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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 <dave.hansen@linux.intel.com> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: paulus@samba.org Cc: acme@ghostprotocols.net Cc: Dave Hansen <dave@sr71.net> Signed-off-by: Ingo Molnar <mingo@kernel.org>
44 lines
1.5 KiB
Plaintext
44 lines
1.5 KiB
Plaintext
NMI Trace Events
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These events normally show up here:
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/sys/kernel/debug/tracing/events/nmi
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--
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nmi_handler:
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You might want to use this tracepoint if you suspect that your
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NMI handlers are hogging large amounts of CPU time. The kernel
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will warn if it sees long-running handlers:
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INFO: NMI handler took too long to run: 9.207 msecs
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and this tracepoint will allow you to drill down and get some
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more details.
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Let's say you suspect that perf_event_nmi_handler() is causing
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you some problems and you only want to trace that handler
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specifically. You need to find its address:
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$ grep perf_event_nmi_handler /proc/kallsyms
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ffffffff81625600 t perf_event_nmi_handler
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Let's also say you are only interested in when that function is
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really hogging a lot of CPU time, like a millisecond at a time.
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Note that the kernel's output is in milliseconds, but the input
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to the filter is in nanoseconds! You can filter on 'delta_ns':
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cd /sys/kernel/debug/tracing/events/nmi/nmi_handler
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echo 'handler==0xffffffff81625600 && delta_ns>1000000' > filter
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echo 1 > enable
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Your output would then look like:
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$ cat /sys/kernel/debug/tracing/trace_pipe
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<idle>-0 [000] d.h3 505.397558: nmi_handler: perf_event_nmi_handler() delta_ns: 3236765 handled: 1
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<idle>-0 [000] d.h3 505.805893: nmi_handler: perf_event_nmi_handler() delta_ns: 3174234 handled: 1
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<idle>-0 [000] d.h3 506.158206: nmi_handler: perf_event_nmi_handler() delta_ns: 3084642 handled: 1
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<idle>-0 [000] d.h3 506.334346: nmi_handler: perf_event_nmi_handler() delta_ns: 3080351 handled: 1
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