.' - normal context.
716 
717   preempt-depth: The level of preempt_disabled
718 
719 The above is mostly meaningful for kernel developers.
720 
721   time: When the latency-format option is enabled, the trace file
722         output includes a timestamp relative to the start of the
723         trace. This differs from the output when latency-format
724         is disabled, which includes an absolute timestamp.
725 
726   delay: This is just to help catch your eye a bit better. And
727          needs to be fixed to be only relative to the same CPU.
728          The marks are determined by the difference between this
729          current trace and the next trace.
730           '$' - greater than 1 second
731           '@' - greater than 100 milisecond
732           '*' - greater than 10 milisecond
733           '#' - greater than 1000 microsecond
734           '!' - greater than 100 microsecond
735           '+' - greater than 10 microsecond
736           ' ' - less than or equal to 10 microsecond.
737 
738   The rest is the same as the 'trace' file.
739 
740   Note, the latency tracers will usually end with a back trace
741   to easily find where the latency occurred.
742 
743 trace_options
744 -------------
745 
746 The trace_options file (or the options directory) is used to control
747 what gets printed in the trace output, or manipulate the tracers.
748 To see what is available, simply cat the file:
749 
750   cat trace_options
751 print-parent
752 nosym-offset
753 nosym-addr
754 noverbose
755 noraw
756 nohex
757 nobin
758 noblock
759 trace_printk
760 nobranch
761 annotate
762 nouserstacktrace
763 nosym-userobj
764 noprintk-msg-only
765 context-info
766 nolatency-format
767 sleep-time
768 graph-time
769 record-cmd
770 overwrite
771 nodisable_on_free
772 irq-info
773 markers
774 noevent-fork
775 function-trace
776 nodisplay-graph
777 nostacktrace
778 
779 To disable one of the options, echo in the option prepended with
780 "no".
781 
782   echo noprint-parent > trace_options
783 
784 To enable an option, leave off the "no".
785 
786   echo sym-offset > trace_options
787 
788 Here are the available options:
789 
790   print-parent - On function traces, display the calling (parent)
791                  function as well as the function being traced.
792 
793   print-parent:
794    bash-4000  [01]  1477.606694: simple_strtoul <-kstrtoul
795 
796   noprint-parent:
797    bash-4000  [01]  1477.606694: simple_strtoul
798 
799 
800   sym-offset - Display not only the function name, but also the
801                offset in the function. For example, instead of
802                seeing just "ktime_get", you will see
803                "ktime_get+0xb/0x20".
804 
805   sym-offset:
806    bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0
807 
808   sym-addr - this will also display the function address as well
809              as the function name.
810 
811   sym-addr:
812    bash-4000  [01]  1477.606694: simple_strtoul <c0339346>
813 
814   verbose - This deals with the trace file when the
815             latency-format option is enabled.
816 
817     bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
818     (+0.000ms): simple_strtoul (kstrtoul)
819 
820   raw - This will display raw numbers. This option is best for
821         use with user applications that can translate the raw
822         numbers better than having it done in the kernel.
823 
824   hex - Similar to raw, but the numbers will be in a hexadecimal
825         format.
826 
827   bin - This will print out the formats in raw binary.
828 
829   block - When set, reading trace_pipe will not block when polled.
830 
831   trace_printk - Can disable trace_printk() from writing into the buffer.
832 
833   branch - Enable branch tracing with the tracer.
834 
835   annotate - It is sometimes confusing when the CPU buffers are full
836              and one CPU buffer had a lot of events recently, thus
837              a shorter time frame, were another CPU may have only had
838              a few events, which lets it have older events. When
839              the trace is reported, it shows the oldest events first,
840              and it may look like only one CPU ran (the one with the
841              oldest events). When the annotate option is set, it will
842              display when a new CPU buffer started:
843 
844           <idle>-0     [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
845           <idle>-0     [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
846           <idle>-0     [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
847 ##### CPU 2 buffer started ####
848           <idle>-0     [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
849           <idle>-0     [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
850           <idle>-0     [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
851 
852   userstacktrace - This option changes the trace. It records a
853                    stacktrace of the current userspace thread.
854 
855   sym-userobj - when user stacktrace are enabled, look up which
856                 object the address belongs to, and print a
857                 relative address. This is especially useful when
858                 ASLR is on, otherwise you don't get a chance to
859                 resolve the address to object/file/line after
860                 the app is no longer running
861 
862                 The lookup is performed when you read
863                 trace,trace_pipe. Example:
864 
865                 a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
866 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
867 
868 
869   printk-msg-only - When set, trace_printk()s will only show the format
870                     and not their parameters (if trace_bprintk() or
871                     trace_bputs() was used to save the trace_printk()).
872 
873   context-info - Show only the event data. Hides the comm, PID,
874                  timestamp, CPU, and other useful data.
875 
876   latency-format - This option changes the trace. When
877                    it is enabled, the trace displays
878                    additional information about the
879                    latencies, as described in "Latency
880                    trace format".
881 
882   sleep-time - When running function graph tracer, to include
883                the time a task schedules out in its function.
884                When enabled, it will account time the task has been
885                scheduled out as part of the function call.
886 
887   graph-time - When running function profiler with function graph tracer,
888                to include the time to call nested functions. When this is
889                not set, the time reported for the function will only
890                include the time the function itself executed for, not the
891                time for functions that it called.
892 
893   record-cmd - When any event or tracer is enabled, a hook is enabled
894                in the sched_switch trace point to fill comm cache
895                with mapped pids and comms. But this may cause some
896                overhead, and if you only care about pids, and not the
897                name of the task, disabling this option can lower the
898                impact of tracing.
899 
900   overwrite - This controls what happens when the trace buffer is
901               full. If "1" (default), the oldest events are
902               discarded and overwritten. If "0", then the newest
903               events are discarded.
904                 (see per_cpu/cpu0/stats for overrun and dropped)
905 
906   disable_on_free - When the free_buffer is closed, tracing will
907                     stop (tracing_on set to 0).
908 
909   irq-info - Shows the interrupt, preempt count, need resched data.
910              When disabled, the trace looks like:
911 
912 # tracer: function
913 #
914 # entries-in-buffer/entries-written: 144405/9452052   #P:4
915 #
916 #           TASK-PID   CPU#      TIMESTAMP  FUNCTION
917 #              | |       |          |         |
918           <idle>-0     [002]  23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
919           <idle>-0     [002]  23636.756054: activate_task <-ttwu_do_activate.constprop.89
920           <idle>-0     [002]  23636.756055: enqueue_task <-activate_task
921 
922 
923   markers - When set, the trace_marker is writable (only by root).
924             When disabled, the trace_marker will error with EINVAL
925             on write.
926 
927   event-fork - When set, tasks with PIDs listed in set_event_pid will have
928                the PIDs of their children added to set_event_pid when those
929                tasks fork. Also, when tasks with PIDs in set_event_pid exit,
930                their PIDs will be removed from the file.
931 
932   function-trace - The latency tracers will enable function tracing
933             if this option is enabled (default it is). When
934             it is disabled, the latency tracers do not trace
935             functions. This keeps the overhead of the tracer down
936             when performing latency tests.
937 
938   display-graph - When set, the latency tracers (irqsoff, wakeup, etc) will
939                   use function graph tracing instead of function tracing.
940 
941   stacktrace - This is one of the options that changes the trace
942                itself. When a trace is recorded, so is the stack
943                of functions. This allows for back traces of
944                trace sites.
945 
946  Note: Some tracers have their own options. They only appear in this
947        file when the tracer is active. They always appear in the
948        options directory.
949 
950 
951 
952 irqsoff
953 -------
954 
955 When interrupts are disabled, the CPU can not react to any other
956 external event (besides NMIs and SMIs). This prevents the timer
957 interrupt from triggering or the mouse interrupt from letting
958 the kernel know of a new mouse event. The result is a latency
959 with the reaction time.
960 
961 The irqsoff tracer tracks the time for which interrupts are
962 disabled. When a new maximum latency is hit, the tracer saves
963 the trace leading up to that latency point so that every time a
964 new maximum is reached, the old saved trace is discarded and the
965 new trace is saved.
966 
967 To reset the maximum, echo 0 into tracing_max_latency. Here is
968 an example:
969 
970  # echo 0 > options/function-trace
971  # echo irqsoff > current_tracer
972  # echo 1 > tracing_on
973  # echo 0 > tracing_max_latency
974  # ls -ltr
975  [...]
976  # echo 0 > tracing_on
977  # cat trace
978 # tracer: irqsoff
979 #
980 # irqsoff latency trace v1.1.5 on 3.8.0-test+
981 # --------------------------------------------------------------------
982 # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
983 #    -----------------
984 #    | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
985 #    -----------------
986 #  => started at: run_timer_softirq
987 #  => ended at:   run_timer_softirq
988 #
989 #
990 #                  _------=> CPU#            
991 #                 / _-----=> irqs-off        
992 #                | / _----=> need-resched    
993 #                || / _---=> hardirq/softirq 
994 #                ||| / _--=> preempt-depth   
995 #                |||| /     delay             
996 #  cmd     pid   ||||| time  |   caller      
997 #     \   /      |||||  \    |   /           
998   <idle>-0       0d.s2    0us+: _raw_spin_lock_irq <-run_timer_softirq
999   <idle>-0       0dNs3   17us : _raw_spin_unlock_irq <-run_timer_softirq
1000   <idle>-0       0dNs3   17us+: trace_hardirqs_on <-run_timer_softirq
1001   <idle>-0       0dNs3   25us : <stack trace>
1002  => _raw_spin_unlock_irq
1003  => run_timer_softirq
1004  => __do_softirq
1005  => call_softirq
1006  => do_softirq
1007  => irq_exit
1008  => smp_apic_timer_interrupt
1009  => apic_timer_interrupt
1010  => rcu_idle_exit
1011  => cpu_idle
1012  => rest_init
1013  => start_kernel
1014  => x86_64_start_reservations
1015  => x86_64_start_kernel
1016 
1017 Here we see that that we had a latency of 16 microseconds (which is
1018 very good). The _raw_spin_lock_irq in run_timer_softirq disabled
1019 interrupts. The difference between the 16 and the displayed
1020 timestamp 25us occurred because the clock was incremented
1021 between the time of recording the max latency and the time of
1022 recording the function that had that latency.
1023 
1024 Note the above example had function-trace not set. If we set
1025 function-trace, we get a much larger output:
1026 
1027  with echo 1 > options/function-trace
1028 
1029 # tracer: irqsoff
1030 #
1031 # irqsoff latency trace v1.1.5 on 3.8.0-test+
1032 # --------------------------------------------------------------------
1033 # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1034 #    -----------------
1035 #    | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
1036 #    -----------------
1037 #  => started at: ata_scsi_queuecmd
1038 #  => ended at:   ata_scsi_queuecmd
1039 #
1040 #
1041 #                  _------=> CPU#            
1042 #                 / _-----=> irqs-off        
1043 #                | / _----=> need-resched    
1044 #                || / _---=> hardirq/softirq 
1045 #                ||| / _--=> preempt-depth   
1046 #                |||| /     delay             
1047 #  cmd     pid   ||||| time  |   caller      
1048 #     \   /      |||||  \    |   /           
1049     bash-2042    3d...    0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1050     bash-2042    3d...    0us : add_preempt_count <-_raw_spin_lock_irqsave
1051     bash-2042    3d..1    1us : ata_scsi_find_dev <-ata_scsi_queuecmd
1052     bash-2042    3d..1    1us : __ata_scsi_find_dev <-ata_scsi_find_dev
1053     bash-2042    3d..1    2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
1054     bash-2042    3d..1    2us : ata_qc_new_init <-__ata_scsi_queuecmd
1055     bash-2042    3d..1    3us : ata_sg_init <-__ata_scsi_queuecmd
1056     bash-2042    3d..1    4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
1057     bash-2042    3d..1    4us : ata_build_rw_tf <-ata_scsi_rw_xlat
1058 [...]
