1 ftrace - Function Tracer
2 ========================
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
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.
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.
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.
33 Implementation Details
34 ----------------------
36 See ftrace-design.txt for details for arch porters and such.
42 Ftrace uses the debugfs file system to hold the control files as
43 well as the files to display output.
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:
49 debugfs /sys/kernel/debug debugfs defaults 0 0
51 Or you can mount it at run time with:
53 mount -t debugfs nodev /sys/kernel/debug
55 For quicker access to that directory you may want to make a soft link to
58 ln -s /sys/kernel/debug /debug
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.
66 That's it! (assuming that you have ftrace configured into your kernel)
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:
73 Note: all time values are in microseconds.
77 This is used to set or display the current tracer
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.
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
97 This file holds the output of the trace in a human
98 readable format (described below).
102 The output is the same as the "trace" file but this
103 file is meant to be streamed with live tracing.
104 Reads from this file will block until new data is
105 retrieved. Unlike the "trace" file, this file is a
106 consumer. This means reading from this file causes
107 sequential reads to display more current data. Once
108 data is read from this file, it is consumed, and
109 will not be read again with a sequential read. The
110 "trace" file is static, and if the tracer is not
111 adding more data,they will display the same
112 information every time they are read.
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).
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.
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)
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.
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. )
159 buffer_total_size_kb:
161 This displays the total combined size of all the trace buffers.
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
174 It may also stop tracing if disable_on_free option is set.
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.
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.
193 This interface also allows for commands to be used. See the
194 "Filter commands" section for more details.
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.
205 Have the function tracer only trace a single thread.
209 Set a "trigger" function where tracing should start
210 with the function graph tracer (See the section
211 "dynamic ftrace" for more details).
213 available_filter_functions:
215 This lists the functions that ftrace
216 has processed and can trace. These are the function
217 names that you can pass to "set_ftrace_filter" or
218 "set_ftrace_notrace". (See the section "dynamic ftrace"
219 below for more details.)
223 This file is more for debugging ftrace, but can also be useful
224 in seeing if any function has a callback attached to it.
225 Not only does the trace infrastructure use ftrace function
226 trace utility, but other subsystems might too. This file
227 displays all functions that have a callback attached to them
228 as well as the number of callbacks that have been attached.
229 Note, a callback may also call multiple functions which will
230 not be listed in this count.
232 If the callback registered to be traced by a function with
233 the "save regs" attribute (thus even more overhead), a 'R'
234 will be displayed on the same line as the function that
235 is returning registers.
237 function_profile_enabled:
239 When set it will enable all functions with either the function
240 tracer, or if enabled, the function graph tracer. It will
241 keep a histogram of the number of functions that were called
242 and if run with the function graph tracer, it will also keep
243 track of the time spent in those functions. The histogram
244 content can be displayed in the files:
246 trace_stats/function<cpu> ( function0, function1, etc).
250 A directory that holds different tracing stats.
254 Enable dynamic trace points. See kprobetrace.txt.
258 Dynamic trace points stats. See kprobetrace.txt.
262 Used with the function graph tracer. This is the max depth
263 it will trace into a function. Setting this to a value of
264 one will show only the first kernel function that is called
269 This is for tools that read the raw format files. If an event in
270 the ring buffer references a string (currently only trace_printk()
271 does this), only a pointer to the string is recorded into the buffer
272 and not the string itself. This prevents tools from knowing what
273 that string was. This file displays the string and address for
274 the string allowing tools to map the pointers to what the
279 Only the pid of the task is recorded in a trace event unless
280 the event specifically saves the task comm as well. Ftrace
281 makes a cache of pid mappings to comms to try to display
282 comms for events. If a pid for a comm is not listed, then
283 "<...>" is displayed in the output.
287 This displays the "snapshot" buffer and also lets the user
288 take a snapshot of the current running trace.
289 See the "Snapshot" section below for more details.
293 When the stack tracer is activated, this will display the
294 maximum stack size it has encountered.
295 See the "Stack Trace" section below.
299 This displays the stack back trace of the largest stack
300 that was encountered when the stack tracer is activated.
301 See the "Stack Trace" section below.
305 This is similar to "set_ftrace_filter" but it limits what
306 functions the stack tracer will check.
310 Whenever an event is recorded into the ring buffer, a
311 "timestamp" is added. This stamp comes from a specified
312 clock. By default, ftrace uses the "local" clock. This
313 clock is very fast and strictly per cpu, but on some
314 systems it may not be monotonic with respect to other
315 CPUs. In other words, the local clocks may not be in sync
316 with local clocks on other CPUs.
318 Usual clocks for tracing:
321 [local] global counter x86-tsc
323 local: Default clock, but may not be in sync across CPUs
325 global: This clock is in sync with all CPUs but may
326 be a bit slower than the local clock.
328 counter: This is not a clock at all, but literally an atomic
329 counter. It counts up one by one, but is in sync
330 with all CPUs. This is useful when you need to
331 know exactly the order events occurred with respect to
332 each other on different CPUs.
334 uptime: This uses the jiffies counter and the time stamp
335 is relative to the time since boot up.
337 perf: This makes ftrace use the same clock that perf uses.
338 Eventually perf will be able to read ftrace buffers
339 and this will help out in interleaving the data.
341 x86-tsc: Architectures may define their own clocks. For
342 example, x86 uses its own TSC cycle clock here.
344 To set a clock, simply echo the clock name into this file.
346 echo global > trace_clock
350 This is a very useful file for synchronizing user space
351 with events happening in the kernel. Writing strings into
352 this file will be written into the ftrace buffer.
354 It is useful in applications to open this file at the start
355 of the application and just reference the file descriptor
358 void trace_write(const char *fmt, ...)
368 n = vsnprintf(buf, 256, fmt, ap);
371 write(trace_fd, buf, n);
376 trace_fd = open("trace_marker", WR_ONLY);
380 Add dynamic tracepoints in programs.
385 Uprobe statistics. See uprobetrace.txt
389 This is a way to make multiple trace buffers where different
390 events can be recorded in different buffers.
391 See "Instances" section below.
395 This is the trace event directory. It holds event tracepoints
396 (also known as static tracepoints) that have been compiled
397 into the kernel. It shows what event tracepoints exist
398 and how they are grouped by system. There are "enable"
399 files at various levels that can enable the tracepoints
400 when a "1" is written to them.
402 See events.txt for more information.
406 This is a directory that contains the trace per_cpu information.
408 per_cpu/cpu0/buffer_size_kb:
410 The ftrace buffer is defined per_cpu. That is, there's a separate
411 buffer for each CPU to allow writes to be done atomically,
412 and free from cache bouncing. These buffers may have different
413 size buffers. This file is similar to the buffer_size_kb
414 file, but it only displays or sets the buffer size for the
415 specific CPU. (here cpu0).
419 This is similar to the "trace" file, but it will only display
420 the data specific for the CPU. If written to, it only clears
421 the specific CPU buffer.
423 per_cpu/cpu0/trace_pipe
425 This is similar to the "trace_pipe" file, and is a consuming
426 read, but it will only display (and consume) the data specific
429 per_cpu/cpu0/trace_pipe_raw
431 For tools that can parse the ftrace ring buffer binary format,
432 the trace_pipe_raw file can be used to extract the data
433 from the ring buffer directly. With the use of the splice()
434 system call, the buffer data can be quickly transferred to
435 a file or to the network where a server is collecting the
438 Like trace_pipe, this is a consuming reader, where multiple
439 reads will always produce different data.
441 per_cpu/cpu0/snapshot:
443 This is similar to the main "snapshot" file, but will only
444 snapshot the current CPU (if supported). It only displays
445 the content of the snapshot for a given CPU, and if
446 written to, only clears this CPU buffer.
448 per_cpu/cpu0/snapshot_raw:
450 Similar to the trace_pipe_raw, but will read the binary format
451 from the snapshot buffer for the given CPU.
455 This displays certain stats about the ring buffer:
457 entries: The number of events that are still in the buffer.
459 overrun: The number of lost events due to overwriting when
462 commit overrun: Should always be zero.
463 This gets set if so many events happened within a nested
464 event (ring buffer is re-entrant), that it fills the
465 buffer and starts dropping events.
467 bytes: Bytes actually read (not overwritten).
469 oldest event ts: The oldest timestamp in the buffer
471 now ts: The current timestamp
473 dropped events: Events lost due to overwrite option being off.
475 read events: The number of events read.
480 Here is the list of current tracers that may be configured.
484 Function call tracer to trace all kernel functions.
488 Similar to the function tracer except that the
489 function tracer probes the functions on their entry
490 whereas the function graph tracer traces on both entry
491 and exit of the functions. It then provides the ability
492 to draw a graph of function calls similar to C code
497 Traces the areas that disable interrupts and saves
498 the trace with the longest max latency.
499 See tracing_max_latency. When a new max is recorded,
500 it replaces the old trace. It is best to view this
501 trace with the latency-format option enabled.
505 Similar to irqsoff but traces and records the amount of
506 time for which preemption is disabled.
510 Similar to irqsoff and preemptoff, but traces and
511 records the largest time for which irqs and/or preemption
516 Traces and records the max latency that it takes for
517 the highest priority task to get scheduled after
518 it has been woken up.
519 Traces all tasks as an average developer would expect.
523 Traces and records the max latency that it takes for just
524 RT tasks (as the current "wakeup" does). This is useful
525 for those interested in wake up timings of RT tasks.
529 This is the "trace nothing" tracer. To remove all
530 tracers from tracing simply echo "nop" into
534 Examples of using the tracer
535 ----------------------------
537 Here are typical examples of using the tracers when controlling
538 them only with the debugfs interface (without using any
539 user-land utilities).
