linux/Documentation/trace/events-nmi.txt
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   1NMI Trace Events
   2
   3These events normally show up here:
   4
   5        /sys/kernel/debug/tracing/events/nmi
   6
   7--
   8
   9nmi_handler:
  10
  11You might want to use this tracepoint if you suspect that your
  12NMI handlers are hogging large amounts of CPU time.  The kernel
  13will warn if it sees long-running handlers:
  14
  15        INFO: NMI handler took too long to run: 9.207 msecs
  16
  17and this tracepoint will allow you to drill down and get some
  18more details.
  19
  20Let's say you suspect that perf_event_nmi_handler() is causing
  21you some problems and you only want to trace that handler
  22specifically.  You need to find its address:
  23
  24        $ grep perf_event_nmi_handler /proc/kallsyms
  25        ffffffff81625600 t perf_event_nmi_handler
  26
  27Let's also say you are only interested in when that function is
  28really hogging a lot of CPU time, like a millisecond at a time.
  29Note that the kernel's output is in milliseconds, but the input
  30to the filter is in nanoseconds!  You can filter on 'delta_ns':
  31
  32cd /sys/kernel/debug/tracing/events/nmi/nmi_handler
  33echo 'handler==0xffffffff81625600 && delta_ns>1000000' > filter
  34echo 1 > enable
  35
  36Your output would then look like:
  37
  38$ cat /sys/kernel/debug/tracing/trace_pipe
  39<idle>-0     [000] d.h3   505.397558: nmi_handler: perf_event_nmi_handler() delta_ns: 3236765 handled: 1
  40<idle>-0     [000] d.h3   505.805893: nmi_handler: perf_event_nmi_handler() delta_ns: 3174234 handled: 1
  41<idle>-0     [000] d.h3   506.158206: nmi_handler: perf_event_nmi_handler() delta_ns: 3084642 handled: 1
  42<idle>-0     [000] d.h3   506.334346: nmi_handler: perf_event_nmi_handler() delta_ns: 3080351 handled: 1
  43
  44
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