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x86: Add NMI duration tracepoints

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This patch has been invaluable in my adventures finding
issues in the perf NMI handler.  I'm as big a fan of
printk() as anybody is, but using printk() in NMIs is
deadly when they're happening frequently.

Even hacking in trace_printk() ended up eating enough
CPU to throw off some of the measurements I was making.

Signed-off-by: Dave Hansen <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Dave Hansen <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
This commit is contained in:
Dave Hansen 2013-06-21 08:51:38 -07:00 • committed by Ingo Molnar
commit 0c4df02d73
3 changed files with 87 additions and 2 deletions
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43
Documentation/trace/events-nmi.txt Normal file
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NMI Trace Events
These events normally show up here:
/sys/kernel/debug/tracing/events/nmi
--
nmi_handler:
You might want to use this tracepoint if you suspect that your
NMI handlers are hogging large amounts of CPU time. The kernel
will warn if it sees long-running handlers:
INFO: NMI handler took too long to run: 9.207 msecs
and this tracepoint will allow you to drill down and get some
more details.
Let's say you suspect that perf_event_nmi_handler() is causing
you some problems and you only want to trace that handler
specifically. You need to find its address:
$ grep perf_event_nmi_handler /proc/kallsyms
ffffffff81625600 t perf_event_nmi_handler
Let's also say you are only interested in when that function is
really hogging a lot of CPU time, like a millisecond at a time.
Note that the kernel's output is in milliseconds, but the input
to the filter is in nanoseconds! You can filter on 'delta_ns':
cd /sys/kernel/debug/tracing/events/nmi/nmi_handler
echo 'handler==0xffffffff81625600 && delta_ns>1000000' > filter
echo 1 > enable
Your output would then look like:
$ cat /sys/kernel/debug/tracing/trace_pipe
<idle>-0 [000] d.h3 505.397558: nmi_handler: perf_event_nmi_handler() delta_ns: 3236765 handled: 1
<idle>-0 [000] d.h3 505.805893: nmi_handler: perf_event_nmi_handler() delta_ns: 3174234 handled: 1
<idle>-0 [000] d.h3 506.158206: nmi_handler: perf_event_nmi_handler() delta_ns: 3084642 handled: 1
<idle>-0 [000] d.h3 506.334346: nmi_handler: perf_event_nmi_handler() delta_ns: 3080351 handled: 1