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================
NMI Trace Events
================
These events normally show up here:
/sys/kernel/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/tracing/events/nmi/nmi_handler
echo 'handler==0xffffffff81625600 && delta_ns>1000000' > filter
echo 1 > enable
Your output would then look like::
$ cat /sys/kernel/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
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