544 Here is an example of the output format of the file "trace"
549 # entries-in-buffer/entries-written: 140080/250280 #P:4
552 # / _----=> need-resched
553 # | / _---=> hardirq/softirq
554 # || / _--=> preempt-depth
556 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
558 bash-1977 [000] .... 17284.993652: sys_close <-system_call_fastpath
559 bash-1977 [000] .... 17284.993653: __close_fd <-sys_close
560 bash-1977 [000] .... 17284.993653: _raw_spin_lock <-__close_fd
561 sshd-1974 [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
562 bash-1977 [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
563 bash-1977 [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
564 bash-1977 [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
565 bash-1977 [000] .... 17284.993657: filp_close <-__close_fd
566 bash-1977 [000] .... 17284.993657: dnotify_flush <-filp_close
567 sshd-1974 [003] .... 17284.993658: sys_select <-system_call_fastpath
570 A header is printed with the tracer name that is represented by
571 the trace. In this case the tracer is "function". Then it shows the
572 number of events in the buffer as well as the total number of entries
573 that were written. The difference is the number of entries that were
574 lost due to the buffer filling up (250280 - 140080 = 110200 events
577 The header explains the content of the events. Task name "bash", the task
578 PID "1977", the CPU that it was running on "000", the latency format
579 (explained below), the timestamp in <secs>.<usecs> format, the
580 function name that was traced "sys_close" and the parent function that
581 called this function "system_call_fastpath". The timestamp is the time
582 at which the function was entered.
587 When the latency-format option is enabled or when one of the latency
588 tracers is set, the trace file gives somewhat more information to see
589 why a latency happened. Here is a typical trace.
593 # irqsoff latency trace v1.1.5 on 3.8.0-test+
594 # --------------------------------------------------------------------
595 # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
597 # | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
599 # => started at: __lock_task_sighand
600 # => ended at: _raw_spin_unlock_irqrestore
604 # / _-----=> irqs-off
605 # | / _----=> need-resched
606 # || / _---=> hardirq/softirq
607 # ||| / _--=> preempt-depth
609 # cmd pid ||||| time | caller
611 ps-6143 2d... 0us!: trace_hardirqs_off <-__lock_task_sighand
612 ps-6143 2d..1 259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
613 ps-6143 2d..1 263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
614 ps-6143 2d..1 306us : <stack trace>
615 => trace_hardirqs_on_caller
617 => _raw_spin_unlock_irqrestore
624 => system_call_fastpath
627 This shows that the current tracer is "irqsoff" tracing the time
628 for which interrupts were disabled. It gives the trace version (which
629 never changes) and the version of the kernel upon which this was executed on
630 (3.10). Then it displays the max latency in microseconds (259 us). The number
631 of trace entries displayed and the total number (both are four: #4/4).
632 VP, KP, SP, and HP are always zero and are reserved for later use.
633 #P is the number of online CPUs (#P:4).
635 The task is the process that was running when the latency
636 occurred. (ps pid: 6143).
638 The start and stop (the functions in which the interrupts were
639 disabled and enabled respectively) that caused the latencies:
641 __lock_task_sighand is where the interrupts were disabled.
642 _raw_spin_unlock_irqrestore is where they were enabled again.
644 The next lines after the header are the trace itself. The header
645 explains which is which.
647 cmd: The name of the process in the trace.
649 pid: The PID of that process.
651 CPU#: The CPU which the process was running on.
653 irqs-off: 'd' interrupts are disabled. '.' otherwise.
654 Note: If the architecture does not support a way to
655 read the irq flags variable, an 'X' will always
658 need-resched: 'N' task need_resched is set, '.' otherwise.
661 'H' - hard irq occurred inside a softirq.
662 'h' - hard irq is running
663 's' - soft irq is running
664 '.' - normal context.
666 preempt-depth: The level of preempt_disabled
668 The above is mostly meaningful for kernel developers.
670 time: When the latency-format option is enabled, the trace file
671 output includes a timestamp relative to the start of the
672 trace. This differs from the output when latency-format
673 is disabled, which includes an absolute timestamp.
675 delay: This is just to help catch your eye a bit better. And
676 needs to be fixed to be only relative to the same CPU.
677 The marks are determined by the difference between this
678 current trace and the next trace.
679 '!' - greater than preempt_mark_thresh (default 100)
680 '+' - greater than 1 microsecond
681 ' ' - less than or equal to 1 microsecond.
683 The rest is the same as the 'trace' file.
685 Note, the latency tracers will usually end with a back trace
686 to easily find where the latency occurred.
691 The trace_options file (or the options directory) is used to control
692 what gets printed in the trace output, or manipulate the tracers.
693 To see what is available, simply cat the file:
723 To disable one of the options, echo in the option prepended with
726 echo noprint-parent > trace_options
728 To enable an option, leave off the "no".
730 echo sym-offset > trace_options
732 Here are the available options:
734 print-parent - On function traces, display the calling (parent)
735 function as well as the function being traced.
738 bash-4000 [01] 1477.606694: simple_strtoul <-kstrtoul
741 bash-4000 [01] 1477.606694: simple_strtoul
744 sym-offset - Display not only the function name, but also the
745 offset in the function. For example, instead of
746 seeing just "ktime_get", you will see
747 "ktime_get+0xb/0x20".
750 bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0
752 sym-addr - this will also display the function address as well
753 as the function name.
756 bash-4000 [01] 1477.606694: simple_strtoul <c0339346>
758 verbose - This deals with the trace file when the
759 latency-format option is enabled.
761 bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
762 (+0.000ms): simple_strtoul (kstrtoul)
764 raw - This will display raw numbers. This option is best for
765 use with user applications that can translate the raw
766 numbers better than having it done in the kernel.
768 hex - Similar to raw, but the numbers will be in a hexadecimal
771 bin - This will print out the formats in raw binary.
773 block - When set, reading trace_pipe will not block when polled.
775 stacktrace - This is one of the options that changes the trace
776 itself. When a trace is recorded, so is the stack
777 of functions. This allows for back traces of
780 trace_printk - Can disable trace_printk() from writing into the buffer.
782 branch - Enable branch tracing with the tracer.
784 annotate - It is sometimes confusing when the CPU buffers are full
785 and one CPU buffer had a lot of events recently, thus
786 a shorter time frame, were another CPU may have only had
787 a few events, which lets it have older events. When
788 the trace is reported, it shows the oldest events first,
789 and it may look like only one CPU ran (the one with the
790 oldest events). When the annotate option is set, it will
791 display when a new CPU buffer started:
793 <idle>-0 [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
794 <idle>-0 [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
795 <idle>-0 [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
796 ##### CPU 2 buffer started ####
797 <idle>-0 [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
798 <idle>-0 [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
799 <idle>-0 [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
801 userstacktrace - This option changes the trace. It records a
802 stacktrace of the current userspace thread.
804 sym-userobj - when user stacktrace are enabled, look up which
805 object the address belongs to, and print a
806 relative address. This is especially useful when
807 ASLR is on, otherwise you don't get a chance to
808 resolve the address to object/file/line after
809 the app is no longer running
811 The lookup is performed when you read
812 trace,trace_pipe. Example:
814 a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
815 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
818 printk-msg-only - When set, trace_printk()s will only show the format
819 and not their parameters (if trace_bprintk() or
820 trace_bputs() was used to save the trace_printk()).
822 context-info - Show only the event data. Hides the comm, PID,
823 timestamp, CPU, and other useful data.
825 latency-format - This option changes the trace. When
826 it is enabled, the trace displays
827 additional information about the
828 latencies, as described in "Latency
831 sleep-time - When running function graph tracer, to include
832 the time a task schedules out in its function.
833 When enabled, it will account time the task has been
834 scheduled out as part of the function call.
836 graph-time - When running function graph tracer, to include the
837 time to call nested functions. When this is not set,
838 the time reported for the function will only include
839 the time the function itself executed for, not the time
840 for functions that it called.
842 record-cmd - When any event or tracer is enabled, a hook is enabled
843 in the sched_switch trace point to fill comm cache
844 with mapped pids and comms. But this may cause some
845 overhead, and if you only care about pids, and not the
846 name of the task, disabling this option can lower the
849 overwrite - This controls what happens when the trace buffer is
850 full. If "1" (default), the oldest events are
851 discarded and overwritten. If "0", then the newest
852 events are discarded.
853 (see per_cpu/cpu0/stats for overrun and dropped)
855 disable_on_free - When the free_buffer is closed, tracing will
856 stop (tracing_on set to 0).
858 irq-info - Shows the interrupt, preempt count, need resched data.
859 When disabled, the trace looks like:
863 # entries-in-buffer/entries-written: 144405/9452052 #P:4
865 # TASK-PID CPU# TIMESTAMP FUNCTION
867 <idle>-0 [002] 23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
868 <idle>-0 [002] 23636.756054: activate_task <-ttwu_do_activate.constprop.89
869 <idle>-0 [002] 23636.756055: enqueue_task <-activate_task
872 markers - When set, the trace_marker is writable (only by root).
873 When disabled, the trace_marker will error with EINVAL
877 function-trace - The latency tracers will enable function tracing
878 if this option is enabled (default it is). When
879 it is disabled, the latency tracers do not trace
880 functions. This keeps the overhead of the tracer down
881 when performing latency tests.
883 Note: Some tracers have their own options. They only appear
884 when the tracer is active.
891 When interrupts are disabled, the CPU can not react to any other
892 external event (besides NMIs and SMIs). This prevents the timer
893 interrupt from triggering or the mouse interrupt from letting
894 the kernel know of a new mouse event. The result is a latency
895 with the reaction time.
897 The irqsoff tracer tracks the time for which interrupts are
898 disabled. When a new maximum latency is hit, the tracer saves
899 the trace leading up to that latency point so that every time a
900 new maximum is reached, the old saved trace is discarded and the
903 To reset the maximum, echo 0 into tracing_max_latency. Here is
906 # echo 0 > options/function-trace
907 # echo irqsoff > current_tracer
908 # echo 1 > tracing_on
909 # echo 0 > tracing_max_latency
912 # echo 0 > tracing_on
916 # irqsoff latency trace v1.1.5 on 3.8.0-test+
917 # --------------------------------------------------------------------
918 # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
920 # | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
922 # => started at: run_timer_softirq
923 # => ended at: run_timer_softirq
927 # / _-----=> irqs-off
928 # | / _----=> need-resched
929 # || / _---=> hardirq/softirq
930 # ||| / _--=> preempt-depth
932 # cmd pid ||||| time | caller
934 <idle>-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq
935 <idle>-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq
936 <idle>-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq
937 <idle>-0 0dNs3 25us : <stack trace>
938 => _raw_spin_unlock_irq
944 => smp_apic_timer_interrupt
945 => apic_timer_interrupt
950 => x86_64_start_reservations
951 => x86_64_start_kernel
953 Here we see that that we had a latency of 16 microseconds (which is
954 very good). The _raw_spin_lock_irq in run_timer_softirq disabled
955 interrupts. The difference between the 16 and the displayed
956 timestamp 25us occurred because the clock was incremented
957 between the time of recording the max latency and the time of
958 recording the function that had that latency.