1059     bash-2042    3d..1   67us : delay_tsc <-__delay
1060     bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
1061     bash-2042    3d..2   67us : sub_preempt_count <-delay_tsc
1062     bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
1063     bash-2042    3d..2   68us : sub_preempt_count <-delay_tsc
1064     bash-2042    3d..1   68us+: ata_bmdma_start <-ata_bmdma_qc_issue
1065     bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1066     bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1067     bash-2042    3d..1   72us+: trace_hardirqs_on <-ata_scsi_queuecmd
1068     bash-2042    3d..1  120us : <stack trace>
1069  => _raw_spin_unlock_irqrestore
1070  => ata_scsi_queuecmd
1071  => scsi_dispatch_cmd
1072  => scsi_request_fn
1073  => __blk_run_queue_uncond
1074  => __blk_run_queue
1075  => blk_queue_bio
1076  => generic_make_request
1077  => submit_bio
1078  => submit_bh
1079  => __ext3_get_inode_loc
1080  => ext3_iget
1081  => ext3_lookup
1082  => lookup_real
1083  => __lookup_hash
1084  => walk_component
1085  => lookup_last
1086  => path_lookupat
1087  => filename_lookup
1088  => user_path_at_empty
1089  => user_path_at
1090  => vfs_fstatat
1091  => vfs_stat
1092  => sys_newstat
1093  => system_call_fastpath
1094 
1095 
1096 Here we traced a 71 microsecond latency. But we also see all the
1097 functions that were called during that time. Note that by
1098 enabling function tracing, we incur an added overhead. This
1099 overhead may extend the latency times. But nevertheless, this
1100 trace has provided some very helpful debugging information.
1101 
1102 
1103 preemptoff
1104 ----------
1105 
1106 When preemption is disabled, we may be able to receive
1107 interrupts but the task cannot be preempted and a higher
1108 priority task must wait for preemption to be enabled again
1109 before it can preempt a lower priority task.
1110 
1111 The preemptoff tracer traces the places that disable preemption.
1112 Like the irqsoff tracer, it records the maximum latency for
1113 which preemption was disabled. The control of preemptoff tracer
1114 is much like the irqsoff tracer.
1115 
1116  # echo 0 > options/function-trace
1117  # echo preemptoff > current_tracer
1118  # echo 1 > tracing_on
1119  # echo 0 > tracing_max_latency
1120  # ls -ltr
1121  [...]
1122  # echo 0 > tracing_on
1123  # cat trace
1124 # tracer: preemptoff
1125 #
1126 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1127 # --------------------------------------------------------------------
1128 # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1129 #    -----------------
1130 #    | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
1131 #    -----------------
1132 #  => started at: do_IRQ
1133 #  => ended at:   do_IRQ
1134 #
1135 #
1136 #                  _------=> CPU#            
1137 #                 / _-----=> irqs-off        
1138 #                | / _----=> need-resched    
1139 #                || / _---=> hardirq/softirq 
1140 #                ||| / _--=> preempt-depth   
1141 #                |||| /     delay             
1142 #  cmd     pid   ||||| time  |   caller      
1143 #     \   /      |||||  \    |   /           
1144     sshd-1991    1d.h.    0us+: irq_enter <-do_IRQ
1145     sshd-1991    1d..1   46us : irq_exit <-do_IRQ
1146     sshd-1991    1d..1   47us+: trace_preempt_on <-do_IRQ
1147     sshd-1991    1d..1   52us : <stack trace>
1148  => sub_preempt_count
1149  => irq_exit
1150  => do_IRQ
1151  => ret_from_intr
1152 
1153 
1154 This has some more changes. Preemption was disabled when an
1155 interrupt came in (notice the 'h'), and was enabled on exit.
1156 But we also see that interrupts have been disabled when entering
1157 the preempt off section and leaving it (the 'd'). We do not know if
1158 interrupts were enabled in the mean time or shortly after this
1159 was over.
1160 
1161 # tracer: preemptoff
1162 #
1163 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1164 # --------------------------------------------------------------------
1165 # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1166 #    -----------------
1167 #    | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
1168 #    -----------------
1169 #  => started at: wake_up_new_task
1170 #  => ended at:   task_rq_unlock
1171 #
1172 #
1173 #                  _------=> CPU#            
1174 #                 / _-----=> irqs-off        
1175 #                | / _----=> need-resched    
1176 #                || / _---=> hardirq/softirq 
1177 #                ||| / _--=> preempt-depth   
1178 #                |||| /     delay             
1179 #  cmd     pid   ||||| time  |   caller      
1180 #     \   /      |||||  \    |   /           
1181     bash-1994    1d..1    0us : _raw_spin_lock_irqsave <-wake_up_new_task
1182     bash-1994    1d..1    0us : select_task_rq_fair <-select_task_rq
1183     bash-1994    1d..1    1us : __rcu_read_lock <-select_task_rq_fair
1184     bash-1994    1d..1    1us : source_load <-select_task_rq_fair
1185     bash-1994    1d..1    1us : source_load <-select_task_rq_fair
1186 [...]
1187     bash-1994    1d..1   12us : irq_enter <-smp_apic_timer_interrupt
1188     bash-1994    1d..1   12us : rcu_irq_enter <-irq_enter
1189     bash-1994    1d..1   13us : add_preempt_count <-irq_enter
1190     bash-1994    1d.h1   13us : exit_idle <-smp_apic_timer_interrupt
1191     bash-1994    1d.h1   13us : hrtimer_interrupt <-smp_apic_timer_interrupt
1192     bash-1994    1d.h1   13us : _raw_spin_lock <-hrtimer_interrupt
1193     bash-1994    1d.h1   14us : add_preempt_count <-_raw_spin_lock
1194     bash-1994    1d.h2   14us : ktime_get_update_offsets <-hrtimer_interrupt
1195 [...]
1196     bash-1994    1d.h1   35us : lapic_next_event <-clockevents_program_event
1197     bash-1994    1d.h1   35us : irq_exit <-smp_apic_timer_interrupt
1198     bash-1994    1d.h1   36us : sub_preempt_count <-irq_exit
1199     bash-1994    1d..2   36us : do_softirq <-irq_exit
1200     bash-1994    1d..2   36us : __do_softirq <-call_softirq
1201     bash-1994    1d..2   36us : __local_bh_disable <-__do_softirq
1202     bash-1994    1d.s2   37us : add_preempt_count <-_raw_spin_lock_irq
1203     bash-1994    1d.s3   38us : _raw_spin_unlock <-run_timer_softirq
1204     bash-1994    1d.s3   39us : sub_preempt_count <-_raw_spin_unlock
1205     bash-1994    1d.s2   39us : call_timer_fn <-run_timer_softirq
1206 [...]
1207     bash-1994    1dNs2   81us : cpu_needs_another_gp <-rcu_process_callbacks
1208     bash-1994    1dNs2   82us : __local_bh_enable <-__do_softirq
1209     bash-1994    1dNs2   82us : sub_preempt_count <-__local_bh_enable
1210     bash-1994    1dN.2   82us : idle_cpu <-irq_exit
1211     bash-1994    1dN.2   83us : rcu_irq_exit <-irq_exit
1212     bash-1994    1dN.2   83us : sub_preempt_count <-irq_exit
1213     bash-1994    1.N.1   84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
1214     bash-1994    1.N.1   84us+: trace_preempt_on <-task_rq_unlock
1215     bash-1994    1.N.1  104us : <stack trace>
1216  => sub_preempt_count
1217  => _raw_spin_unlock_irqrestore
1218  => task_rq_unlock
1219  => wake_up_new_task
1220  => do_fork
1221  => sys_clone
1222  => stub_clone
1223 
1224 
1225 The above is an example of the preemptoff trace with
1226 function-trace set. Here we see that interrupts were not disabled
1227 the entire time. The irq_enter code lets us know that we entered
1228 an interrupt 'h'. Before that, the functions being traced still
1229 show that it is not in an interrupt, but we can see from the
1230 functions themselves that this is not the case.
1231 
1232 preemptirqsoff
1233 --------------
1234 
1235 Knowing the locations that have interrupts disabled or
1236 preemption disabled for the longest times is helpful. But
1237 sometimes we would like to know when either preemption and/or
1238 interrupts are disabled.
1239 
1240 Consider the following code:
1241 
1242     local_irq_disable();
1243     call_function_with_irqs_off();
1244     preempt_disable();
1245     call_function_with_irqs_and_preemption_off();
1246     local_irq_enable();
1247     call_function_with_preemption_off();
1248     preempt_enable();
1249 
1250 The irqsoff tracer will record the total length of
1251 call_function_with_irqs_off() and
1252 call_function_with_irqs_and_preemption_off().
1253 
1254 The preemptoff tracer will record the total length of
1255 call_function_with_irqs_and_preemption_off() and
1256 call_function_with_preemption_off().
1257 
1258 But neither will trace the time that interrupts and/or
1259 preemption is disabled. This total time is the time that we can
1260 not schedule. To record this time, use the preemptirqsoff
1261 tracer.
1262 
1263 Again, using this trace is much like the irqsoff and preemptoff
1264 tracers.
1265 
1266  # echo 0 > options/function-trace
1267  # echo preemptirqsoff > current_tracer
1268  # echo 1 > tracing_on
1269  # echo 0 > tracing_max_latency
1270  # ls -ltr
1271  [...]
1272  # echo 0 > tracing_on
1273  # cat trace
1274 # tracer: preemptirqsoff
1275 #
1276 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1277 # --------------------------------------------------------------------
1278 # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1279 #    -----------------
1280 #    | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
1281 #    -----------------
1282 #  => started at: ata_scsi_queuecmd
1283 #  => ended at:   ata_scsi_queuecmd
1284 #
1285 #
1286 #                  _------=> CPU#            
1287 #                 / _-----=> irqs-off        
1288 #                | / _----=> need-resched    
1289 #                || / _---=> hardirq/softirq 
1290 #                ||| / _--=> preempt-depth   
1291 #                |||| /     delay             
1292 #  cmd     pid   ||||| time  |   caller      
1293 #     \   /      |||||  \    |   /           
1294       ls-2230    3d...    0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1295       ls-2230    3...1  100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1296       ls-2230    3...1  101us+: trace_preempt_on <-ata_scsi_queuecmd
1297       ls-2230    3...1  111us : <stack trace>
1298  => sub_preempt_count
1299  => _raw_spin_unlock_irqrestore
1300  => ata_scsi_queuecmd
1301  => scsi_dispatch_cmd
1302  => scsi_request_fn
1303  => __blk_run_queue_uncond
1304  => __blk_run_queue
1305  => blk_queue_bio
1306  => generic_make_request
1307  => submit_bio
1308  => submit_bh
1309  => ext3_bread
1310  => ext3_dir_bread
1311  => htree_dirblock_to_tree
1312  => ext3_htree_fill_tree
1313  => ext3_readdir
1314  => vfs_readdir
1315  => sys_getdents
1316  => system_call_fastpath
1317 
1318 
1319 The trace_hardirqs_off_thunk is called from assembly on x86 when
1320 interrupts are disabled in the assembly code. Without the
1321 function tracing, we do not know if interrupts were enabled
1322 within the preemption points. We do see that it started with
1323 preemption enabled.
