We have 2 functions using the same sched_task callback:
- PEBS drain for free running counters
- LBR save/store
Both of them are called from intel_pmu_sched_task() and
either of them can be unwillingly triggered when the
other one is configured to run.
Let's say there's PEBS drain configured in sched_task
callback for the event, but in the callback itself
(intel_pmu_sched_task()) we will also run the code for
LBR save/restore, which we did not ask for, but the
code in intel_pmu_sched_task() does not check for that.
This can lead to extra cycles in some perf monitoring,
like when we monitor PEBS event without LBR data.
# perf record --no-timestamp -c 10000 -e cycles:p ./perf bench sched pipe -l 1000000
(We need PEBS, non freq/non timestamp event to enable
the sched_task callback)
The perf stat of cycles and msr:write_msr for above
command before the change:
...
Performance counter stats for './perf record --no-timestamp -c 10000 -e cycles:p \
./perf bench sched pipe -l 1000000' (5 runs):
18,519,557,441 cycles:k
91,195,527 msr:write_msr
29.334476406 seconds time elapsed
And after the change:
...
Performance counter stats for './perf record --no-timestamp -c 10000 -e cycles:p \
./perf bench sched pipe -l 1000000' (5 runs):
18,704,973,540 cycles:k
27,184,720 msr:write_msr
16.977875900 seconds time elapsed
There's no affect on cycles:k because the sched_task happens
with events switched off, however the msr:write_msr tracepoint
counter together with almost 50% of time speedup show the
improvement.
Monitoring LBR event and having extra PEBS drain processing
in sched_task callback showed just a little speedup, because
the drain function does not do much extra work in case there
is no PEBS data.
Adding conditions to recognize the configured work that needs
to be done in the x86_pmu's sched_task callback.
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170719075247.GA27506@krava
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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