960 Note the above example had function-trace not set. If we set
961 function-trace, we get a much larger output:
963 with echo 1 > options/function-trace
967 # irqsoff latency trace v1.1.5 on 3.8.0-test+
968 # --------------------------------------------------------------------
969 # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
971 # | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
973 # => started at: ata_scsi_queuecmd
974 # => ended at: ata_scsi_queuecmd
978 # / _-----=> irqs-off
979 # | / _----=> need-resched
980 # || / _---=> hardirq/softirq
981 # ||| / _--=> preempt-depth
983 # cmd pid ||||| time | caller
985 bash-2042 3d... 0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
986 bash-2042 3d... 0us : add_preempt_count <-_raw_spin_lock_irqsave
987 bash-2042 3d..1 1us : ata_scsi_find_dev <-ata_scsi_queuecmd
988 bash-2042 3d..1 1us : __ata_scsi_find_dev <-ata_scsi_find_dev
989 bash-2042 3d..1 2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
990 bash-2042 3d..1 2us : ata_qc_new_init <-__ata_scsi_queuecmd
991 bash-2042 3d..1 3us : ata_sg_init <-__ata_scsi_queuecmd
992 bash-2042 3d..1 4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
993 bash-2042 3d..1 4us : ata_build_rw_tf <-ata_scsi_rw_xlat
995 bash-2042 3d..1 67us : delay_tsc <-__delay
996 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
997 bash-2042 3d..2 67us : sub_preempt_count <-delay_tsc
998 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
999 bash-2042 3d..2 68us : sub_preempt_count <-delay_tsc
1000 bash-2042 3d..1 68us+: ata_bmdma_start <-ata_bmdma_qc_issue
1001 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1002 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1003 bash-2042 3d..1 72us+: trace_hardirqs_on <-ata_scsi_queuecmd
1004 bash-2042 3d..1 120us : <stack trace>
1005 => _raw_spin_unlock_irqrestore
1006 => ata_scsi_queuecmd
1007 => scsi_dispatch_cmd
1009 => __blk_run_queue_uncond
1012 => generic_make_request
1015 => __ext3_get_inode_loc
1024 => user_path_at_empty
1029 => system_call_fastpath
1032 Here we traced a 71 microsecond latency. But we also see all the
1033 functions that were called during that time. Note that by
1034 enabling function tracing, we incur an added overhead. This
1035 overhead may extend the latency times. But nevertheless, this
1036 trace has provided some very helpful debugging information.
1042 When preemption is disabled, we may be able to receive
1043 interrupts but the task cannot be preempted and a higher
1044 priority task must wait for preemption to be enabled again
1045 before it can preempt a lower priority task.
1047 The preemptoff tracer traces the places that disable preemption.
1048 Like the irqsoff tracer, it records the maximum latency for
1049 which preemption was disabled. The control of preemptoff tracer
1050 is much like the irqsoff tracer.
1052 # echo 0 > options/function-trace
1053 # echo preemptoff > current_tracer
1054 # echo 1 > tracing_on
1055 # echo 0 > tracing_max_latency
1058 # echo 0 > tracing_on
1060 # tracer: preemptoff
1062 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1063 # --------------------------------------------------------------------
1064 # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1066 # | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
1068 # => started at: do_IRQ
1069 # => ended at: do_IRQ
1073 # / _-----=> irqs-off
1074 # | / _----=> need-resched
1075 # || / _---=> hardirq/softirq
1076 # ||| / _--=> preempt-depth
1078 # cmd pid ||||| time | caller
1080 sshd-1991 1d.h. 0us+: irq_enter <-do_IRQ
1081 sshd-1991 1d..1 46us : irq_exit <-do_IRQ
1082 sshd-1991 1d..1 47us+: trace_preempt_on <-do_IRQ
1083 sshd-1991 1d..1 52us : <stack trace>
1084 => sub_preempt_count
1090 This has some more changes. Preemption was disabled when an
1091 interrupt came in (notice the 'h'), and was enabled on exit.
1092 But we also see that interrupts have been disabled when entering
1093 the preempt off section and leaving it (the 'd'). We do not know if
1094 interrupts were enabled in the mean time or shortly after this
1097 # tracer: preemptoff
1099 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1100 # --------------------------------------------------------------------
1101 # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1103 # | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
1105 # => started at: wake_up_new_task
1106 # => ended at: task_rq_unlock
1110 # / _-----=> irqs-off
1111 # | / _----=> need-resched
1112 # || / _---=> hardirq/softirq
1113 # ||| / _--=> preempt-depth
1115 # cmd pid ||||| time | caller
1117 bash-1994 1d..1 0us : _raw_spin_lock_irqsave <-wake_up_new_task
1118 bash-1994 1d..1 0us : select_task_rq_fair <-select_task_rq
1119 bash-1994 1d..1 1us : __rcu_read_lock <-select_task_rq_fair
1120 bash-1994 1d..1 1us : source_load <-select_task_rq_fair
1121 bash-1994 1d..1 1us : source_load <-select_task_rq_fair
1123 bash-1994 1d..1 12us : irq_enter <-smp_apic_timer_interrupt
1124 bash-1994 1d..1 12us : rcu_irq_enter <-irq_enter
1125 bash-1994 1d..1 13us : add_preempt_count <-irq_enter
1126 bash-1994 1d.h1 13us : exit_idle <-smp_apic_timer_interrupt
1127 bash-1994 1d.h1 13us : hrtimer_interrupt <-smp_apic_timer_interrupt
1128 bash-1994 1d.h1 13us : _raw_spin_lock <-hrtimer_interrupt
1129 bash-1994 1d.h1 14us : add_preempt_count <-_raw_spin_lock
1130 bash-1994 1d.h2 14us : ktime_get_update_offsets <-hrtimer_interrupt
1132 bash-1994 1d.h1 35us : lapic_next_event <-clockevents_program_event
1133 bash-1994 1d.h1 35us : irq_exit <-smp_apic_timer_interrupt
1134 bash-1994 1d.h1 36us : sub_preempt_count <-irq_exit
1135 bash-1994 1d..2 36us : do_softirq <-irq_exit
1136 bash-1994 1d..2 36us : __do_softirq <-call_softirq
1137 bash-1994 1d..2 36us : __local_bh_disable <-__do_softirq
1138 bash-1994 1d.s2 37us : add_preempt_count <-_raw_spin_lock_irq
1139 bash-1994 1d.s3 38us : _raw_spin_unlock <-run_timer_softirq
1140 bash-1994 1d.s3 39us : sub_preempt_count <-_raw_spin_unlock
1141 bash-1994 1d.s2 39us : call_timer_fn <-run_timer_softirq
1143 bash-1994 1dNs2 81us : cpu_needs_another_gp <-rcu_process_callbacks
1144 bash-1994 1dNs2 82us : __local_bh_enable <-__do_softirq
1145 bash-1994 1dNs2 82us : sub_preempt_count <-__local_bh_enable
1146 bash-1994 1dN.2 82us : idle_cpu <-irq_exit
1147 bash-1994 1dN.2 83us : rcu_irq_exit <-irq_exit
1148 bash-1994 1dN.2 83us : sub_preempt_count <-irq_exit
1149 bash-1994 1.N.1 84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
1150 bash-1994 1.N.1 84us+: trace_preempt_on <-task_rq_unlock
1151 bash-1994 1.N.1 104us : <stack trace>
1152 => sub_preempt_count
1153 => _raw_spin_unlock_irqrestore
1161 The above is an example of the preemptoff trace with
1162 function-trace set. Here we see that interrupts were not disabled
1163 the entire time. The irq_enter code lets us know that we entered
1164 an interrupt 'h'. Before that, the functions being traced still
1165 show that it is not in an interrupt, but we can see from the
1166 functions themselves that this is not the case.
1171 Knowing the locations that have interrupts disabled or
1172 preemption disabled for the longest times is helpful. But
1173 sometimes we would like to know when either preemption and/or
1174 interrupts are disabled.
1176 Consider the following code:
1178 local_irq_disable();
1179 call_function_with_irqs_off();
1181 call_function_with_irqs_and_preemption_off();
1183 call_function_with_preemption_off();
1186 The irqsoff tracer will record the total length of
1187 call_function_with_irqs_off() and
1188 call_function_with_irqs_and_preemption_off().
1190 The preemptoff tracer will record the total length of
1191 call_function_with_irqs_and_preemption_off() and
1192 call_function_with_preemption_off().