1324 
1325 Here is a trace with function-trace set:
1326 
1327 # tracer: preemptirqsoff
1328 #
1329 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1330 # --------------------------------------------------------------------
1331 # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1332 #    -----------------
1333 #    | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
1334 #    -----------------
1335 #  => started at: schedule
1336 #  => ended at:   mutex_unlock
1337 #
1338 #
1339 #                  _------=> CPU#            
1340 #                 / _-----=> irqs-off        
1341 #                | / _----=> need-resched    
1342 #                || / _---=> hardirq/softirq 
1343 #                ||| / _--=> preempt-depth   
1344 #                |||| /     delay             
1345 #  cmd     pid   ||||| time  |   caller      
1346 #     \   /      |||||  \    |   /           
1347 kworker/-59      3...1    0us : __schedule <-schedule
1348 kworker/-59      3d..1    0us : rcu_preempt_qs <-rcu_note_context_switch
1349 kworker/-59      3d..1    1us : add_preempt_count <-_raw_spin_lock_irq
1350 kworker/-59      3d..2    1us : deactivate_task <-__schedule
1351 kworker/-59      3d..2    1us : dequeue_task <-deactivate_task
1352 kworker/-59      3d..2    2us : update_rq_clock <-dequeue_task
1353 kworker/-59      3d..2    2us : dequeue_task_fair <-dequeue_task
1354 kworker/-59      3d..2    2us : update_curr <-dequeue_task_fair
1355 kworker/-59      3d..2    2us : update_min_vruntime <-update_curr
1356 kworker/-59      3d..2    3us : cpuacct_charge <-update_curr
1357 kworker/-59      3d..2    3us : __rcu_read_lock <-cpuacct_charge
1358 kworker/-59      3d..2    3us : __rcu_read_unlock <-cpuacct_charge
1359 kworker/-59      3d..2    3us : update_cfs_rq_blocked_load <-dequeue_task_fair
1360 kworker/-59      3d..2    4us : clear_buddies <-dequeue_task_fair
1361 kworker/-59      3d..2    4us : account_entity_dequeue <-dequeue_task_fair
1362 kworker/-59      3d..2    4us : update_min_vruntime <-dequeue_task_fair
1363 kworker/-59      3d..2    4us : update_cfs_shares <-dequeue_task_fair
1364 kworker/-59      3d..2    5us : hrtick_update <-dequeue_task_fair
1365 kworker/-59      3d..2    5us : wq_worker_sleeping <-__schedule
1366 kworker/-59      3d..2    5us : kthread_data <-wq_worker_sleeping
1367 kworker/-59      3d..2    5us : put_prev_task_fair <-__schedule
1368 kworker/-59      3d..2    6us : pick_next_task_fair <-pick_next_task
1369 kworker/-59      3d..2    6us : clear_buddies <-pick_next_task_fair
1370 kworker/-59      3d..2    6us : set_next_entity <-pick_next_task_fair
1371 kworker/-59      3d..2    6us : update_stats_wait_end <-set_next_entity
1372       ls-2269    3d..2    7us : finish_task_switch <-__schedule
1373       ls-2269    3d..2    7us : _raw_spin_unlock_irq <-finish_task_switch
1374       ls-2269    3d..2    8us : do_IRQ <-ret_from_intr
1375       ls-2269    3d..2    8us : irq_enter <-do_IRQ
1376       ls-2269    3d..2    8us : rcu_irq_enter <-irq_enter
1377       ls-2269    3d..2    9us : add_preempt_count <-irq_enter
1378       ls-2269    3d.h2    9us : exit_idle <-do_IRQ
1379 [...]
1380       ls-2269    3d.h3   20us : sub_preempt_count <-_raw_spin_unlock
1381       ls-2269    3d.h2   20us : irq_exit <-do_IRQ
1382       ls-2269    3d.h2   21us : sub_preempt_count <-irq_exit
1383       ls-2269    3d..3   21us : do_softirq <-irq_exit
1384       ls-2269    3d..3   21us : __do_softirq <-call_softirq
1385       ls-2269    3d..3   21us+: __local_bh_disable <-__do_softirq
1386       ls-2269    3d.s4   29us : sub_preempt_count <-_local_bh_enable_ip
1387       ls-2269    3d.s5   29us : sub_preempt_count <-_local_bh_enable_ip
1388       ls-2269    3d.s5   31us : do_IRQ <-ret_from_intr
1389       ls-2269    3d.s5   31us : irq_enter <-do_IRQ
1390       ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
1391 [...]
1392       ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
1393       ls-2269    3d.s5   32us : add_preempt_count <-irq_enter
1394       ls-2269    3d.H5   32us : exit_idle <-do_IRQ
1395       ls-2269    3d.H5   32us : handle_irq <-do_IRQ
1396       ls-2269    3d.H5   32us : irq_to_desc <-handle_irq
1397       ls-2269    3d.H5   33us : handle_fasteoi_irq <-handle_irq
1398 [...]
1399       ls-2269    3d.s5  158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
1400       ls-2269    3d.s3  158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
1401       ls-2269    3d.s3  159us : __local_bh_enable <-__do_softirq
1402       ls-2269    3d.s3  159us : sub_preempt_count <-__local_bh_enable
1403       ls-2269    3d..3  159us : idle_cpu <-irq_exit
1404       ls-2269    3d..3  159us : rcu_irq_exit <-irq_exit
1405       ls-2269    3d..3  160us : sub_preempt_count <-irq_exit
1406       ls-2269    3d...  161us : __mutex_unlock_slowpath <-mutex_unlock
1407       ls-2269    3d...  162us+: trace_hardirqs_on <-mutex_unlock
1408       ls-2269    3d...  186us : <stack trace>
1409  => __mutex_unlock_slowpath
1410  => mutex_unlock
1411  => process_output
1412  => n_tty_write
1413  => tty_write
1414  => vfs_write
1415  => sys_write
1416  => system_call_fastpath
1417 
1418 This is an interesting trace. It started with kworker running and
1419 scheduling out and ls taking over. But as soon as ls released the
1420 rq lock and enabled interrupts (but not preemption) an interrupt
1421 triggered. When the interrupt finished, it started running softirqs.
1422 But while the softirq was running, another interrupt triggered.
1423 When an interrupt is running inside a softirq, the annotation is 'H'.
1424 
1425 
1426 wakeup
1427 ------
1428 
1429 One common case that people are interested in tracing is the
1430 time it takes for a task that is woken to actually wake up.
1431 Now for non Real-Time tasks, this can be arbitrary. But tracing
1432 it none the less can be interesting. 
1433 
1434 Without function tracing:
1435 
1436  # echo 0 > options/function-trace
1437  # echo wakeup > current_tracer
1438  # echo 1 > tracing_on
1439  # echo 0 > tracing_max_latency
1440  # chrt -f 5 sleep 1
1441  # echo 0 > tracing_on
1442  # cat trace
1443 # tracer: wakeup
1444 #
1445 # wakeup latency trace v1.1.5 on 3.8.0-test+
1446 # --------------------------------------------------------------------
1447 # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1448 #    -----------------
1449 #    | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
1450 #    -----------------
1451 #
1452 #                  _------=> CPU#            
1453 #                 / _-----=> irqs-off        
1454 #                | / _----=> need-resched    
1455 #                || / _---=> hardirq/softirq 
1456 #                ||| / _--=> preempt-depth   
1457 #                |||| /     delay             
1458 #  cmd     pid   ||||| time  |   caller      
1459 #     \   /      |||||  \    |   /           
1460   <idle>-0       3dNs7    0us :      0:120:R   + [003]   312:100:R kworker/3:1H
1461   <idle>-0       3dNs7    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1462   <idle>-0       3d..3   15us : __schedule <-schedule
1463   <idle>-0       3d..3   15us :      0:120:R ==> [003]   312:100:R kworker/3:1H
1464 
1465 The tracer only traces the highest priority task in the system
1466 to avoid tracing the normal circumstances. Here we see that
1467 the kworker with a nice priority of -20 (not very nice), took
1468 just 15 microseconds from the time it woke up, to the time it
1469 ran.
1470 
1471 Non Real-Time tasks are not that interesting. A more interesting
1472 trace is to concentrate only on Real-Time tasks.
1473 
1474 wakeup_rt
1475 ---------
1476 
1477 In a Real-Time environment it is very important to know the
1478 wakeup time it takes for the highest priority task that is woken
1479 up to the time that it executes. This is also known as "schedule
1480 latency". I stress the point that this is about RT tasks. It is
1481 also important to know the scheduling latency of non-RT tasks,
1482 but the average schedule latency is better for non-RT tasks.
1483 Tools like LatencyTop are more appropriate for such
1484 measurements.
1485 
1486 Real-Time environments are interested in the worst case latency.
1487 That is the longest latency it takes for something to happen,
1488 and not the average. We can have a very fast scheduler that may
1489 only have a large latency once in a while, but that would not
1490 work well with Real-Time tasks.  The wakeup_rt tracer was designed
1491 to record the worst case wakeups of RT tasks. Non-RT tasks are
1492 not recorded because the tracer only records one worst case and
1493 tracing non-RT tasks that are unpredictable will overwrite the
1494 worst case latency of RT tasks (just run the normal wakeup
1495 tracer for a while to see that effect).
1496 
1497 Since this tracer only deals with RT tasks, we will run this
1498 slightly differently than we did with the previous tracers.
1499 Instead of performing an 'ls', we will run 'sleep 1' under
1500 'chrt' which changes the priority of the task.
1501 
1502  # echo 0 > options/function-trace
1503  # echo wakeup_rt > current_tracer
1504  # echo 1 > tracing_on
1505  # echo 0 > tracing_max_latency
1506  # chrt -f 5 sleep 1
1507  # echo 0 > tracing_on
1508  # cat trace
1509 # tracer: wakeup
1510 #
1511 # tracer: wakeup_rt
1512 #
1513 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1514 # --------------------------------------------------------------------
1515 # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1516 #    -----------------
1517 #    | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
1518 #    -----------------
1519 #
1520 #                  _------=> CPU#            
1521 #                 / _-----=> irqs-off        
1522 #                | / _----=> need-resched    
1523 #                || / _---=> hardirq/softirq 
1524 #                ||| / _--=> preempt-depth   
1525 #                |||| /     delay             
1526 #  cmd     pid   ||||| time  |   caller      
1527 #     \   /      |||||  \    |   /           
1528   <idle>-0       3d.h4    0us :      0:120:R   + [003]  2389: 94:R sleep
1529   <idle>-0       3d.h4    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1530   <idle>-0       3d..3    5us : __schedule <-schedule
1531   <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
1532 
1533 
1534 Running this on an idle system, we see that it only took 5 microseconds
1535 to perform the task switch.  Note, since the trace point in the schedule
1536 is before the actual "switch", we stop the tracing when the recorded task
1537 is about to schedule in. This may change if we add a new marker at the
1538 end of the scheduler.
1539 
1540 Notice that the recorded task is 'sleep' with the PID of 2389
1541 and it has an rt_prio of 5. This priority is user-space priority
1542 and not the internal kernel priority. The policy is 1 for
1543 SCHED_FIFO and 2 for SCHED_RR.
1544 
1545 Note, that the trace data shows the internal priority (99 - rtprio).
1546 
1547   <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
1548 
1549 The 0:120:R means idle was running with a nice priority of 0 (120 - 20)
1550 and in the running state 'R'. The sleep task was scheduled in with
1551 2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
1552 and it too is in the running state.
