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Commit e2ea5399 authored by Markus Metzger's avatar Markus Metzger Committed by Ingo Molnar
Browse files

x86, ftrace, hw-branch-tracer: documentation 



Document the hw-branch-tracer in the ftrace documentation.

Signed-off-by: default avatarMarkus Metzger <markus.t.metzger@intel.com>
Signed-off-by: default avatarSteven Rostedt <srostedt@redhat.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 11edda06

Documentation/ftrace.txt

+74 −0
Original line number Diff line number Diff line
@@ -165,6 +165,8 @@ Here is the list of current tracers that may be configured. 
  nop - This is not a tracer. To remove all tracers from tracing

		simply echo "nop" into current_tracer.



  hw-branch-tracer - traces branches on all cpu's in a circular buffer.





Examples of using the tracer

----------------------------
@@ -1152,6 +1154,78 @@ int main (int argc, char **argv) 
        return 0;

}





hw-branch-tracer (x86 only)

---------------------------



This tracer uses the x86 last branch tracing hardware feature to

collect a branch trace on all cpus with relatively low overhead.



The tracer uses a fixed-size circular buffer per cpu and only

traces ring 0 branches. The trace file dumps that buffer in the

following format:



# tracer: hw-branch-tracer

#

# CPU#        TO  <-  FROM

   0  scheduler_tick+0xb5/0x1bf	  <-  task_tick_idle+0x5/0x6

   2  run_posix_cpu_timers+0x2b/0x72a	  <-  run_posix_cpu_timers+0x25/0x72a

   0  scheduler_tick+0x139/0x1bf	  <-  scheduler_tick+0xed/0x1bf

   0  scheduler_tick+0x17c/0x1bf	  <-  scheduler_tick+0x148/0x1bf

   2  run_posix_cpu_timers+0x9e/0x72a	  <-  run_posix_cpu_timers+0x5e/0x72a

   0  scheduler_tick+0x1b6/0x1bf	  <-  scheduler_tick+0x1aa/0x1bf





The tracer may be used to dump the trace for the oops'ing cpu on a

kernel oops into the system log. To enable this, ftrace_dump_on_oops

must be set. To set ftrace_dump_on_oops, one can either use the sysctl

function or set it via the proc system interface.



  sysctl kernel.ftrace_dump_on_oops=1



or



  echo 1 > /proc/sys/kernel/ftrace_dump_on_oops





Here's an example of such a dump after a null pointer dereference in a

kernel module:



[57848.105921] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000

[57848.106019] IP: [<ffffffffa0000006>] open+0x6/0x14 [oops]

[57848.106019] PGD 2354e9067 PUD 2375e7067 PMD 0

[57848.106019] Oops: 0002 [#1] SMP

[57848.106019] last sysfs file: /sys/devices/pci0000:00/0000:00:1e.0/0000:20:05.0/local_cpus

[57848.106019] Dumping ftrace buffer:

[57848.106019] ---------------------------------

[...]

[57848.106019]    0  chrdev_open+0xe6/0x165	  <-  cdev_put+0x23/0x24

[57848.106019]    0  chrdev_open+0x117/0x165	  <-  chrdev_open+0xfa/0x165

[57848.106019]    0  chrdev_open+0x120/0x165	  <-  chrdev_open+0x11c/0x165

[57848.106019]    0  chrdev_open+0x134/0x165	  <-  chrdev_open+0x12b/0x165

[57848.106019]    0  open+0x0/0x14 [oops]	  <-  chrdev_open+0x144/0x165

[57848.106019]    0  page_fault+0x0/0x30	  <-  open+0x6/0x14 [oops]

[57848.106019]    0  error_entry+0x0/0x5b	  <-  page_fault+0x4/0x30

[57848.106019]    0  error_kernelspace+0x0/0x31	  <-  error_entry+0x59/0x5b

[57848.106019]    0  error_sti+0x0/0x1	  <-  error_kernelspace+0x2d/0x31

[57848.106019]    0  page_fault+0x9/0x30	  <-  error_sti+0x0/0x1

[57848.106019]    0  do_page_fault+0x0/0x881	  <-  page_fault+0x1a/0x30

[...]

[57848.106019]    0  do_page_fault+0x66b/0x881	  <-  is_prefetch+0x1ee/0x1f2

[57848.106019]    0  do_page_fault+0x6e0/0x881	  <-  do_page_fault+0x67a/0x881

[57848.106019]    0  oops_begin+0x0/0x96	  <-  do_page_fault+0x6e0/0x881

[57848.106019]    0  trace_hw_branch_oops+0x0/0x2d	  <-  oops_begin+0x9/0x96

[...]

[57848.106019]    0  ds_suspend_bts+0x2a/0xe3	  <-  ds_suspend_bts+0x1a/0xe3

[57848.106019] ---------------------------------

[57848.106019] CPU 0

[57848.106019] Modules linked in: oops

[57848.106019] Pid: 5542, comm: cat Tainted: G        W  2.6.28 #23

[57848.106019] RIP: 0010:[<ffffffffa0000006>]  [<ffffffffa0000006>] open+0x6/0x14 [oops]

[57848.106019] RSP: 0018:ffff880235457d48  EFLAGS: 00010246

[...]





dynamic ftrace

--------------