1194 But neither will trace the time that interrupts and/or
1195 preemption is disabled. This total time is the time that we can
1196 not schedule. To record this time, use the preemptirqsoff
1199 Again, using this trace is much like the irqsoff and preemptoff
1202 # echo 0 > options/function-trace
1203 # echo preemptirqsoff > current_tracer
1204 # echo 1 > tracing_on
1205 # echo 0 > tracing_max_latency
1208 # echo 0 > tracing_on
1210 # tracer: preemptirqsoff
1212 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1213 # --------------------------------------------------------------------
1214 # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1216 # | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
1218 # => started at: ata_scsi_queuecmd
1219 # => ended at: ata_scsi_queuecmd
1223 # / _-----=> irqs-off
1224 # | / _----=> need-resched
1225 # || / _---=> hardirq/softirq
1226 # ||| / _--=> preempt-depth
1228 # cmd pid ||||| time | caller
1230 ls-2230 3d... 0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1231 ls-2230 3...1 100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1232 ls-2230 3...1 101us+: trace_preempt_on <-ata_scsi_queuecmd
1233 ls-2230 3...1 111us : <stack trace>
1234 => sub_preempt_count
1235 => _raw_spin_unlock_irqrestore
1236 => ata_scsi_queuecmd
1237 => scsi_dispatch_cmd
1239 => __blk_run_queue_uncond
1242 => generic_make_request
1247 => htree_dirblock_to_tree
1248 => ext3_htree_fill_tree
1252 => system_call_fastpath
1255 The trace_hardirqs_off_thunk is called from assembly on x86 when
1256 interrupts are disabled in the assembly code. Without the
1257 function tracing, we do not know if interrupts were enabled
1258 within the preemption points. We do see that it started with
1261 Here is a trace with function-trace set:
1263 # tracer: preemptirqsoff
1265 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1266 # --------------------------------------------------------------------
1267 # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1269 # | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
1271 # => started at: schedule
1272 # => ended at: mutex_unlock
1276 # / _-----=> irqs-off
1277 # | / _----=> need-resched
1278 # || / _---=> hardirq/softirq
1279 # ||| / _--=> preempt-depth
1281 # cmd pid ||||| time | caller
1283 kworker/-59 3...1 0us : __schedule <-schedule
1284 kworker/-59 3d..1 0us : rcu_preempt_qs <-rcu_note_context_switch
1285 kworker/-59 3d..1 1us : add_preempt_count <-_raw_spin_lock_irq
1286 kworker/-59 3d..2 1us : deactivate_task <-__schedule
1287 kworker/-59 3d..2 1us : dequeue_task <-deactivate_task
1288 kworker/-59 3d..2 2us : update_rq_clock <-dequeue_task
1289 kworker/-59 3d..2 2us : dequeue_task_fair <-dequeue_task
1290 kworker/-59 3d..2 2us : update_curr <-dequeue_task_fair
1291 kworker/-59 3d..2 2us : update_min_vruntime <-update_curr
1292 kworker/-59 3d..2 3us : cpuacct_charge <-update_curr
1293 kworker/-59 3d..2 3us : __rcu_read_lock <-cpuacct_charge
1294 kworker/-59 3d..2 3us : __rcu_read_unlock <-cpuacct_charge
1295 kworker/-59 3d..2 3us : update_cfs_rq_blocked_load <-dequeue_task_fair
1296 kworker/-59 3d..2 4us : clear_buddies <-dequeue_task_fair
1297 kworker/-59 3d..2 4us : account_entity_dequeue <-dequeue_task_fair
1298 kworker/-59 3d..2 4us : update_min_vruntime <-dequeue_task_fair
1299 kworker/-59 3d..2 4us : update_cfs_shares <-dequeue_task_fair
1300 kworker/-59 3d..2 5us : hrtick_update <-dequeue_task_fair
1301 kworker/-59 3d..2 5us : wq_worker_sleeping <-__schedule
1302 kworker/-59 3d..2 5us : kthread_data <-wq_worker_sleeping
1303 kworker/-59 3d..2 5us : put_prev_task_fair <-__schedule
1304 kworker/-59 3d..2 6us : pick_next_task_fair <-pick_next_task
1305 kworker/-59 3d..2 6us : clear_buddies <-pick_next_task_fair
1306 kworker/-59 3d..2 6us : set_next_entity <-pick_next_task_fair
1307 kworker/-59 3d..2 6us : update_stats_wait_end <-set_next_entity
1308 ls-2269 3d..2 7us : finish_task_switch <-__schedule
1309 ls-2269 3d..2 7us : _raw_spin_unlock_irq <-finish_task_switch
1310 ls-2269 3d..2 8us : do_IRQ <-ret_from_intr
1311 ls-2269 3d..2 8us : irq_enter <-do_IRQ
1312 ls-2269 3d..2 8us : rcu_irq_enter <-irq_enter
1313 ls-2269 3d..2 9us : add_preempt_count <-irq_enter
1314 ls-2269 3d.h2 9us : exit_idle <-do_IRQ
1316 ls-2269 3d.h3 20us : sub_preempt_count <-_raw_spin_unlock
1317 ls-2269 3d.h2 20us : irq_exit <-do_IRQ
1318 ls-2269 3d.h2 21us : sub_preempt_count <-irq_exit
1319 ls-2269 3d..3 21us : do_softirq <-irq_exit
1320 ls-2269 3d..3 21us : __do_softirq <-call_softirq
1321 ls-2269 3d..3 21us+: __local_bh_disable <-__do_softirq
1322 ls-2269 3d.s4 29us : sub_preempt_count <-_local_bh_enable_ip
1323 ls-2269 3d.s5 29us : sub_preempt_count <-_local_bh_enable_ip
1324 ls-2269 3d.s5 31us : do_IRQ <-ret_from_intr
1325 ls-2269 3d.s5 31us : irq_enter <-do_IRQ
1326 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
1328 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
1329 ls-2269 3d.s5 32us : add_preempt_count <-irq_enter
1330 ls-2269 3d.H5 32us : exit_idle <-do_IRQ
1331 ls-2269 3d.H5 32us : handle_irq <-do_IRQ
1332 ls-2269 3d.H5 32us : irq_to_desc <-handle_irq
1333 ls-2269 3d.H5 33us : handle_fasteoi_irq <-handle_irq
1335 ls-2269 3d.s5 158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
1336 ls-2269 3d.s3 158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
1337 ls-2269 3d.s3 159us : __local_bh_enable <-__do_softirq
1338 ls-2269 3d.s3 159us : sub_preempt_count <-__local_bh_enable
1339 ls-2269 3d..3 159us : idle_cpu <-irq_exit
1340 ls-2269 3d..3 159us : rcu_irq_exit <-irq_exit
1341 ls-2269 3d..3 160us : sub_preempt_count <-irq_exit
1342 ls-2269 3d... 161us : __mutex_unlock_slowpath <-mutex_unlock
1343 ls-2269 3d... 162us+: trace_hardirqs_on <-mutex_unlock
1344 ls-2269 3d... 186us : <stack trace>
1345 => __mutex_unlock_slowpath
1352 => system_call_fastpath
1354 This is an interesting trace. It started with kworker running and
1355 scheduling out and ls taking over. But as soon as ls released the
1356 rq lock and enabled interrupts (but not preemption) an interrupt
1357 triggered. When the interrupt finished, it started running softirqs.
1358 But while the softirq was running, another interrupt triggered.
1359 When an interrupt is running inside a softirq, the annotation is 'H'.
1365 One common case that people are interested in tracing is the
1366 time it takes for a task that is woken to actually wake up.
1367 Now for non Real-Time tasks, this can be arbitrary. But tracing
1368 it none the less can be interesting.
1370 Without function tracing:
1372 # echo 0 > options/function-trace
1373 # echo wakeup > current_tracer
1374 # echo 1 > tracing_on
1375 # echo 0 > tracing_max_latency
1377 # echo 0 > tracing_on
1381 # wakeup latency trace v1.1.5 on 3.8.0-test+
1382 # --------------------------------------------------------------------
1383 # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1385 # | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
1389 # / _-----=> irqs-off
1390 # | / _----=> need-resched
1391 # || / _---=> hardirq/softirq
1392 # ||| / _--=> preempt-depth
1394 # cmd pid ||||| time | caller
1396 <idle>-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H
1397 <idle>-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1398 <idle>-0 3d..3 15us : __schedule <-schedule
1399 <idle>-0 3d..3 15us : 0:120:R ==> [003] 312:100:R kworker/3:1H
1401 The tracer only traces the highest priority task in the system
1402 to avoid tracing the normal circumstances. Here we see that
1403 the kworker with a nice priority of -20 (not very nice), took
1404 just 15 microseconds from the time it woke up, to the time it
1407 Non Real-Time tasks are not that interesting. A more interesting
1408 trace is to concentrate only on Real-Time tasks.
1413 In a Real-Time environment it is very important to know the
1414 wakeup time it takes for the highest priority task that is woken
1415 up to the time that it executes. This is also known as "schedule
1416 latency". I stress the point that this is about RT tasks. It is
1417 also important to know the scheduling latency of non-RT tasks,
1418 but the average schedule latency is better for non-RT tasks.
1419 Tools like LatencyTop are more appropriate for such
1422 Real-Time environments are interested in the worst case latency.
1423 That is the longest latency it takes for something to happen,
1424 and not the average. We can have a very fast scheduler that may
1425 only have a large latency once in a while, but that would not
1426 work well with Real-Time tasks. The wakeup_rt tracer was designed
1427 to record the worst case wakeups of RT tasks. Non-RT tasks are
1428 not recorded because the tracer only records one worst case and
1429 tracing non-RT tasks that are unpredictable will overwrite the
1430 worst case latency of RT tasks (just run the normal wakeup
1431 tracer for a while to see that effect).
1433 Since this tracer only deals with RT tasks, we will run this
1434 slightly differently than we did with the previous tracers.
1435 Instead of performing an 'ls', we will run 'sleep 1' under
1436 'chrt' which changes the priority of the task.
1438 # echo 0 > options/function-trace
1439 # echo wakeup_rt > current_tracer
1440 # echo 1 > tracing_on
1441 # echo 0 > tracing_max_latency
1443 # echo 0 > tracing_on
1449 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1450 # --------------------------------------------------------------------
1451 # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1453 # | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
1457 # / _-----=> irqs-off
1458 # | / _----=> need-resched
1459 # || / _---=> hardirq/softirq
1460 # ||| / _--=> preempt-depth
1462 # cmd pid ||||| time | caller
1464 <idle>-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep
1465 <idle>-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1466 <idle>-0 3d..3 5us : __schedule <-schedule
1467 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep
1470 Running this on an idle system, we see that it only took 5 microseconds
1471 to perform the task switch. Note, since the trace point in the schedule
1472 is before the actual "switch", we stop the tracing when the recorded task
1473 is about to schedule in. This may change if we add a new marker at the
1474 end of the scheduler.
1476 Notice that the recorded task is 'sleep' with the PID of 2389
1477 and it has an rt_prio of 5. This priority is user-space priority
1478 and not the internal kernel priority. The policy is 1 for
1479 SCHED_FIFO and 2 for SCHED_RR.
1481 Note, that the trace data shows the internal priority (99 - rtprio).
1483 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep
1485 The 0:120:R means idle was running with a nice priority of 0 (120 - 20)
1486 and in the running state 'R'. The sleep task was scheduled in with
1487 2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
1488 and it too is in the running state.
