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The canonical location for the tracefs filesystem is at /sys/kernel/tracing. But, from Documentation/trace/ftrace.rst: Before 4.1, all ftrace tracing control files were within the debugfs file system, which is typically located at /sys/kernel/debug/tracing. For backward compatibility, when mounting the debugfs file system, the tracefs file system will be automatically mounted at: /sys/kernel/debug/tracing Many parts of Documentation still reference this older debugfs path, so let's update them to avoid confusion. Signed-off-by: Ross Zwisler <zwisler@google.com> Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org> Link: https://lore.kernel.org/r/20230125213251.2013791-1-zwisler@google.com Signed-off-by: Jonathan Corbet <corbet@lwn.net>
45 lines
1.6 KiB
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45 lines
1.6 KiB
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NMI Trace Events
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================
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These events normally show up here:
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/sys/kernel/tracing/events/nmi
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nmi_handler
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-----------
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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/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/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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