1553 
1554 Doing the same with chrt -r 5 and function-trace set.
1555 
1556   echo 1 > options/function-trace
1557 
1558 # tracer: wakeup_rt
1559 #
1560 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1561 # --------------------------------------------------------------------
1562 # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1563 #    -----------------
1564 #    | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
1565 #    -----------------
1566 #
1567 #                  _------=> CPU#            
1568 #                 / _-----=> irqs-off        
1569 #                | / _----=> need-resched    
1570 #                || / _---=> hardirq/softirq 
1571 #                ||| / _--=> preempt-depth   
1572 #                |||| /     delay             
1573 #  cmd     pid   ||||| time  |   caller      
1574 #     \   /      |||||  \    |   /           
1575   <idle>-0       3d.h4    1us+:      0:120:R   + [003]  2448: 94:R sleep
1576   <idle>-0       3d.h4    2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
1577   <idle>-0       3d.h3    3us : check_preempt_curr <-ttwu_do_wakeup
1578   <idle>-0       3d.h3    3us : resched_curr <-check_preempt_curr
1579   <idle>-0       3dNh3    4us : task_woken_rt <-ttwu_do_wakeup
1580   <idle>-0       3dNh3    4us : _raw_spin_unlock <-try_to_wake_up
1581   <idle>-0       3dNh3    4us : sub_preempt_count <-_raw_spin_unlock
1582   <idle>-0       3dNh2    5us : ttwu_stat <-try_to_wake_up
1583   <idle>-0       3dNh2    5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
1584   <idle>-0       3dNh2    6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1585   <idle>-0       3dNh1    6us : _raw_spin_lock <-__run_hrtimer
1586   <idle>-0       3dNh1    6us : add_preempt_count <-_raw_spin_lock
1587   <idle>-0       3dNh2    7us : _raw_spin_unlock <-hrtimer_interrupt
1588   <idle>-0       3dNh2    7us : sub_preempt_count <-_raw_spin_unlock
1589   <idle>-0       3dNh1    7us : tick_program_event <-hrtimer_interrupt
1590   <idle>-0       3dNh1    7us : clockevents_program_event <-tick_program_event
1591   <idle>-0       3dNh1    8us : ktime_get <-clockevents_program_event
1592   <idle>-0       3dNh1    8us : lapic_next_event <-clockevents_program_event
1593   <idle>-0       3dNh1    8us : irq_exit <-smp_apic_timer_interrupt
1594   <idle>-0       3dNh1    9us : sub_preempt_count <-irq_exit
1595   <idle>-0       3dN.2    9us : idle_cpu <-irq_exit
1596   <idle>-0       3dN.2    9us : rcu_irq_exit <-irq_exit
1597   <idle>-0       3dN.2   10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
1598   <idle>-0       3dN.2   10us : sub_preempt_count <-irq_exit
1599   <idle>-0       3.N.1   11us : rcu_idle_exit <-cpu_idle
1600   <idle>-0       3dN.1   11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
1601   <idle>-0       3.N.1   11us : tick_nohz_idle_exit <-cpu_idle
1602   <idle>-0       3dN.1   12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
1603   <idle>-0       3dN.1   12us : ktime_get <-tick_nohz_idle_exit
1604   <idle>-0       3dN.1   12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
1605   <idle>-0       3dN.1   13us : cpu_load_update_nohz <-tick_nohz_idle_exit
1606   <idle>-0       3dN.1   13us : _raw_spin_lock <-cpu_load_update_nohz
1607   <idle>-0       3dN.1   13us : add_preempt_count <-_raw_spin_lock
1608   <idle>-0       3dN.2   13us : __cpu_load_update <-cpu_load_update_nohz
1609   <idle>-0       3dN.2   14us : sched_avg_update <-__cpu_load_update
1610   <idle>-0       3dN.2   14us : _raw_spin_unlock <-cpu_load_update_nohz
1611   <idle>-0       3dN.2   14us : sub_preempt_count <-_raw_spin_unlock
1612   <idle>-0       3dN.1   15us : calc_load_exit_idle <-tick_nohz_idle_exit
1613   <idle>-0       3dN.1   15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
1614   <idle>-0       3dN.1   15us : hrtimer_cancel <-tick_nohz_idle_exit
1615   <idle>-0       3dN.1   15us : hrtimer_try_to_cancel <-hrtimer_cancel
1616   <idle>-0       3dN.1   16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
1617   <idle>-0       3dN.1   16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
1618   <idle>-0       3dN.1   16us : add_preempt_count <-_raw_spin_lock_irqsave
1619   <idle>-0       3dN.2   17us : __remove_hrtimer <-remove_hrtimer.part.16
1620   <idle>-0       3dN.2   17us : hrtimer_force_reprogram <-__remove_hrtimer
1621   <idle>-0       3dN.2   17us : tick_program_event <-hrtimer_force_reprogram
1622   <idle>-0       3dN.2   18us : clockevents_program_event <-tick_program_event
1623   <idle>-0       3dN.2   18us : ktime_get <-clockevents_program_event
1624   <idle>-0       3dN.2   18us : lapic_next_event <-clockevents_program_event
1625   <idle>-0       3dN.2   19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
1626   <idle>-0       3dN.2   19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1627   <idle>-0       3dN.1   19us : hrtimer_forward <-tick_nohz_idle_exit
1628   <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
1629   <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
1630   <idle>-0       3dN.1   20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
1631   <idle>-0       3dN.1   20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
1632   <idle>-0       3dN.1   21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
1633   <idle>-0       3dN.1   21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
1634   <idle>-0       3dN.1   21us : add_preempt_count <-_raw_spin_lock_irqsave
1635   <idle>-0       3dN.2   22us : ktime_add_safe <-__hrtimer_start_range_ns
1636   <idle>-0       3dN.2   22us : enqueue_hrtimer <-__hrtimer_start_range_ns
1637   <idle>-0       3dN.2   22us : tick_program_event <-__hrtimer_start_range_ns
1638   <idle>-0       3dN.2   23us : clockevents_program_event <-tick_program_event
1639   <idle>-0       3dN.2   23us : ktime_get <-clockevents_program_event
1640   <idle>-0       3dN.2   23us : lapic_next_event <-clockevents_program_event
1641   <idle>-0       3dN.2   24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
1642   <idle>-0       3dN.2   24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1643   <idle>-0       3dN.1   24us : account_idle_ticks <-tick_nohz_idle_exit
1644   <idle>-0       3dN.1   24us : account_idle_time <-account_idle_ticks
1645   <idle>-0       3.N.1   25us : sub_preempt_count <-cpu_idle
1646   <idle>-0       3.N..   25us : schedule <-cpu_idle
1647   <idle>-0       3.N..   25us : __schedule <-preempt_schedule
1648   <idle>-0       3.N..   26us : add_preempt_count <-__schedule
1649   <idle>-0       3.N.1   26us : rcu_note_context_switch <-__schedule
1650   <idle>-0       3.N.1   26us : rcu_sched_qs <-rcu_note_context_switch
1651   <idle>-0       3dN.1   27us : rcu_preempt_qs <-rcu_note_context_switch
1652   <idle>-0       3.N.1   27us : _raw_spin_lock_irq <-__schedule
1653   <idle>-0       3dN.1   27us : add_preempt_count <-_raw_spin_lock_irq
1654   <idle>-0       3dN.2   28us : put_prev_task_idle <-__schedule
1655   <idle>-0       3dN.2   28us : pick_next_task_stop <-pick_next_task
1656   <idle>-0       3dN.2   28us : pick_next_task_rt <-pick_next_task
1657   <idle>-0       3dN.2   29us : dequeue_pushable_task <-pick_next_task_rt
1658   <idle>-0       3d..3   29us : __schedule <-preempt_schedule
1659   <idle>-0       3d..3   30us :      0:120:R ==> [003]  2448: 94:R sleep
1660 
1661 This isn't that big of a trace, even with function tracing enabled,
1662 so I included the entire trace.
1663 
1664 The interrupt went off while when the system was idle. Somewhere
1665 before task_woken_rt() was called, the NEED_RESCHED flag was set,
1666 this is indicated by the first occurrence of the 'N' flag.
1667 
1668 Latency tracing and events
1669 --------------------------
1670 As function tracing can induce a much larger latency, but without
1671 seeing what happens within the latency it is hard to know what
1672 caused it. There is a middle ground, and that is with enabling
1673 events.
1674 
1675  # echo 0 > options/function-trace
1676  # echo wakeup_rt > current_tracer
1677  # echo 1 > events/enable
1678  # echo 1 > tracing_on
1679  # echo 0 > tracing_max_latency
1680  # chrt -f 5 sleep 1
1681  # echo 0 > tracing_on
1682  # cat trace
1683 # tracer: wakeup_rt
1684 #
1685 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1686 # --------------------------------------------------------------------
1687 # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1688 #    -----------------
1689 #    | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
1690 #    -----------------
1691 #
1692 #                  _------=> CPU#            
1693 #                 / _-----=> irqs-off        
1694 #                | / _----=> need-resched    
1695 #                || / _---=> hardirq/softirq 
1696 #                ||| / _--=> preempt-depth   
1697 #                |||| /     delay             
1698 #  cmd     pid   ||||| time  |   caller      
1699 #     \   /      |||||  \    |   /           
1700   <idle>-0       2d.h4    0us :      0:120:R   + [002]  5882: 94:R sleep
1701   <idle>-0       2d.h4    0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
1702   <idle>-0       2d.h4    1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
1703   <idle>-0       2dNh2    1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
1704   <idle>-0       2.N.2    2us : power_end: cpu_id=2
1705   <idle>-0       2.N.2    3us : cpu_idle: state=4294967295 cpu_id=2
1706   <idle>-0       2dN.3    4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
1707   <idle>-0       2dN.3    4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
1708   <idle>-0       2.N.2    5us : rcu_utilization: Start context switch
1709   <idle>-0       2.N.2    5us : rcu_utilization: End context switch
1710   <idle>-0       2d..3    6us : __schedule <-schedule
1711   <idle>-0       2d..3    6us :      0:120:R ==> [002]  5882: 94:R sleep
1712 
1713 
1714 function
1715 --------
1716 
1717 This tracer is the function tracer. Enabling the function tracer
1718 can be done from the debug file system. Make sure the
1719 ftrace_enabled is set; otherwise this tracer is a nop.
1720 See the "ftrace_enabled" section below.
1721 
1722  # sysctl kernel.ftrace_enabled=1
1723  # echo function > current_tracer
1724  # echo 1 > tracing_on
1725  # usleep 1
1726  # echo 0 > tracing_on
1727  # cat trace
1728 # tracer: function
1729 #
1730 # entries-in-buffer/entries-written: 24799/24799   #P:4
1731 #
1732 #                              _-----=> irqs-off
1733 #                             / _----=> need-resched
1734 #                            | / _---=> hardirq/softirq
1735 #                            || / _--=> preempt-depth
1736 #                            ||| /     delay
1737 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
1738 #              | |       |   ||||       |         |
1739             bash-1994  [002] ....  3082.063030: mutex_unlock <-rb_simple_write
1740             bash-1994  [002] ....  3082.063031: __mutex_unlock_slowpath <-mutex_unlock
1741             bash-1994  [002] ....  3082.063031: __fsnotify_parent <-fsnotify_modify
1742             bash-1994  [002] ....  3082.063032: fsnotify <-fsnotify_modify
1743             bash-1994  [002] ....  3082.063032: __srcu_read_lock <-fsnotify
1744             bash-1994  [002] ....  3082.063032: add_preempt_count <-__srcu_read_lock
1745             bash-1994  [002] ...1  3082.063032: sub_preempt_count <-__srcu_read_lock
1746             bash-1994  [002] ....  3082.063033: __srcu_read_unlock <-fsnotify
1747 [...]
