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.comSigned-off-by: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Documentation/trace/ftrace.rst

@@ -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
 ----------