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Subject[PATCH 5/5] trace/doc: Add latency tracer funcgraph example
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This add an example about how to use funcgraph with latency tracers.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
---
Documentation/trace/ftrace.rst | 51 ++++++++++++++++++++++++++++++++++
1 file changed, 51 insertions(+)

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 0131df7f5968..6ce2763a2a3e 100644
--- a/Documentation/trace/ftrace.rst
+++ b/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
----------
--
2.17.1
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