1490 Doing the same with chrt -r 5 and function-trace set.
1492 echo 1 > options/function-trace
1496 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1497 # --------------------------------------------------------------------
1498 # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1500 # | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
1504 # / _-----=> irqs-off
1505 # | / _----=> need-resched
1506 # || / _---=> hardirq/softirq
1507 # ||| / _--=> preempt-depth
1509 # cmd pid ||||| time | caller
1511 <idle>-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep
1512 <idle>-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
1513 <idle>-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup
1514 <idle>-0 3d.h3 3us : resched_task <-check_preempt_curr
1515 <idle>-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup
1516 <idle>-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up
1517 <idle>-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock
1518 <idle>-0 3dNh2 5us : ttwu_stat <-try_to_wake_up
1519 <idle>-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
1520 <idle>-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1521 <idle>-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer
1522 <idle>-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock
1523 <idle>-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt
1524 <idle>-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock
1525 <idle>-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt
1526 <idle>-0 3dNh1 7us : clockevents_program_event <-tick_program_event
1527 <idle>-0 3dNh1 8us : ktime_get <-clockevents_program_event
1528 <idle>-0 3dNh1 8us : lapic_next_event <-clockevents_program_event
1529 <idle>-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt
1530 <idle>-0 3dNh1 9us : sub_preempt_count <-irq_exit
1531 <idle>-0 3dN.2 9us : idle_cpu <-irq_exit
1532 <idle>-0 3dN.2 9us : rcu_irq_exit <-irq_exit
1533 <idle>-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
1534 <idle>-0 3dN.2 10us : sub_preempt_count <-irq_exit
1535 <idle>-0 3.N.1 11us : rcu_idle_exit <-cpu_idle
1536 <idle>-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
1537 <idle>-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle
1538 <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
1539 <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit
1540 <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
1541 <idle>-0 3dN.1 13us : update_cpu_load_nohz <-tick_nohz_idle_exit
1542 <idle>-0 3dN.1 13us : _raw_spin_lock <-update_cpu_load_nohz
1543 <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock
1544 <idle>-0 3dN.2 13us : __update_cpu_load <-update_cpu_load_nohz
1545 <idle>-0 3dN.2 14us : sched_avg_update <-__update_cpu_load
1546 <idle>-0 3dN.2 14us : _raw_spin_unlock <-update_cpu_load_nohz
1547 <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock
1548 <idle>-0 3dN.1 15us : calc_load_exit_idle <-tick_nohz_idle_exit
1549 <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
1550 <idle>-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit
1551 <idle>-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel
1552 <idle>-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
1553 <idle>-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
1554 <idle>-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave
1555 <idle>-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16
1556 <idle>-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer
1557 <idle>-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram
1558 <idle>-0 3dN.2 18us : clockevents_program_event <-tick_program_event
1559 <idle>-0 3dN.2 18us : ktime_get <-clockevents_program_event
1560 <idle>-0 3dN.2 18us : lapic_next_event <-clockevents_program_event
1561 <idle>-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
1562 <idle>-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1563 <idle>-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit
1564 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
1565 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
1566 <idle>-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
1567 <idle>-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
1568 <idle>-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
1569 <idle>-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
1570 <idle>-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave
1571 <idle>-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns
1572 <idle>-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns
1573 <idle>-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns
1574 <idle>-0 3dN.2 23us : clockevents_program_event <-tick_program_event
1575 <idle>-0 3dN.2 23us : ktime_get <-clockevents_program_event
1576 <idle>-0 3dN.2 23us : lapic_next_event <-clockevents_program_event
1577 <idle>-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
1578 <idle>-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1579 <idle>-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit
1580 <idle>-0 3dN.1 24us : account_idle_time <-account_idle_ticks
1581 <idle>-0 3.N.1 25us : sub_preempt_count <-cpu_idle
1582 <idle>-0 3.N.. 25us : schedule <-cpu_idle
1583 <idle>-0 3.N.. 25us : __schedule <-preempt_schedule
1584 <idle>-0 3.N.. 26us : add_preempt_count <-__schedule
1585 <idle>-0 3.N.1 26us : rcu_note_context_switch <-__schedule
1586 <idle>-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch
1587 <idle>-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch
1588 <idle>-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule
1589 <idle>-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq
1590 <idle>-0 3dN.2 28us : put_prev_task_idle <-__schedule
1591 <idle>-0 3dN.2 28us : pick_next_task_stop <-pick_next_task
1592 <idle>-0 3dN.2 28us : pick_next_task_rt <-pick_next_task
1593 <idle>-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt
1594 <idle>-0 3d..3 29us : __schedule <-preempt_schedule
1595 <idle>-0 3d..3 30us : 0:120:R ==> [003] 2448: 94:R sleep
1597 This isn't that big of a trace, even with function tracing enabled,
1598 so I included the entire trace.
1600 The interrupt went off while when the system was idle. Somewhere
1601 before task_woken_rt() was called, the NEED_RESCHED flag was set,
1602 this is indicated by the first occurrence of the 'N' flag.
1604 Latency tracing and events
1605 --------------------------
1606 As function tracing can induce a much larger latency, but without
1607 seeing what happens within the latency it is hard to know what
1608 caused it. There is a middle ground, and that is with enabling
1611 # echo 0 > options/function-trace
1612 # echo wakeup_rt > current_tracer
1613 # echo 1 > events/enable
1614 # echo 1 > tracing_on
1615 # echo 0 > tracing_max_latency
1617 # echo 0 > tracing_on
1621 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1622 # --------------------------------------------------------------------
1623 # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1625 # | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
1629 # / _-----=> irqs-off
1630 # | / _----=> need-resched
1631 # || / _---=> hardirq/softirq
1632 # ||| / _--=> preempt-depth
1634 # cmd pid ||||| time | caller
1636 <idle>-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep
1637 <idle>-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
1638 <idle>-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
1639 <idle>-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
1640 <idle>-0 2.N.2 2us : power_end: cpu_id=2
1641 <idle>-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2
1642 <idle>-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
1643 <idle>-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
1644 <idle>-0 2.N.2 5us : rcu_utilization: Start context switch
1645 <idle>-0 2.N.2 5us : rcu_utilization: End context switch
1646 <idle>-0 2d..3 6us : __schedule <-schedule
1647 <idle>-0 2d..3 6us : 0:120:R ==> [002] 5882: 94:R sleep
1653 This tracer is the function tracer. Enabling the function tracer
1654 can be done from the debug file system. Make sure the
1655 ftrace_enabled is set; otherwise this tracer is a nop.
1656 See the "ftrace_enabled" section below.
1658 # sysctl kernel.ftrace_enabled=1
1659 # echo function > current_tracer
1660 # echo 1 > tracing_on
1662 # echo 0 > tracing_on
1666 # entries-in-buffer/entries-written: 24799/24799 #P:4
1669 # / _----=> need-resched
1670 # | / _---=> hardirq/softirq
1671 # || / _--=> preempt-depth
1673 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
1675 bash-1994 [002] .... 3082.063030: mutex_unlock <-rb_simple_write
1676 bash-1994 [002] .... 3082.063031: __mutex_unlock_slowpath <-mutex_unlock
1677 bash-1994 [002] .... 3082.063031: __fsnotify_parent <-fsnotify_modify
1678 bash-1994 [002] .... 3082.063032: fsnotify <-fsnotify_modify
1679 bash-1994 [002] .... 3082.063032: __srcu_read_lock <-fsnotify
1680 bash-1994 [002] .... 3082.063032: add_preempt_count <-__srcu_read_lock
1681 bash-1994 [002] ...1 3082.063032: sub_preempt_count <-__srcu_read_lock
1682 bash-1994 [002] .... 3082.063033: __srcu_read_unlock <-fsnotify
1686 Note: function tracer uses ring buffers to store the above
1687 entries. The newest data may overwrite the oldest data.
1688 Sometimes using echo to stop the trace is not sufficient because
1689 the tracing could have overwritten the data that you wanted to
1690 record. For this reason, it is sometimes better to disable
1691 tracing directly from a program. This allows you to stop the
1692 tracing at the point that you hit the part that you are
1693 interested in. To disable the tracing directly from a C program,
1694 something like following code snippet can be used:
1698 int main(int argc, char *argv[]) {
1700 trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
1702 if (condition_hit()) {
1703 write(trace_fd, "0", 1);
1709 Single thread tracing
1710 ---------------------
1712 By writing into set_ftrace_pid you can trace a
1713 single thread. For example:
1715 # cat set_ftrace_pid
1717 # echo 3111 > set_ftrace_pid
1718 # cat set_ftrace_pid
1720 # echo function > current_tracer
1724 # TASK-PID CPU# TIMESTAMP FUNCTION
1726 yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return
1727 yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
1728 yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
1729 yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
1730 yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll
1731 yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll
1732 # echo -1 > set_ftrace_pid
1736 # TASK-PID CPU# TIMESTAMP FUNCTION
1738 ##### CPU 3 buffer started ####
1739 yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait
1740 yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry
1741 yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry
1742 yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit
1743 yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit
1745 If you want to trace a function when executing, you could use
1746 something like this simple program:
1750 #include <sys/types.h>
1751 #include <sys/stat.h>
1757 #define STR(x) _STR(x)
1758 #define MAX_PATH 256
1760 const char *find_debugfs(void)
1762 static char debugfs[MAX_PATH+1];
1763 static int debugfs_found;
1770 if ((fp = fopen("/proc/mounts","r")) == NULL) {
1771 perror("/proc/mounts");
1775 while (fscanf(fp, "%*s %"
1777 "s %99s %*s %*d %*d\n",
1778 debugfs, type) == 2) {
1779 if (strcmp(type, "debugfs") == 0)
1784 if (strcmp(type, "debugfs") != 0) {
1785 fprintf(stderr, "debugfs not mounted");
1789 strcat(debugfs, "/tracing/");
1795 const char *tracing_file(const char *file_name)
1797 static char trace_file[MAX_PATH+1];
1798 snprintf(trace_file, MAX_PATH, "%s/%s", find_debugfs(), file_name);
1802 int main (int argc, char **argv)
1812 ffd = open(tracing_file("current_tracer"), O_WRONLY);
1815 write(ffd, "nop", 3);
1817 fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
1818 s = sprintf(line, "%d\n", getpid());
1821 write(ffd, "function", 8);
1826 execvp(argv[1], argv+1);