1748 
1749 
1750 Note: function tracer uses ring buffers to store the above
1751 entries. The newest data may overwrite the oldest data.
1752 Sometimes using echo to stop the trace is not sufficient because
1753 the tracing could have overwritten the data that you wanted to
1754 record. For this reason, it is sometimes better to disable
1755 tracing directly from a program. This allows you to stop the
1756 tracing at the point that you hit the part that you are
1757 interested in. To disable the tracing directly from a C program,
1758 something like following code snippet can be used:
1759 
1760 int trace_fd;
1761 [...]
1762 int main(int argc, char *argv[]) {
1763         [...]
1764         trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
1765         [...]
1766         if (condition_hit()) {
1767                 write(trace_fd, "0", 1);
1768         }
1769         [...]
1770 }
1771 
1772 
1773 Single thread tracing
1774 ---------------------
1775 
1776 By writing into set_ftrace_pid you can trace a
1777 single thread. For example:
1778 
1779 # cat set_ftrace_pid
1780 no pid
1781 # echo 3111 > set_ftrace_pid
1782 # cat set_ftrace_pid
1783 3111
1784 # echo function > current_tracer
1785 # cat trace | head
1786  # tracer: function
1787  #
1788  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
1789  #              | |       |          |         |
1790      yum-updatesd-3111  [003]  1637.254676: finish_task_switch <-thread_return
1791      yum-updatesd-3111  [003]  1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
1792      yum-updatesd-3111  [003]  1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
1793      yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
1794      yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
1795      yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll
1796 # echo > set_ftrace_pid
1797 # cat trace |head
1798  # tracer: function
1799  #
1800  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
1801  #              | |       |          |         |
1802  ##### CPU 3 buffer started ####
1803      yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
1804      yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
1805      yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
1806      yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
1807      yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit
1808 
1809 If you want to trace a function when executing, you could use
1810 something like this simple program:
1811 
1812 #include <stdio.h>
1813 #include <stdlib.h>
1814 #include <sys/types.h>
1815 #include <sys/stat.h>
1816 #include <fcntl.h>
1817 #include <unistd.h>
1818 #include <string.h>
1819 
1820 #define _STR(x) #x
1821 #define STR(x) _STR(x)
1822 #define MAX_PATH 256
1823 
1824 const char *find_debugfs(void)
1825 {
1826        static char debugfs[MAX_PATH+1];
1827        static int debugfs_found;
1828        char type[100];
1829        FILE *fp;
1830 
1831        if (debugfs_found)
1832                return debugfs;
1833 
1834        if ((fp = fopen("/proc/mounts","r")) == NULL) {
1835                perror("/proc/mounts");
1836                return NULL;
1837        }
1838 
1839        while (fscanf(fp, "%*s %"
1840                      STR(MAX_PATH)
1841                      "s %99s %*s %*d %*d\n",
1842                      debugfs, type) == 2) {
1843                if (strcmp(type, "debugfs") == 0)
1844                        break;
1845        }
1846        fclose(fp);
1847 
1848        if (strcmp(type, "debugfs") != 0) {
1849                fprintf(stderr, "debugfs not mounted");
1850                return NULL;
1851        }
1852 
1853        strcat(debugfs, "/tracing/");
1854        debugfs_found = 1;
1855 
1856        return debugfs;
1857 }
1858 
1859 const char *tracing_file(const char *file_name)
1860 {
1861        static char trace_file[MAX_PATH+1];
1862        snprintf(trace_file, MAX_PATH, "%s/%s", find_debugfs(), file_name);
1863        return trace_file;
1864 }
1865 
1866 int main (int argc, char **argv)
1867 {
1868         if (argc < 1)
1869                 exit(-1);
1870 
1871         if (fork() > 0) {
1872                 int fd, ffd;
1873                 char line[64];
1874                 int s;
1875 
1876                 ffd = open(tracing_file("current_tracer"), O_WRONLY);
1877                 if (ffd < 0)
1878                         exit(-1);
1879                 write(ffd, "nop", 3);
1880 
1881                 fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
1882                 s = sprintf(line, "%d\n", getpid());
1883                 write(fd, line, s);
1884 
1885                 write(ffd, "function", 8);
1886 
1887                 close(fd);
1888                 close(ffd);
1889 
1890                 execvp(argv[1], argv+1);
1891         }
1892 
1893         return 0;
1894 }
1895 
1896 Or this simple script!
1897 
1898 ------
1899 #!/bin/bash
1900 
1901 debugfs=`sed -ne 's/^debugfs \(.*\) debugfs.*/\1/p' /proc/mounts`
1902 echo nop > $debugfs/tracing/current_tracer
1903 echo 0 > $debugfs/tracing/tracing_on
1904 echo $$ > $debugfs/tracing/set_ftrace_pid
1905 echo function > $debugfs/tracing/current_tracer
1906 echo 1 > $debugfs/tracing/tracing_on
1907 exec "$@"
1908 ------
1909 
1910 
1911 function graph tracer
1912 ---------------------------
1913 
1914 This tracer is similar to the function tracer except that it
1915 probes a function on its entry and its exit. This is done by
1916 using a dynamically allocated stack of return addresses in each
1917 task_struct. On function entry the tracer overwrites the return
1918 address of each function traced to set a custom probe. Thus the
1919 original return address is stored on the stack of return address
1920 in the task_struct.
1921 
1922 Probing on both ends of a function leads to special features
1923 such as:
1924 
1925 - measure of a function's time execution
1926 - having a reliable call stack to draw function calls graph
1927 
1928 This tracer is useful in several situations:
1929 
1930 - you want to find the reason of a strange kernel behavior and
1931   need to see what happens in detail on any areas (or specific
1932   ones).
1933 
1934 - you are experiencing weird latencies but it's difficult to
1935   find its origin.
1936 
1937 - you want to find quickly which path is taken by a specific
1938   function
1939 
1940 - you just want to peek inside a working kernel and want to see
1941   what happens there.
1942 
1943 # tracer: function_graph
1944 #
1945 # CPU  DURATION                  FUNCTION CALLS
1946 # |     |   |                     |   |   |   |
1947 
1948  0)               |  sys_open() {
1949  0)               |    do_sys_open() {
1950  0)               |      getname() {
1951  0)               |        kmem_cache_alloc() {
1952  0)   1.382 us    |          __might_sleep();
1953  0)   2.478 us    |        }
1954  0)               |        strncpy_from_user() {
1955  0)               |          might_fault() {
1956  0)   1.389 us    |            __might_sleep();
1957  0)   2.553 us    |          }
1958  0)   3.807 us    |        }
1959  0)   7.876 us    |      }
1960  0)               |      alloc_fd() {
1961  0)   0.668 us    |        _spin_lock();
1962  0)   0.570 us    |        expand_files();
1963  0)   0.586 us    |        _spin_unlock();
1964 
1965 
1966 There are several columns that can be dynamically
1967 enabled/disabled. You can use every combination of options you
1968 want, depending on your needs.
1969 
1970 - The cpu number on which the function executed is default
1971   enabled.  It is sometimes better to only trace one cpu (see
1972   tracing_cpu_mask file) or you might sometimes see unordered
1973   function calls while cpu tracing switch.
1974 
1975         hide: echo nofuncgraph-cpu > trace_options
1976         show: echo funcgraph-cpu > trace_options
1977 
1978 - The duration (function's time of execution) is displayed on
1979   the closing bracket line of a function or on the same line
1980   than the current function in case of a leaf one. It is default
1981   enabled.
1982 
1983         hide: echo nofuncgraph-duration > trace_options
1984         show: echo funcgraph-duration > trace_options
1985 
1986 - The overhead field precedes the duration field in case of
1987   reached duration thresholds.
1988 
1989         hide: echo nofuncgraph-overhead > trace_options
1990         show: echo funcgraph-overhead > trace_options
1991         depends on: funcgraph-duration
1992 
1993   ie:
1994 
1995   3) # 1837.709 us |          } /* __switch_to */
1996   3)               |          finish_task_switch() {
1997   3)   0.313 us    |            _raw_spin_unlock_irq();
1998   3)   3.177 us    |          }
1999   3) # 1889.063 us |        } /* __schedule */
2000   3) ! 140.417 us  |      } /* __schedule */
2001   3) # 2034.948 us |    } /* schedule */
2002   3) * 33998.59 us |  } /* schedule_preempt_disabled */
2003 
2004   [...]
2005 
2006   1)   0.260 us    |              msecs_to_jiffies();
2007   1)   0.313 us    |              __rcu_read_unlock();
2008   1) + 61.770 us   |            }
2009   1) + 64.479 us   |          }
2010   1)   0.313 us    |          rcu_bh_qs();
2011   1)   0.313 us    |          __local_bh_enable();
2012   1) ! 217.240 us  |        }
2013   1)   0.365 us    |        idle_cpu();
2014   1)               |        rcu_irq_exit() {
2015   1)   0.417 us    |          rcu_eqs_enter_common.isra.47();
2016   1)   3.125 us    |        }
2017   1) ! 227.812 us  |      }
2018   1) ! 457.395 us  |    }
2019   1) @ 119760.2 us |  }
2020 
2021   [...]
2022 
2023   2)               |    handle_IPI() {
2024   1)   6.979 us    |                  }
2025   2)   0.417 us    |      scheduler_ipi();
2026   1)   9.791 us    |                }
2027   1) + 12.917 us   |              }
2028   2)   3.490 us    |    }
2029   1) + 15.729 us   |            }
2030   1) + 18.542 us   |          }
2031   2) $ 3594274 us  |  }
2032 
2033   + means that the function exceeded 10 usecs.
2034   ! means that the function exceeded 100 usecs.
2035   # means that the function exceeded 1000 usecs.
2036   * means that the function exceeded 10 msecs.
2037   @ means that the function exceeded 100 msecs.
2038   $ means that the function exceeded 1 sec.
2039 
2040 
2041 - The task/pid field displays the thread cmdline and pid which
2042   executed the function. It is default disabled.
