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Commit 88d380eb authored by Changbin Du's avatar Changbin Du Committed by Steven Rostedt (VMware)
Browse files

tracing/doc: Add latency tracer funcgraph example 

This add an example about how to use funcgraph with latency tracers.

Link: http://lkml.kernel.org/r/20190101154614.8887-6-changbin.du@gmail.com



Signed-off-by: default avatarChangbin Du <changbin.du@gmail.com>
Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
parent afbab501

Documentation/trace/ftrace.rst

+51 −0
Original line number Diff line number Diff line
@@ -1396,6 +1396,57 @@ enabling function tracing, we incur an added overhead. This 
overhead may extend the latency times. But nevertheless, this

trace has provided some very helpful debugging information.



If we prefer function graph output instead of function, we can set

display-graph option::

 with echo 1 > options/display-graph



  # tracer: irqsoff

  #

  # irqsoff latency trace v1.1.5 on 4.20.0-rc6+

  # --------------------------------------------------------------------

  # latency: 3751 us, #274/274, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4)

  #    -----------------

  #    | task: bash-1507 (uid:0 nice:0 policy:0 rt_prio:0)

  #    -----------------

  #  => started at: free_debug_processing

  #  => ended at:   return_to_handler

  #

  #

  #                                       _-----=> irqs-off

  #                                      / _----=> need-resched

  #                                     | / _---=> hardirq/softirq

  #                                     || / _--=> preempt-depth

  #                                     ||| /

  #   REL TIME      CPU  TASK/PID       ||||     DURATION                  FUNCTION CALLS

  #      |          |     |    |        ||||      |   |                     |   |   |   |

          0 us |   0)   bash-1507    |  d... |   0.000 us    |  _raw_spin_lock_irqsave();

          0 us |   0)   bash-1507    |  d..1 |   0.378 us    |    do_raw_spin_trylock();

          1 us |   0)   bash-1507    |  d..2 |               |    set_track() {

          2 us |   0)   bash-1507    |  d..2 |               |      save_stack_trace() {

          2 us |   0)   bash-1507    |  d..2 |               |        __save_stack_trace() {

          3 us |   0)   bash-1507    |  d..2 |               |          __unwind_start() {

          3 us |   0)   bash-1507    |  d..2 |               |            get_stack_info() {

          3 us |   0)   bash-1507    |  d..2 |   0.351 us    |              in_task_stack();

          4 us |   0)   bash-1507    |  d..2 |   1.107 us    |            }

  [...]

       3750 us |   0)   bash-1507    |  d..1 |   0.516 us    |      do_raw_spin_unlock();

       3750 us |   0)   bash-1507    |  d..1 |   0.000 us    |  _raw_spin_unlock_irqrestore();

       3764 us |   0)   bash-1507    |  d..1 |   0.000 us    |  tracer_hardirqs_on();

      bash-1507    0d..1 3792us : <stack trace>

   => free_debug_processing

   => __slab_free

   => kmem_cache_free

   => vm_area_free

   => remove_vma

   => exit_mmap

   => mmput

   => flush_old_exec

   => load_elf_binary

   => search_binary_handler

   => __do_execve_file.isra.32

   => __x64_sys_execve

   => do_syscall_64

   => entry_SYSCALL_64_after_hwframe



preemptoff

----------