1832 Or this simple script!
1837 debugfs=`sed -ne 's/^debugfs \(.*\) debugfs.*/\1/p' /proc/mounts`
1838 echo nop > $debugfs/tracing/current_tracer
1839 echo 0 > $debugfs/tracing/tracing_on
1840 echo $$ > $debugfs/tracing/set_ftrace_pid
1841 echo function > $debugfs/tracing/current_tracer
1842 echo 1 > $debugfs/tracing/tracing_on
1847 function graph tracer
1848 ---------------------------
1850 This tracer is similar to the function tracer except that it
1851 probes a function on its entry and its exit. This is done by
1852 using a dynamically allocated stack of return addresses in each
1853 task_struct. On function entry the tracer overwrites the return
1854 address of each function traced to set a custom probe. Thus the
1855 original return address is stored on the stack of return address
1858 Probing on both ends of a function leads to special features
1861 - measure of a function's time execution
1862 - having a reliable call stack to draw function calls graph
1864 This tracer is useful in several situations:
1866 - you want to find the reason of a strange kernel behavior and
1867 need to see what happens in detail on any areas (or specific
1870 - you are experiencing weird latencies but it's difficult to
1873 - you want to find quickly which path is taken by a specific
1876 - you just want to peek inside a working kernel and want to see
1879 # tracer: function_graph
1881 # CPU DURATION FUNCTION CALLS
1885 0) | do_sys_open() {
1887 0) | kmem_cache_alloc() {
1888 0) 1.382 us | __might_sleep();
1890 0) | strncpy_from_user() {
1891 0) | might_fault() {
1892 0) 1.389 us | __might_sleep();
1897 0) 0.668 us | _spin_lock();
1898 0) 0.570 us | expand_files();
1899 0) 0.586 us | _spin_unlock();
1902 There are several columns that can be dynamically
1903 enabled/disabled. You can use every combination of options you
1904 want, depending on your needs.
1906 - The cpu number on which the function executed is default
1907 enabled. It is sometimes better to only trace one cpu (see
1908 tracing_cpu_mask file) or you might sometimes see unordered
1909 function calls while cpu tracing switch.
1911 hide: echo nofuncgraph-cpu > trace_options
1912 show: echo funcgraph-cpu > trace_options
1914 - The duration (function's time of execution) is displayed on
1915 the closing bracket line of a function or on the same line
1916 than the current function in case of a leaf one. It is default
1919 hide: echo nofuncgraph-duration > trace_options
1920 show: echo funcgraph-duration > trace_options
1922 - The overhead field precedes the duration field in case of
1923 reached duration thresholds.
1925 hide: echo nofuncgraph-overhead > trace_options
1926 show: echo funcgraph-overhead > trace_options
1927 depends on: funcgraph-duration
1932 0) 0.646 us | _spin_lock_irqsave();
1933 0) 0.684 us | _spin_unlock_irqrestore();
1935 0) 0.548 us | fput();
1941 0) | kmem_cache_free() {
1942 0) 0.518 us | __phys_addr();
1948 + means that the function exceeded 10 usecs.
1949 ! means that the function exceeded 100 usecs.
1952 - The task/pid field displays the thread cmdline and pid which
1953 executed the function. It is default disabled.
1955 hide: echo nofuncgraph-proc > trace_options
1956 show: echo funcgraph-proc > trace_options
1960 # tracer: function_graph
1962 # CPU TASK/PID DURATION FUNCTION CALLS
1964 0) sh-4802 | | d_free() {
1965 0) sh-4802 | | call_rcu() {
1966 0) sh-4802 | | __call_rcu() {
1967 0) sh-4802 | 0.616 us | rcu_process_gp_end();
1968 0) sh-4802 | 0.586 us | check_for_new_grace_period();
1969 0) sh-4802 | 2.899 us | }
1970 0) sh-4802 | 4.040 us | }
1971 0) sh-4802 | 5.151 us | }
1972 0) sh-4802 | + 49.370 us | }
1975 - The absolute time field is an absolute timestamp given by the
1976 system clock since it started. A snapshot of this time is
1977 given on each entry/exit of functions
1979 hide: echo nofuncgraph-abstime > trace_options
1980 show: echo funcgraph-abstime > trace_options
1985 # TIME CPU DURATION FUNCTION CALLS
1987 360.774522 | 1) 0.541 us | }
1988 360.774522 | 1) 4.663 us | }
1989 360.774523 | 1) 0.541 us | __wake_up_bit();
1990 360.774524 | 1) 6.796 us | }
1991 360.774524 | 1) 7.952 us | }
1992 360.774525 | 1) 9.063 us | }
1993 360.774525 | 1) 0.615 us | journal_mark_dirty();
1994 360.774527 | 1) 0.578 us | __brelse();
1995 360.774528 | 1) | reiserfs_prepare_for_journal() {
1996 360.774528 | 1) | unlock_buffer() {
1997 360.774529 | 1) | wake_up_bit() {
1998 360.774529 | 1) | bit_waitqueue() {
1999 360.774530 | 1) 0.594 us | __phys_addr();
2002 You can put some comments on specific functions by using
2003 trace_printk() For example, if you want to put a comment inside
2004 the __might_sleep() function, you just have to include
2005 <linux/ftrace.h> and call trace_printk() inside __might_sleep()
2007 trace_printk("I'm a comment!\n")
2011 1) | __might_sleep() {
2012 1) | /* I'm a comment! */
2016 You might find other useful features for this tracer in the
2017 following "dynamic ftrace" section such as tracing only specific
2023 If CONFIG_DYNAMIC_FTRACE is set, the system will run with
2024 virtually no overhead when function tracing is disabled. The way
2025 this works is the mcount function call (placed at the start of
2026 every kernel function, produced by the -pg switch in gcc),
2027 starts of pointing to a simple return. (Enabling FTRACE will
2028 include the -pg switch in the compiling of the kernel.)
2030 At compile time every C file object is run through the
2031 recordmcount program (located in the scripts directory). This
2032 program will parse the ELF headers in the C object to find all
2033 the locations in the .text section that call mcount. (Note, only
2034 white listed .text sections are processed, since processing other
2035 sections like .init.text may cause races due to those sections
2036 being freed unexpectedly).
2038 A new section called "__mcount_loc" is created that holds
2039 references to all the mcount call sites in the .text section.
2040 The recordmcount program re-links this section back into the
2041 original object. The final linking stage of the kernel will add all these
2042 references into a single table.
2044 On boot up, before SMP is initialized, the dynamic ftrace code
2045 scans this table and updates all the locations into nops. It
2046 also records the locations, which are added to the
2047 available_filter_functions list. Modules are processed as they
2048 are loaded and before they are executed. When a module is
2049 unloaded, it also removes its functions from the ftrace function
2050 list. This is automatic in the module unload code, and the
2051 module author does not need to worry about it.
2053 When tracing is enabled, the process of modifying the function
2054 tracepoints is dependent on architecture. The old method is to use
2055 kstop_machine to prevent races with the CPUs executing code being
2056 modified (which can cause the CPU to do undesirable things, especially
2057 if the modified code crosses cache (or page) boundaries), and the nops are
2058 patched back to calls. But this time, they do not call mcount
2059 (which is just a function stub). They now call into the ftrace
2062 The new method of modifying the function tracepoints is to place
2063 a breakpoint at the location to be modified, sync all CPUs, modify
2064 the rest of the instruction not covered by the breakpoint. Sync
2065 all CPUs again, and then remove the breakpoint with the finished
2066 version to the ftrace call site.
2068 Some archs do not even need to monkey around with the synchronization,
2069 and can just slap the new code on top of the old without any
2070 problems with other CPUs executing it at the same time.
2072 One special side-effect to the recording of the functions being
2073 traced is that we can now selectively choose which functions we
2074 wish to trace and which ones we want the mcount calls to remain
2077 Two files are used, one for enabling and one for disabling the
2078 tracing of specified functions. They are:
2086 A list of available functions that you can add to these files is
2089 available_filter_functions
2091 # cat available_filter_functions
2100 If I am only interested in sys_nanosleep and hrtimer_interrupt:
2102 # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
2103 # echo function > current_tracer
2104 # echo 1 > tracing_on
2106 # echo 0 > tracing_on
2110 # entries-in-buffer/entries-written: 5/5 #P:4
2113 # / _----=> need-resched
2114 # | / _---=> hardirq/softirq
2115 # || / _--=> preempt-depth
2117 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2119 usleep-2665 [001] .... 4186.475355: sys_nanosleep <-system_call_fastpath
2120 <idle>-0 [001] d.h1 4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
2121 usleep-2665 [001] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2122 <idle>-0 [003] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2123 <idle>-0 [002] d.h1 4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
2125 To see which functions are being traced, you can cat the file:
2127 # cat set_ftrace_filter
2132 Perhaps this is not enough. The filters also allow simple wild
2133 cards. Only the following are currently available
2135 <match>* - will match functions that begin with <match>
2136 *<match> - will match functions that end with <match>
2137 *<match>* - will match functions that have <match> in it
2139 These are the only wild cards which are supported.
2141 <match>*<match> will not work.
2143 Note: It is better to use quotes to enclose the wild cards,
2144 otherwise the shell may expand the parameters into names
2145 of files in the local directory.