2043 
2044         hide: echo nofuncgraph-proc > trace_options
2045         show: echo funcgraph-proc > trace_options
2046 
2047   ie:
2048 
2049   # tracer: function_graph
2050   #
2051   # CPU  TASK/PID        DURATION                  FUNCTION CALLS
2052   # |    |    |           |   |                     |   |   |   |
2053   0)    sh-4802     |               |                  d_free() {
2054   0)    sh-4802     |               |                    call_rcu() {
2055   0)    sh-4802     |               |                      __call_rcu() {
2056   0)    sh-4802     |   0.616 us    |                        rcu_process_gp_end();
2057   0)    sh-4802     |   0.586 us    |                        check_for_new_grace_period();
2058   0)    sh-4802     |   2.899 us    |                      }
2059   0)    sh-4802     |   4.040 us    |                    }
2060   0)    sh-4802     |   5.151 us    |                  }
2061   0)    sh-4802     | + 49.370 us   |                }
2062 
2063 
2064 - The absolute time field is an absolute timestamp given by the
2065   system clock since it started. A snapshot of this time is
2066   given on each entry/exit of functions
2067 
2068         hide: echo nofuncgraph-abstime > trace_options
2069         show: echo funcgraph-abstime > trace_options
2070 
2071   ie:
2072 
2073   #
2074   #      TIME       CPU  DURATION                  FUNCTION CALLS
2075   #       |         |     |   |                     |   |   |   |
2076   360.774522 |   1)   0.541 us    |                                          }
2077   360.774522 |   1)   4.663 us    |                                        }
2078   360.774523 |   1)   0.541 us    |                                        __wake_up_bit();
2079   360.774524 |   1)   6.796 us    |                                      }
2080   360.774524 |   1)   7.952 us    |                                    }
2081   360.774525 |   1)   9.063 us    |                                  }
2082   360.774525 |   1)   0.615 us    |                                  journal_mark_dirty();
2083   360.774527 |   1)   0.578 us    |                                  __brelse();
2084   360.774528 |   1)               |                                  reiserfs_prepare_for_journal() {
2085   360.774528 |   1)               |                                    unlock_buffer() {
2086   360.774529 |   1)               |                                      wake_up_bit() {
2087   360.774529 |   1)               |                                        bit_waitqueue() {
2088   360.774530 |   1)   0.594 us    |                                          __phys_addr();
2089 
2090 
2091 The function name is always displayed after the closing bracket
2092 for a function if the start of that function is not in the
2093 trace buffer.
2094 
2095 Display of the function name after the closing bracket may be
2096 enabled for functions whose start is in the trace buffer,
2097 allowing easier searching with grep for function durations.
2098 It is default disabled.
2099 
2100         hide: echo nofuncgraph-tail > trace_options
2101         show: echo funcgraph-tail > trace_options
2102 
2103   Example with nofuncgraph-tail (default):
2104   0)               |      putname() {
2105   0)               |        kmem_cache_free() {
2106   0)   0.518 us    |          __phys_addr();
2107   0)   1.757 us    |        }
2108   0)   2.861 us    |      }
2109 
2110   Example with funcgraph-tail:
2111   0)               |      putname() {
2112   0)               |        kmem_cache_free() {
2113   0)   0.518 us    |          __phys_addr();
2114   0)   1.757 us    |        } /* kmem_cache_free() */
2115   0)   2.861 us    |      } /* putname() */
2116 
2117 You can put some comments on specific functions by using
2118 trace_printk() For example, if you want to put a comment inside
2119 the __might_sleep() function, you just have to include
2120 <linux/ftrace.h> and call trace_printk() inside __might_sleep()
2121 
2122 trace_printk("I'm a comment!\n")
2123 
2124 will produce:
2125 
2126  1)               |             __might_sleep() {
2127  1)               |                /* I'm a comment! */
2128  1)   1.449 us    |             }
2129 
2130 
2131 You might find other useful features for this tracer in the
2132 following "dynamic ftrace" section such as tracing only specific
2133 functions or tasks.
2134 
2135 dynamic ftrace
2136 --------------
2137 
2138 If CONFIG_DYNAMIC_FTRACE is set, the system will run with
2139 virtually no overhead when function tracing is disabled. The way
2140 this works is the mcount function call (placed at the start of
2141 every kernel function, produced by the -pg switch in gcc),
2142 starts of pointing to a simple return. (Enabling FTRACE will
2143 include the -pg switch in the compiling of the kernel.)
2144 
2145 At compile time every C file object is run through the
2146 recordmcount program (located in the scripts directory). This
2147 program will parse the ELF headers in the C object to find all
2148 the locations in the .text section that call mcount. (Note, only
2149 white listed .text sections are processed, since processing other
2150 sections like .init.text may cause races due to those sections
2151 being freed unexpectedly).
2152 
2153 A new section called "__mcount_loc" is created that holds
2154 references to all the mcount call sites in the .text section.
2155 The recordmcount program re-links this section back into the
2156 original object. The final linking stage of the kernel will add all these
2157 references into a single table.
2158 
2159 On boot up, before SMP is initialized, the dynamic ftrace code
2160 scans this table and updates all the locations into nops. It
2161 also records the locations, which are added to the
2162 available_filter_functions list.  Modules are processed as they
2163 are loaded and before they are executed.  When a module is
2164 unloaded, it also removes its functions from the ftrace function
2165 list. This is automatic in the module unload code, and the
2166 module author does not need to worry about it.
2167 
2168 When tracing is enabled, the process of modifying the function
2169 tracepoints is dependent on architecture. The old method is to use
2170 kstop_machine to prevent races with the CPUs executing code being
2171 modified (which can cause the CPU to do undesirable things, especially
2172 if the modified code crosses cache (or page) boundaries), and the nops are
2173 patched back to calls. But this time, they do not call mcount
2174 (which is just a function stub). They now call into the ftrace
2175 infrastructure.
2176 
2177 The new method of modifying the function tracepoints is to place
2178 a breakpoint at the location to be modified, sync all CPUs, modify
2179 the rest of the instruction not covered by the breakpoint. Sync
2180 all CPUs again, and then remove the breakpoint with the finished
2181 version to the ftrace call site.
2182 
2183 Some archs do not even need to monkey around with the synchronization,
2184 and can just slap the new code on top of the old without any
2185 problems with other CPUs executing it at the same time.
2186 
2187 One special side-effect to the recording of the functions being
2188 traced is that we can now selectively choose which functions we
2189 wish to trace and which ones we want the mcount calls to remain
2190 as nops.
2191 
2192 Two files are used, one for enabling and one for disabling the
2193 tracing of specified functions. They are:
2194 
2195   set_ftrace_filter
2196 
2197 and
2198 
2199   set_ftrace_notrace
2200 
2201 A list of available functions that you can add to these files is
2202 listed in:
2203 
2204    available_filter_functions
2205 
2206  # cat available_filter_functions
2207 put_prev_task_idle
2208 kmem_cache_create
2209 pick_next_task_rt
2210 get_online_cpus
2211 pick_next_task_fair
2212 mutex_lock
2213 [...]
2214 
2215 If I am only interested in sys_nanosleep and hrtimer_interrupt:
2216 
2217  # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
2218  # echo function > current_tracer
2219  # echo 1 > tracing_on
2220  # usleep 1
2221  # echo 0 > tracing_on
2222  # cat trace
2223 # tracer: function
2224 #
2225 # entries-in-buffer/entries-written: 5/5   #P:4
2226 #
2227 #                              _-----=> irqs-off
2228 #                             / _----=> need-resched
2229 #                            | / _---=> hardirq/softirq
2230 #                            || / _--=> preempt-depth
2231 #                            ||| /     delay
2232 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2233 #              | |       |   ||||       |         |
2234           usleep-2665  [001] ....  4186.475355: sys_nanosleep <-system_call_fastpath
2235           <idle>-0     [001] d.h1  4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
2236           usleep-2665  [001] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2237           <idle>-0     [003] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2238           <idle>-0     [002] d.h1  4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
2239 
2240 To see which functions are being traced, you can cat the file:
2241 
2242  # cat set_ftrace_filter
2243 hrtimer_interrupt
2244 sys_nanosleep
2245 
2246 
2247 Perhaps this is not enough. The filters also allow glob(7) matching.
2248 
2249   <match>*  - will match functions that begin with <match>
2250   *<match>  - will match functions that end with <match>
2251   *<match>* - will match functions that have <match> in it
2252   <match1>*<match2> - will match functions that begin with
2253                       <match1> and end with <match2>
2254 
2255 Note: It is better to use quotes to enclose the wild cards,
2256       otherwise the shell may expand the parameters into names
2257       of files in the local directory.
2258 
2259  # echo 'hrtimer_*' > set_ftrace_filter
2260 
2261 Produces:
2262 
2263 # tracer: function
2264 #
2265 # entries-in-buffer/entries-written: 897/897   #P:4
2266 #
2267 #                              _-----=> irqs-off
2268 #                             / _----=> need-resched
2269 #                            | / _---=> hardirq/softirq
2270 #                            || / _--=> preempt-depth
2271 #                            ||| /     delay
2272 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2273 #              | |       |   ||||       |         |
2274           <idle>-0     [003] dN.1  4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
2275           <idle>-0     [003] dN.1  4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
2276           <idle>-0     [003] dN.2  4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
2277           <idle>-0     [003] dN.1  4228.547805: hrtimer_forward <-tick_nohz_idle_exit
2278           <idle>-0     [003] dN.1  4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
2279           <idle>-0     [003] d..1  4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
2280           <idle>-0     [003] d..1  4228.547859: hrtimer_start <-__tick_nohz_idle_enter
2281           <idle>-0     [003] d..2  4228.547860: hrtimer_force_reprogram <-__rem
2282 
2283 Notice that we lost the sys_nanosleep.
2284 
2285  # cat set_ftrace_filter
2286 hrtimer_run_queues
2287 hrtimer_run_pending
2288 hrtimer_init
2289 hrtimer_cancel
2290 hrtimer_try_to_cancel
2291 hrtimer_forward
2292 hrtimer_start
2293 hrtimer_reprogram
2294 hrtimer_force_reprogram
2295 hrtimer_get_next_event
2296 hrtimer_interrupt
2297 hrtimer_nanosleep
2298 hrtimer_wakeup
2299 hrtimer_get_remaining
2300 hrtimer_get_res
2301 hrtimer_init_sleeper
2302 
2303 
2304 This is because the '>' and '>>' act just like they do in bash.
2305 To rewrite the filters, use '>'
2306 To append to the filters, use '>>'
2307 
2308 To clear out a filter so that all functions will be recorded
2309 again:
2310 
2311  # echo > set_ftrace_filter
2312  # cat set_ftrace_filter
2313  #
2314 
2315 Again, now we want to append.
2316 
2317  # echo sys_nanosleep > set_ftrace_filter
2318  # cat set_ftrace_filter
2319 sys_nanosleep
2320  # echo 'hrtimer_*' >> set_ftrace_filter
2321  # cat set_ftrace_filter
2322 hrtimer_run_queues
2323 hrtimer_run_pending
2324 hrtimer_init
2325 hrtimer_cancel
2326 hrtimer_try_to_cancel
2327 hrtimer_forward
2328 hrtimer_start
2329 hrtimer_reprogram
2330 hrtimer_force_reprogram
2331 hrtimer_get_next_event
2332 hrtimer_interrupt
2333 sys_nanosleep
2334 hrtimer_nanosleep
2335 hrtimer_wakeup
2336 hrtimer_get_remaining
2337 hrtimer_get_res
2338 hrtimer_init_sleeper
2339 
2340 
2341 The set_ftrace_notrace prevents those functions from being
2342 traced.
