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x86, ftrace, hw-branch-tracer: documentation

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

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Markus Metzger 2009-01-19 10:35:58 +01:00 committed by Ingo Molnar
parent 11edda0628
commit e2ea5399bb
1 changed files with 74 additions and 0 deletions
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74
Documentation/ftrace.txt
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@ -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
--------------