2147 # echo 'hrtimer_*' > set_ftrace_filter
2153 # entries-in-buffer/entries-written: 897/897 #P:4
2156 # / _----=> need-resched
2157 # | / _---=> hardirq/softirq
2158 # || / _--=> preempt-depth
2160 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2162 <idle>-0 [003] dN.1 4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
2163 <idle>-0 [003] dN.1 4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
2164 <idle>-0 [003] dN.2 4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
2165 <idle>-0 [003] dN.1 4228.547805: hrtimer_forward <-tick_nohz_idle_exit
2166 <idle>-0 [003] dN.1 4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
2167 <idle>-0 [003] d..1 4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
2168 <idle>-0 [003] d..1 4228.547859: hrtimer_start <-__tick_nohz_idle_enter
2169 <idle>-0 [003] d..2 4228.547860: hrtimer_force_reprogram <-__rem
2171 Notice that we lost the sys_nanosleep.
2173 # cat set_ftrace_filter
2178 hrtimer_try_to_cancel
2182 hrtimer_force_reprogram
2183 hrtimer_get_next_event
2187 hrtimer_get_remaining
2189 hrtimer_init_sleeper
2192 This is because the '>' and '>>' act just like they do in bash.
2193 To rewrite the filters, use '>'
2194 To append to the filters, use '>>'
2196 To clear out a filter so that all functions will be recorded
2199 # echo > set_ftrace_filter
2200 # cat set_ftrace_filter
2203 Again, now we want to append.
2205 # echo sys_nanosleep > set_ftrace_filter
2206 # cat set_ftrace_filter
2208 # echo 'hrtimer_*' >> set_ftrace_filter
2209 # cat set_ftrace_filter
2214 hrtimer_try_to_cancel
2218 hrtimer_force_reprogram
2219 hrtimer_get_next_event
2224 hrtimer_get_remaining
2226 hrtimer_init_sleeper
2229 The set_ftrace_notrace prevents those functions from being
2232 # echo '*preempt*' '*lock*' > set_ftrace_notrace
2238 # entries-in-buffer/entries-written: 39608/39608 #P:4
2241 # / _----=> need-resched
2242 # | / _---=> hardirq/softirq
2243 # || / _--=> preempt-depth
2245 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2247 bash-1994 [000] .... 4342.324896: file_ra_state_init <-do_dentry_open
2248 bash-1994 [000] .... 4342.324897: open_check_o_direct <-do_last
2249 bash-1994 [000] .... 4342.324897: ima_file_check <-do_last
2250 bash-1994 [000] .... 4342.324898: process_measurement <-ima_file_check
2251 bash-1994 [000] .... 4342.324898: ima_get_action <-process_measurement
2252 bash-1994 [000] .... 4342.324898: ima_match_policy <-ima_get_action
2253 bash-1994 [000] .... 4342.324899: do_truncate <-do_last
2254 bash-1994 [000] .... 4342.324899: should_remove_suid <-do_truncate
2255 bash-1994 [000] .... 4342.324899: notify_change <-do_truncate
2256 bash-1994 [000] .... 4342.324900: current_fs_time <-notify_change
2257 bash-1994 [000] .... 4342.324900: current_kernel_time <-current_fs_time
2258 bash-1994 [000] .... 4342.324900: timespec_trunc <-current_fs_time
2260 We can see that there's no more lock or preempt tracing.
2263 Dynamic ftrace with the function graph tracer
2264 ---------------------------------------------
2266 Although what has been explained above concerns both the
2267 function tracer and the function-graph-tracer, there are some
2268 special features only available in the function-graph tracer.
2270 If you want to trace only one function and all of its children,
2271 you just have to echo its name into set_graph_function:
2273 echo __do_fault > set_graph_function
2275 will produce the following "expanded" trace of the __do_fault()
2279 0) | filemap_fault() {
2280 0) | find_lock_page() {
2281 0) 0.804 us | find_get_page();
2282 0) | __might_sleep() {
2286 0) 0.653 us | _spin_lock();
2287 0) 0.578 us | page_add_file_rmap();
2288 0) 0.525 us | native_set_pte_at();
2289 0) 0.585 us | _spin_unlock();
2290 0) | unlock_page() {
2291 0) 0.541 us | page_waitqueue();
2292 0) 0.639 us | __wake_up_bit();
2296 0) | filemap_fault() {
2297 0) | find_lock_page() {
2298 0) 0.698 us | find_get_page();
2299 0) | __might_sleep() {
2303 0) 0.631 us | _spin_lock();
2304 0) 0.571 us | page_add_file_rmap();
2305 0) 0.526 us | native_set_pte_at();
2306 0) 0.586 us | _spin_unlock();
2307 0) | unlock_page() {
2308 0) 0.533 us | page_waitqueue();
2309 0) 0.638 us | __wake_up_bit();
2313 You can also expand several functions at once:
2315 echo sys_open > set_graph_function
2316 echo sys_close >> set_graph_function
2318 Now if you want to go back to trace all functions you can clear
2319 this special filter via:
2321 echo > set_graph_function
2327 Note, the proc sysctl ftrace_enable is a big on/off switch for the
2328 function tracer. By default it is enabled (when function tracing is
2329 enabled in the kernel). If it is disabled, all function tracing is
2330 disabled. This includes not only the function tracers for ftrace, but
2331 also for any other uses (perf, kprobes, stack tracing, profiling, etc).
2333 Please disable this with care.
2335 This can be disable (and enabled) with:
2337 sysctl kernel.ftrace_enabled=0
2338 sysctl kernel.ftrace_enabled=1
2342 echo 0 > /proc/sys/kernel/ftrace_enabled
2343 echo 1 > /proc/sys/kernel/ftrace_enabled
2349 A few commands are supported by the set_ftrace_filter interface.
2350 Trace commands have the following format:
2352 <function>:<command>:<parameter>
2354 The following commands are supported:
2357 This command enables function filtering per module. The
2358 parameter defines the module. For example, if only the write*
2359 functions in the ext3 module are desired, run:
2361 echo 'write*:mod:ext3' > set_ftrace_filter
2363 This command interacts with the filter in the same way as
2364 filtering based on function names. Thus, adding more functions
2365 in a different module is accomplished by appending (>>) to the
2366 filter file. Remove specific module functions by prepending
2369 echo '!writeback*:mod:ext3' >> set_ftrace_filter
2372 These commands turn tracing on and off when the specified
2373 functions are hit. The parameter determines how many times the
2374 tracing system is turned on and off. If unspecified, there is
2375 no limit. For example, to disable tracing when a schedule bug
2376 is hit the first 5 times, run:
2378 echo '__schedule_bug:traceoff:5' > set_ftrace_filter
2380 To always disable tracing when __schedule_bug is hit:
2382 echo '__schedule_bug:traceoff' > set_ftrace_filter
2384 These commands are cumulative whether or not they are appended
2385 to set_ftrace_filter. To remove a command, prepend it by '!'
2386 and drop the parameter:
2388 echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
2390 The above removes the traceoff command for __schedule_bug
2391 that have a counter. To remove commands without counters:
2393 echo '!__schedule_bug:traceoff' > set_ftrace_filter
2396 Will cause a snapshot to be triggered when the function is hit.
2398 echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
2400 To only snapshot once:
2402 echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
2404 To remove the above commands:
2406 echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
2407 echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
2409 - enable_event/disable_event
2410 These commands can enable or disable a trace event. Note, because
2411 function tracing callbacks are very sensitive, when these commands
2412 are registered, the trace point is activated, but disabled in
2413 a "soft" mode. That is, the tracepoint will be called, but
2414 just will not be traced. The event tracepoint stays in this mode
2415 as long as there's a command that triggers it.
2417 echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
2422 <function>:enable_event:<system>:<event>[:count]
2423 <function>:disable_event:<system>:<event>[:count]
2425 To remove the events commands:
2428 echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
2430 echo '!schedule:disable_event:sched:sched_switch' > \
2434 When the function is hit, it will dump the contents of the ftrace
2435 ring buffer to the console. This is useful if you need to debug
2436 something, and want to dump the trace when a certain function
2437 is hit. Perhaps its a function that is called before a tripple
2438 fault happens and does not allow you to get a regular dump.
2441 When the function is hit, it will dump the contents of the ftrace
2442 ring buffer for the current CPU to the console. Unlike the "dump"
2443 command, it only prints out the contents of the ring buffer for the
2444 CPU that executed the function that triggered the dump.
2449 The trace_pipe outputs the same content as the trace file, but
2450 the effect on the tracing is different. Every read from
2451 trace_pipe is consumed. This means that subsequent reads will be
2452 different. The trace is live.
2454 # echo function > current_tracer
2455 # cat trace_pipe > /tmp/trace.out &
2457 # echo 1 > tracing_on
2459 # echo 0 > tracing_on
2463 # entries-in-buffer/entries-written: 0/0 #P:4
2466 # / _----=> need-resched
2467 # | / _---=> hardirq/softirq
2468 # || / _--=> preempt-depth
2470 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2474 # cat /tmp/trace.out
2475 bash-1994 [000] .... 5281.568961: mutex_unlock <-rb_simple_write
2476 bash-1994 [000] .... 5281.568963: __mutex_unlock_slowpath <-mutex_unlock
2477 bash-1994 [000] .... 5281.568963: __fsnotify_parent <-fsnotify_modify
2478 bash-1994 [000] .... 5281.568964: fsnotify <-fsnotify_modify
2479 bash-1994 [000] .... 5281.568964: __srcu_read_lock <-fsnotify
2480 bash-1994 [000] .... 5281.568964: add_preempt_count <-__srcu_read_lock
2481 bash-1994 [000] ...1 5281.568965: sub_preempt_count <-__srcu_read_lock
2482 bash-1994 [000] .... 5281.568965: __srcu_read_unlock <-fsnotify
2483 bash-1994 [000] .... 5281.568967: sys_dup2 <-system_call_fastpath
2486 Note, reading the trace_pipe file will block until more input is
2492 Having too much or not enough data can be troublesome in
2493 diagnosing an issue in the kernel. The file buffer_size_kb is
2494 used to modify the size of the internal trace buffers. The
2495 number listed is the number of entries that can be recorded per
2496 CPU. To know the full size, multiply the number of possible CPUs
2497 with the number of entries.
2499 # cat buffer_size_kb
2500 1408 (units kilobytes)
2502 Or simply read buffer_total_size_kb
2504 # cat buffer_total_size_kb
2507 To modify the buffer, simple echo in a number (in 1024 byte segments).
2509 # echo 10000 > buffer_size_kb
2510 # cat buffer_size_kb
2511 10000 (units kilobytes)
2513 It will try to allocate as much as possible. If you allocate too
2514 much, it can cause Out-Of-Memory to trigger.