2343 
2344  # echo '*preempt*' '*lock*' > set_ftrace_notrace
2345 
2346 Produces:
2347 
2348 # tracer: function
2349 #
2350 # entries-in-buffer/entries-written: 39608/39608   #P:4
2351 #
2352 #                              _-----=> irqs-off
2353 #                             / _----=> need-resched
2354 #                            | / _---=> hardirq/softirq
2355 #                            || / _--=> preempt-depth
2356 #                            ||| /     delay
2357 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2358 #              | |       |   ||||       |         |
2359             bash-1994  [000] ....  4342.324896: file_ra_state_init <-do_dentry_open
2360             bash-1994  [000] ....  4342.324897: open_check_o_direct <-do_last
2361             bash-1994  [000] ....  4342.324897: ima_file_check <-do_last
2362             bash-1994  [000] ....  4342.324898: process_measurement <-ima_file_check
2363             bash-1994  [000] ....  4342.324898: ima_get_action <-process_measurement
2364             bash-1994  [000] ....  4342.324898: ima_match_policy <-ima_get_action
2365             bash-1994  [000] ....  4342.324899: do_truncate <-do_last
2366             bash-1994  [000] ....  4342.324899: should_remove_suid <-do_truncate
2367             bash-1994  [000] ....  4342.324899: notify_change <-do_truncate
2368             bash-1994  [000] ....  4342.324900: current_fs_time <-notify_change
2369             bash-1994  [000] ....  4342.324900: current_kernel_time <-current_fs_time
2370             bash-1994  [000] ....  4342.324900: timespec_trunc <-current_fs_time
2371 
2372 We can see that there's no more lock or preempt tracing.
2373 
2374 
2375 Dynamic ftrace with the function graph tracer
2376 ---------------------------------------------
2377 
2378 Although what has been explained above concerns both the
2379 function tracer and the function-graph-tracer, there are some
2380 special features only available in the function-graph tracer.
2381 
2382 If you want to trace only one function and all of its children,
2383 you just have to echo its name into set_graph_function:
2384 
2385  echo __do_fault > set_graph_function
2386 
2387 will produce the following "expanded" trace of the __do_fault()
2388 function:
2389 
2390  0)               |  __do_fault() {
2391  0)               |    filemap_fault() {
2392  0)               |      find_lock_page() {
2393  0)   0.804 us    |        find_get_page();
2394  0)               |        __might_sleep() {
2395  0)   1.329 us    |        }
2396  0)   3.904 us    |      }
2397  0)   4.979 us    |    }
2398  0)   0.653 us    |    _spin_lock();
2399  0)   0.578 us    |    page_add_file_rmap();
2400  0)   0.525 us    |    native_set_pte_at();
2401  0)   0.585 us    |    _spin_unlock();
2402  0)               |    unlock_page() {
2403  0)   0.541 us    |      page_waitqueue();
2404  0)   0.639 us    |      __wake_up_bit();
2405  0)   2.786 us    |    }
2406  0) + 14.237 us   |  }
2407  0)               |  __do_fault() {
2408  0)               |    filemap_fault() {
2409  0)               |      find_lock_page() {
2410  0)   0.698 us    |        find_get_page();
2411  0)               |        __might_sleep() {
2412  0)   1.412 us    |        }
2413  0)   3.950 us    |      }
2414  0)   5.098 us    |    }
2415  0)   0.631 us    |    _spin_lock();
2416  0)   0.571 us    |    page_add_file_rmap();
2417  0)   0.526 us    |    native_set_pte_at();
2418  0)   0.586 us    |    _spin_unlock();
2419  0)               |    unlock_page() {
2420  0)   0.533 us    |      page_waitqueue();
2421  0)   0.638 us    |      __wake_up_bit();
2422  0)   2.793 us    |    }
2423  0) + 14.012 us   |  }
2424 
2425 You can also expand several functions at once:
2426 
2427  echo sys_open > set_graph_function
2428  echo sys_close >> set_graph_function
2429 
2430 Now if you want to go back to trace all functions you can clear
2431 this special filter via:
2432 
2433  echo > set_graph_function
2434 
2435 
2436 ftrace_enabled
2437 --------------
2438 
2439 Note, the proc sysctl ftrace_enable is a big on/off switch for the
2440 function tracer. By default it is enabled (when function tracing is
2441 enabled in the kernel). If it is disabled, all function tracing is
2442 disabled. This includes not only the function tracers for ftrace, but
2443 also for any other uses (perf, kprobes, stack tracing, profiling, etc).
2444 
2445 Please disable this with care.
2446 
2447 This can be disable (and enabled) with:
2448 
2449   sysctl kernel.ftrace_enabled=0
2450   sysctl kernel.ftrace_enabled=1
2451 
2452  or
2453 
2454   echo 0 > /proc/sys/kernel/ftrace_enabled
2455   echo 1 > /proc/sys/kernel/ftrace_enabled
2456 
2457 
2458 Filter commands
2459 ---------------
2460 
2461 A few commands are supported by the set_ftrace_filter interface.
2462 Trace commands have the following format:
2463 
2464 <function>:<command>:<parameter>
2465 
2466 The following commands are supported:
2467 
2468 - mod
2469   This command enables function filtering per module. The
2470   parameter defines the module. For example, if only the write*
2471   functions in the ext3 module are desired, run:
2472 
2473    echo 'write*:mod:ext3' > set_ftrace_filter
2474 
2475   This command interacts with the filter in the same way as
2476   filtering based on function names. Thus, adding more functions
2477   in a different module is accomplished by appending (>>) to the
2478   filter file. Remove specific module functions by prepending
2479   '!':
2480 
2481    echo '!writeback*:mod:ext3' >> set_ftrace_filter
2482 
2483   Mod command supports module globbing. Disable tracing for all
2484   functions except a specific module:
2485 
2486    echo '!*:mod:!ext3' >> set_ftrace_filter
2487 
2488   Disable tracing for all modules, but still trace kernel:
2489 
2490    echo '!*:mod:*' >> set_ftrace_filter
2491 
2492   Enable filter only for kernel:
2493 
2494    echo '*write*:mod:!*' >> set_ftrace_filter
2495 
2496   Enable filter for module globbing:
2497 
2498    echo '*write*:mod:*snd*' >> set_ftrace_filter
2499 
2500 - traceon/traceoff
2501   These commands turn tracing on and off when the specified
2502   functions are hit. The parameter determines how many times the
2503   tracing system is turned on and off. If unspecified, there is
2504   no limit. For example, to disable tracing when a schedule bug
2505   is hit the first 5 times, run:
2506 
2507    echo '__schedule_bug:traceoff:5' > set_ftrace_filter
2508 
2509   To always disable tracing when __schedule_bug is hit:
2510 
2511    echo '__schedule_bug:traceoff' > set_ftrace_filter
2512 
2513   These commands are cumulative whether or not they are appended
2514   to set_ftrace_filter. To remove a command, prepend it by '!'
2515   and drop the parameter:
2516 
2517    echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
2518 
2519     The above removes the traceoff command for __schedule_bug
2520     that have a counter. To remove commands without counters:
2521 
2522    echo '!__schedule_bug:traceoff' > set_ftrace_filter
2523 
2524 - snapshot
2525   Will cause a snapshot to be triggered when the function is hit.
2526 
2527    echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
2528 
2529   To only snapshot once:
2530 
2531    echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
2532 
2533   To remove the above commands:
2534 
2535    echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
2536    echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
2537 
2538 - enable_event/disable_event
2539   These commands can enable or disable a trace event. Note, because
2540   function tracing callbacks are very sensitive, when these commands
2541   are registered, the trace point is activated, but disabled in
2542   a "soft" mode. That is, the tracepoint will be called, but
2543   just will not be traced. The event tracepoint stays in this mode
2544   as long as there's a command that triggers it.
2545 
2546    echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
2547          set_ftrace_filter
2548 
2549   The format is:
2550 
2551     <function>:enable_event:<system>:<event>[:count]
2552     <function>:disable_event:<system>:<event>[:count]
2553 
2554   To remove the events commands:
2555 
2556 
2557    echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
2558          set_ftrace_filter
2559    echo '!schedule:disable_event:sched:sched_switch' > \
2560          set_ftrace_filter
2561 
2562 - dump
2563   When the function is hit, it will dump the contents of the ftrace
2564   ring buffer to the console. This is useful if you need to debug
2565   something, and want to dump the trace when a certain function
2566   is hit. Perhaps its a function that is called before a tripple
2567   fault happens and does not allow you to get a regular dump.
2568 
2569 - cpudump
2570   When the function is hit, it will dump the contents of the ftrace
2571   ring buffer for the current CPU to the console. Unlike the "dump"
2572   command, it only prints out the contents of the ring buffer for the
2573   CPU that executed the function that triggered the dump.
2574 
2575 trace_pipe
2576 ----------
2577 
2578 The trace_pipe outputs the same content as the trace file, but
2579 the effect on the tracing is different. Every read from
2580 trace_pipe is consumed. This means that subsequent reads will be
2581 different. The trace is live.
2582 
2583  # echo function > current_tracer
2584  # cat trace_pipe > /tmp/trace.out &
2585 [1] 4153
2586  # echo 1 > tracing_on
2587  # usleep 1
2588  # echo 0 > tracing_on
2589  # cat trace
2590 # tracer: function
2591 #
2592 # entries-in-buffer/entries-written: 0/0   #P:4
2593 #
2594 #                              _-----=> irqs-off
2595 #                             / _----=> need-resched
2596 #                            | / _---=> hardirq/softirq
2597 #                            || / _--=> preempt-depth
2598 #                            ||| /     delay
2599 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2600 #              | |       |   ||||       |         |
2601 
2602  #
2603  # cat /tmp/trace.out
2604             bash-1994  [000] ....  5281.568961: mutex_unlock <-rb_simple_write
2605             bash-1994  [000] ....  5281.568963: __mutex_unlock_slowpath <-mutex_unlock
2606             bash-1994  [000] ....  5281.568963: __fsnotify_parent <-fsnotify_modify
2607             bash-1994  [000] ....  5281.568964: fsnotify <-fsnotify_modify
2608             bash-1994  [000] ....  5281.568964: __srcu_read_lock <-fsnotify
2609             bash-1994  [000] ....  5281.568964: add_preempt_count <-__srcu_read_lock
2610             bash-1994  [000] ...1  5281.568965: sub_preempt_count <-__srcu_read_lock
2611             bash-1994  [000] ....  5281.568965: __srcu_read_unlock <-fsnotify
2612             bash-1994  [000] ....  5281.568967: sys_dup2 <-system_call_fastpath
2613 
2614 
2615 Note, reading the trace_pipe file will block until more input is
2616 added.
2617 
2618 trace entries
2619 -------------
2620 
2621 Having too much or not enough data can be troublesome in
2622 diagnosing an issue in the kernel. The file buffer_size_kb is
2623 used to modify the size of the internal trace buffers. The
2624 number listed is the number of entries that can be recorded per
2625 CPU. To know the full size, multiply the number of possible CPUs
2626 with the number of entries.
2627 
2628  # cat buffer_size_kb
2629 1408 (units kilobytes)
2630 
2631 Or simply read buffer_total_size_kb
2632 
2633  # cat buffer_total_size_kb 
2634 5632
2635 
2636 To modify the buffer, simple echo in a number (in 1024 byte segments).
2637 
2638  # echo 10000 > buffer_size_kb
2639  # cat buffer_size_kb
2640 10000 (units kilobytes)
2641 
2642 It will try to allocate as much as possible. If you allocate too
2643 much, it can cause Out-Of-Memory to trigger.
2644 
2645  # echo 1000000000000 > buffer_size_kb
2646 -bash: echo: write error: Cannot allocate memory
2647  # cat buffer_size_kb
2648 85
2649 
2650 The per_cpu buffers can be changed individually as well:
2651 
2652  # echo 10000 > per_cpu/cpu0/buffer_size_kb
2653  # echo 100 > per_cpu/cpu1/buffer_size_kb
2654 
2655 When the per_cpu buffers are not the same, the buffer_size_kb
2656 at the top level will just show an X
2657 
2658  # cat buffer_size_kb
2659 X
2660 
2661 This is where the buffer_total_size_kb is useful:
2662 
2663  # cat buffer_total_size_kb 
2664 12916
2665 
2666 Writing to the top level buffer_size_kb will reset all the buffers
2667 to be the same again.
