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