2516 # echo 1000000000000 > buffer_size_kb
2517 -bash: echo: write error: Cannot allocate memory
2518 # cat buffer_size_kb
2521 The per_cpu buffers can be changed individually as well:
2523 # echo 10000 > per_cpu/cpu0/buffer_size_kb
2524 # echo 100 > per_cpu/cpu1/buffer_size_kb
2526 When the per_cpu buffers are not the same, the buffer_size_kb
2527 at the top level will just show an X
2529 # cat buffer_size_kb
2532 This is where the buffer_total_size_kb is useful:
2534 # cat buffer_total_size_kb
2537 Writing to the top level buffer_size_kb will reset all the buffers
2538 to be the same again.
2542 CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
2543 available to all non latency tracers. (Latency tracers which
2544 record max latency, such as "irqsoff" or "wakeup", can't use
2545 this feature, since those are already using the snapshot
2546 mechanism internally.)
2548 Snapshot preserves a current trace buffer at a particular point
2549 in time without stopping tracing. Ftrace swaps the current
2550 buffer with a spare buffer, and tracing continues in the new
2551 current (=previous spare) buffer.
2553 The following debugfs files in "tracing" are related to this
2558 This is used to take a snapshot and to read the output
2559 of the snapshot. Echo 1 into this file to allocate a
2560 spare buffer and to take a snapshot (swap), then read
2561 the snapshot from this file in the same format as
2562 "trace" (described above in the section "The File
2563 System"). Both reads snapshot and tracing are executable
2564 in parallel. When the spare buffer is allocated, echoing
2565 0 frees it, and echoing else (positive) values clear the
2567 More details are shown in the table below.
2569 status\input | 0 | 1 | else |
2570 --------------+------------+------------+------------+
2571 not allocated |(do nothing)| alloc+swap |(do nothing)|
2572 --------------+------------+------------+------------+
2573 allocated | free | swap | clear |
2574 --------------+------------+------------+------------+
2576 Here is an example of using the snapshot feature.
2578 # echo 1 > events/sched/enable
2583 # entries-in-buffer/entries-written: 71/71 #P:8
2586 # / _----=> need-resched
2587 # | / _---=> hardirq/softirq
2588 # || / _--=> preempt-depth
2590 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2592 <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
2593 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
2595 <idle>-0 [002] d... 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120
2600 # entries-in-buffer/entries-written: 77/77 #P:8
2603 # / _----=> need-resched
2604 # | / _---=> hardirq/softirq
2605 # || / _--=> preempt-depth
2607 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2609 <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
2610 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
2614 If you try to use this snapshot feature when current tracer is
2615 one of the latency tracers, you will get the following results.
2617 # echo wakeup > current_tracer
2619 bash: echo: write error: Device or resource busy
2621 cat: snapshot: Device or resource busy
2626 In the debugfs tracing directory is a directory called "instances".
2627 This directory can have new directories created inside of it using
2628 mkdir, and removing directories with rmdir. The directory created
2629 with mkdir in this directory will already contain files and other
2630 directories after it is created.
2632 # mkdir instances/foo
2634 buffer_size_kb buffer_total_size_kb events free_buffer per_cpu
2635 set_event snapshot trace trace_clock trace_marker trace_options
2636 trace_pipe tracing_on
2638 As you can see, the new directory looks similar to the tracing directory
2639 itself. In fact, it is very similar, except that the buffer and
2640 events are agnostic from the main director, or from any other
2641 instances that are created.
2643 The files in the new directory work just like the files with the
2644 same name in the tracing directory except the buffer that is used
2645 is a separate and new buffer. The files affect that buffer but do not
2646 affect the main buffer with the exception of trace_options. Currently,
2647 the trace_options affect all instances and the top level buffer
2648 the same, but this may change in future releases. That is, options
2649 may become specific to the instance they reside in.
2651 Notice that none of the function tracer files are there, nor is
2652 current_tracer and available_tracers. This is because the buffers
2653 can currently only have events enabled for them.
2655 # mkdir instances/foo
2656 # mkdir instances/bar
2657 # mkdir instances/zoot
2658 # echo 100000 > buffer_size_kb
2659 # echo 1000 > instances/foo/buffer_size_kb
2660 # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
2661 # echo function > current_trace
2662 # echo 1 > instances/foo/events/sched/sched_wakeup/enable
2663 # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
2664 # echo 1 > instances/foo/events/sched/sched_switch/enable
2665 # echo 1 > instances/bar/events/irq/enable
2666 # echo 1 > instances/zoot/events/syscalls/enable
2668 CPU:2 [LOST 11745 EVENTS]
2669 bash-2044 [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
2670 bash-2044 [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
2671 bash-2044 [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
2672 bash-2044 [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
2673 bash-2044 [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
2674 bash-2044 [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
2675 bash-2044 [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
2676 bash-2044 [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
2677 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics
2678 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics
2679 bash-2044 [002] .... 10594.481035: arch_dup_task_struct <-copy_process
2682 # cat instances/foo/trace_pipe
2683 bash-1998 [000] d..4 136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
2684 bash-1998 [000] dN.4 136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
2685 <idle>-0 [003] d.h3 136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
2686 <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
2687 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
2688 bash-1998 [000] d..4 136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
2689 bash-1998 [000] dN.4 136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
2690 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
2691 kworker/0:1-59 [000] d..4 136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
2692 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
2695 # cat instances/bar/trace_pipe
2696 migration/1-14 [001] d.h3 138.732674: softirq_raise: vec=3 [action=NET_RX]
2697 <idle>-0 [001] dNh3 138.732725: softirq_raise: vec=3 [action=NET_RX]
2698 bash-1998 [000] d.h1 138.733101: softirq_raise: vec=1 [action=TIMER]
2699 bash-1998 [000] d.h1 138.733102: softirq_raise: vec=9 [action=RCU]
2700 bash-1998 [000] ..s2 138.733105: softirq_entry: vec=1 [action=TIMER]
2701 bash-1998 [000] ..s2 138.733106: softirq_exit: vec=1 [action=TIMER]
2702 bash-1998 [000] ..s2 138.733106: softirq_entry: vec=9 [action=RCU]
2703 bash-1998 [000] ..s2 138.733109: softirq_exit: vec=9 [action=RCU]
2704 sshd-1995 [001] d.h1 138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
2705 sshd-1995 [001] d.h1 138.733280: irq_handler_exit: irq=21 ret=unhandled
2706 sshd-1995 [001] d.h1 138.733281: irq_handler_entry: irq=21 name=eth0
2707 sshd-1995 [001] d.h1 138.733283: irq_handler_exit: irq=21 ret=handled
2710 # cat instances/zoot/trace
2713 # entries-in-buffer/entries-written: 18996/18996 #P:4
2716 # / _----=> need-resched
2717 # | / _---=> hardirq/softirq
2718 # || / _--=> preempt-depth
2720 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2722 bash-1998 [000] d... 140.733501: sys_write -> 0x2
2723 bash-1998 [000] d... 140.733504: sys_dup2(oldfd: a, newfd: 1)
2724 bash-1998 [000] d... 140.733506: sys_dup2 -> 0x1
2725 bash-1998 [000] d... 140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
2726 bash-1998 [000] d... 140.733509: sys_fcntl -> 0x1
2727 bash-1998 [000] d... 140.733510: sys_close(fd: a)
2728 bash-1998 [000] d... 140.733510: sys_close -> 0x0
2729 bash-1998 [000] d... 140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
2730 bash-1998 [000] d... 140.733515: sys_rt_sigprocmask -> 0x0
2731 bash-1998 [000] d... 140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
2732 bash-1998 [000] d... 140.733516: sys_rt_sigaction -> 0x0
2734 You can see that the trace of the top most trace buffer shows only
2735 the function tracing. The foo instance displays wakeups and task
2738 To remove the instances, simply delete their directories:
2740 # rmdir instances/foo
2741 # rmdir instances/bar
2742 # rmdir instances/zoot
2744 Note, if a process has a trace file open in one of the instance
2745 directories, the rmdir will fail with EBUSY.
2750 Since the kernel has a fixed sized stack, it is important not to
2751 waste it in functions. A kernel developer must be conscience of
2752 what they allocate on the stack. If they add too much, the system
2753 can be in danger of a stack overflow, and corruption will occur,
2754 usually leading to a system panic.
2756 There are some tools that check this, usually with interrupts
2757 periodically checking usage. But if you can perform a check
2758 at every function call that will become very useful. As ftrace provides
2759 a function tracer, it makes it convenient to check the stack size
2760 at every function call. This is enabled via the stack tracer.
2762 CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
2763 To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
2765 # echo 1 > /proc/sys/kernel/stack_tracer_enabled
2767 You can also enable it from the kernel command line to trace
2768 the stack size of the kernel during boot up, by adding "stacktrace"
2769 to the kernel command line parameter.
2771 After running it for a few minutes, the output looks like:
2773 # cat stack_max_size
2777 Depth Size Location (18 entries)
2779 0) 2928 224 update_sd_lb_stats+0xbc/0x4ac
2780 1) 2704 160 find_busiest_group+0x31/0x1f1
2781 2) 2544 256 load_balance+0xd9/0x662
2782 3) 2288 80 idle_balance+0xbb/0x130
2783 4) 2208 128 __schedule+0x26e/0x5b9
2784 5) 2080 16 schedule+0x64/0x66
2785 6) 2064 128 schedule_timeout+0x34/0xe0
2786 7) 1936 112 wait_for_common+0x97/0xf1
2787 8) 1824 16 wait_for_completion+0x1d/0x1f
2788 9) 1808 128 flush_work+0xfe/0x119
2789 10) 1680 16 tty_flush_to_ldisc+0x1e/0x20
2790 11) 1664 48 input_available_p+0x1d/0x5c
2791 12) 1616 48 n_tty_poll+0x6d/0x134
2792 13) 1568 64 tty_poll+0x64/0x7f
2793 14) 1504 880 do_select+0x31e/0x511
2794 15) 624 400 core_sys_select+0x177/0x216
2795 16) 224 96 sys_select+0x91/0xb9
2796 17) 128 128 system_call_fastpath+0x16/0x1b
2798 Note, if -mfentry is being used by gcc, functions get traced before
2799 they set up the stack frame. This means that leaf level functions
2800 are not tested by the stack tracer when -mfentry is used.
2802 Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
2806 More details can be found in the source code, in the
2807 kernel/trace/*.c files.
projects / karo-tx-linux.git / blob