2668 
2669 Snapshot
2670 --------
2671 CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
2672 available to all non latency tracers. (Latency tracers which
2673 record max latency, such as "irqsoff" or "wakeup", can't use
2674 this feature, since those are already using the snapshot
2675 mechanism internally.)
2676 
2677 Snapshot preserves a current trace buffer at a particular point
2678 in time without stopping tracing. Ftrace swaps the current
2679 buffer with a spare buffer, and tracing continues in the new
2680 current (=previous spare) buffer.
2681 
2682 The following debugfs files in "tracing" are related to this
2683 feature:
2684 
2685   snapshot:
2686 
2687         This is used to take a snapshot and to read the output
2688         of the snapshot. Echo 1 into this file to allocate a
2689         spare buffer and to take a snapshot (swap), then read
2690         the snapshot from this file in the same format as
2691         "trace" (described above in the section "The File
2692         System"). Both reads snapshot and tracing are executable
2693         in parallel. When the spare buffer is allocated, echoing
2694         0 frees it, and echoing else (positive) values clear the
2695         snapshot contents.
2696         More details are shown in the table below.
2697 
2698         status\input  |     0      |     1      |    else    |
2699         --------------+------------+------------+------------+
2700         not allocated |(do nothing)| alloc+swap |(do nothing)|
2701         --------------+------------+------------+------------+
2702         allocated     |    free    |    swap    |   clear    |
2703         --------------+------------+------------+------------+
2704 
2705 Here is an example of using the snapshot feature.
2706 
2707  # echo 1 > events/sched/enable
2708  # echo 1 > snapshot
2709  # cat snapshot
2710 # tracer: nop
2711 #
2712 # entries-in-buffer/entries-written: 71/71   #P:8
2713 #
2714 #                              _-----=> irqs-off
2715 #                             / _----=> need-resched
2716 #                            | / _---=> hardirq/softirq
2717 #                            || / _--=> preempt-depth
2718 #                            ||| /     delay
2719 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2720 #              | |       |   ||||       |         |
2721           <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
2722            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
2723 [...]
2724           <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
2725 
2726  # cat trace
2727 # tracer: nop
2728 #
2729 # entries-in-buffer/entries-written: 77/77   #P:8
2730 #
2731 #                              _-----=> irqs-off
2732 #                             / _----=> need-resched
2733 #                            | / _---=> hardirq/softirq
2734 #                            || / _--=> preempt-depth
2735 #                            ||| /     delay
2736 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2737 #              | |       |   ||||       |         |
2738           <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
2739  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
2740 [...]
2741 
2742 
2743 If you try to use this snapshot feature when current tracer is
2744 one of the latency tracers, you will get the following results.
2745 
2746  # echo wakeup > current_tracer
2747  # echo 1 > snapshot
2748 bash: echo: write error: Device or resource busy
2749  # cat snapshot
2750 cat: snapshot: Device or resource busy
2751 
2752 
2753 Instances
2754 ---------
2755 In the debugfs tracing directory is a directory called "instances".
2756 This directory can have new directories created inside of it using
2757 mkdir, and removing directories with rmdir. The directory created
2758 with mkdir in this directory will already contain files and other
2759 directories after it is created.
2760 
2761  # mkdir instances/foo
2762  # ls instances/foo
2763 buffer_size_kb  buffer_total_size_kb  events  free_buffer  per_cpu
2764 set_event  snapshot  trace  trace_clock  trace_marker  trace_options
2765 trace_pipe  tracing_on
2766 
2767 As you can see, the new directory looks similar to the tracing directory
2768 itself. In fact, it is very similar, except that the buffer and
2769 events are agnostic from the main director, or from any other
2770 instances that are created.
2771 
2772 The files in the new directory work just like the files with the
2773 same name in the tracing directory except the buffer that is used
2774 is a separate and new buffer. The files affect that buffer but do not
2775 affect the main buffer with the exception of trace_options. Currently,
2776 the trace_options affect all instances and the top level buffer
2777 the same, but this may change in future releases. That is, options
2778 may become specific to the instance they reside in.
2779 
2780 Notice that none of the function tracer files are there, nor is
2781 current_tracer and available_tracers. This is because the buffers
2782 can currently only have events enabled for them.
2783 
2784  # mkdir instances/foo
2785  # mkdir instances/bar
2786  # mkdir instances/zoot
2787  # echo 100000 > buffer_size_kb
2788  # echo 1000 > instances/foo/buffer_size_kb
2789  # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
2790  # echo function > current_trace
2791  # echo 1 > instances/foo/events/sched/sched_wakeup/enable
2792  # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
2793  # echo 1 > instances/foo/events/sched/sched_switch/enable
2794  # echo 1 > instances/bar/events/irq/enable
2795  # echo 1 > instances/zoot/events/syscalls/enable
2796  # cat trace_pipe
2797 CPU:2 [LOST 11745 EVENTS]
2798             bash-2044  [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
2799             bash-2044  [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
2800             bash-2044  [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
2801             bash-2044  [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
2802             bash-2044  [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
2803             bash-2044  [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
2804             bash-2044  [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
2805             bash-2044  [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
2806             bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
2807             bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
2808             bash-2044  [002] .... 10594.481035: arch_dup_task_struct <-copy_process
2809 [...]
2810 
2811  # cat instances/foo/trace_pipe
2812             bash-1998  [000] d..4   136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
2813             bash-1998  [000] dN.4   136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
2814           <idle>-0     [003] d.h3   136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
2815           <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
2816      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
2817             bash-1998  [000] d..4   136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
2818             bash-1998  [000] dN.4   136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
2819             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
2820      kworker/0:1-59    [000] d..4   136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
2821      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
2822 [...]
2823 
2824  # cat instances/bar/trace_pipe
2825      migration/1-14    [001] d.h3   138.732674: softirq_raise: vec=3 [action=NET_RX]
2826           <idle>-0     [001] dNh3   138.732725: softirq_raise: vec=3 [action=NET_RX]
2827             bash-1998  [000] d.h1   138.733101: softirq_raise: vec=1 [action=TIMER]
2828             bash-1998  [000] d.h1   138.733102: softirq_raise: vec=9 [action=RCU]
2829             bash-1998  [000] ..s2   138.733105: softirq_entry: vec=1 [action=TIMER]
2830             bash-1998  [000] ..s2   138.733106: softirq_exit: vec=1 [action=TIMER]
2831             bash-1998  [000] ..s2   138.733106: softirq_entry: vec=9 [action=RCU]
2832             bash-1998  [000] ..s2   138.733109: softirq_exit: vec=9 [action=RCU]
2833             sshd-1995  [001] d.h1   138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
2834             sshd-1995  [001] d.h1   138.733280: irq_handler_exit: irq=21 ret=unhandled
2835             sshd-1995  [001] d.h1   138.733281: irq_handler_entry: irq=21 name=eth0
2836             sshd-1995  [001] d.h1   138.733283: irq_handler_exit: irq=21 ret=handled
2837 [...]
2838 
2839  # cat instances/zoot/trace
2840 # tracer: nop
2841 #
2842 # entries-in-buffer/entries-written: 18996/18996   #P:4
2843 #
2844 #                              _-----=> irqs-off
2845 #                             / _----=> need-resched
2846 #                            | / _---=> hardirq/softirq
2847 #                            || / _--=> preempt-depth
2848 #                            ||| /     delay
2849 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2850 #              | |       |   ||||       |         |
2851             bash-1998  [000] d...   140.733501: sys_write -> 0x2
2852             bash-1998  [000] d...   140.733504: sys_dup2(oldfd: a, newfd: 1)
2853             bash-1998  [000] d...   140.733506: sys_dup2 -> 0x1
2854             bash-1998  [000] d...   140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
2855             bash-1998  [000] d...   140.733509: sys_fcntl -> 0x1
2856             bash-1998  [000] d...   140.733510: sys_close(fd: a)
2857             bash-1998  [000] d...   140.733510: sys_close -> 0x0
2858             bash-1998  [000] d...   140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
2859             bash-1998  [000] d...   140.733515: sys_rt_sigprocmask -> 0x0
2860             bash-1998  [000] d...   140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
2861             bash-1998  [000] d...   140.733516: sys_rt_sigaction -> 0x0
2862 
2863 You can see that the trace of the top most trace buffer shows only
2864 the function tracing. The foo instance displays wakeups and task
2865 switches.
2866 
2867 To remove the instances, simply delete their directories:
2868 
2869  # rmdir instances/foo
2870  # rmdir instances/bar
2871  # rmdir instances/zoot
2872 
2873 Note, if a process has a trace file open in one of the instance
2874 directories, the rmdir will fail with EBUSY.
2875 
2876 
2877 Stack trace
2878 -----------
2879 Since the kernel has a fixed sized stack, it is important not to
2880 waste it in functions. A kernel developer must be conscience of
2881 what they allocate on the stack. If they add too much, the system
2882 can be in danger of a stack overflow, and corruption will occur,
2883 usually leading to a system panic.
2884 
2885 There are some tools that check this, usually with interrupts
2886 periodically checking usage. But if you can perform a check
2887 at every function call that will become very useful. As ftrace provides
2888 a function tracer, it makes it convenient to check the stack size
2889 at every function call. This is enabled via the stack tracer.
2890 
2891 CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
2892 To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
2893 
2894  # echo 1 > /proc/sys/kernel/stack_tracer_enabled
2895 
2896 You can also enable it from the kernel command line to trace
2897 the stack size of the kernel during boot up, by adding "stacktrace"
2898 to the kernel command line parameter.
2899 
2900 After running it for a few minutes, the output looks like:
2901 
2902  # cat stack_max_size
2903 2928
2904 
2905  # cat stack_trace
2906         Depth    Size   Location    (18 entries)
2907         -----    ----   --------
2908   0)     2928     224   update_sd_lb_stats+0xbc/0x4ac
2909   1)     2704     160   find_busiest_group+0x31/0x1f1
2910   2)     2544     256   load_balance+0xd9/0x662
2911   3)     2288      80   idle_balance+0xbb/0x130
2912   4)     2208     128   __schedule+0x26e/0x5b9
2913   5)     2080      16   schedule+0x64/0x66
2914   6)     2064     128   schedule_timeout+0x34/0xe0
2915   7)     1936     112   wait_for_common+0x97/0xf1
2916   8)     1824      16   wait_for_completion+0x1d/0x1f
2917   9)     1808     128   flush_work+0xfe/0x119
2918  10)     1680      16   tty_flush_to_ldisc+0x1e/0x20
2919  11)     1664      48   input_available_p+0x1d/0x5c
2920  12)     1616      48   n_tty_poll+0x6d/0x134
2921  13)     1568      64   tty_poll+0x64/0x7f
2922  14)     1504     880   do_select+0x31e/0x511
2923  15)      624     400   core_sys_select+0x177/0x216
2924  16)      224      96   sys_select+0x91/0xb9
2925  17)      128     128   system_call_fastpath+0x16/0x1b
2926 
2927 Note, if -mfentry is being used by gcc, functions get traced before
2928 they set up the stack frame. This means that leaf level functions
2929 are not tested by the stack tracer when -mfentry is used.
2930 
2931 Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
2932 
2933 ---------
2934 
2935 More details can be found in the source code, in the
2936 kernel/trace/*.c files.

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