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perf/core: Reduce PMU access to adjust sample freq

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In perf_adjust_freq_unthr_context(), it first starts the event and then
stop unnecessarily to adjust the sampling frequency if the event is
throttled.

For a throttled non-frequency event, it doesn't have a freq so no need
to adjust.  Just starting the event would be ok.

For a frequency event, whether it's throttled or not, it needs to stop
before adjusting the frequency.  That means it should not start the
even if it was throttled.  I tried to skip calling the stop callback,
but it didn't work well since the event count might not be up to date.
It should call the stop callback with PERF_EF_UPDATE anyway.

However not calling start would prevent unnecessary MSR accesses (which
can be costly) for already stopped events as stop state is saved in the
hw config.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Ian Rogers <irogers@google.com>
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Link: https://lore.kernel.org/r/20240207050545.2727923-2-namhyung@kernel.org
This commit is contained in:
Namhyung Kim 2024-02-06 21:05:45 -08:00 committed by Ingo Molnar
parent 0259bf63f7
commit f38628b06c
1 changed files with 2 additions and 1 deletions
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3
kernel/events/core.c
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@ -4146,7 +4146,8 @@ static void perf_adjust_freq_unthr_events(struct list_head *event_list)
if (hwc->interrupts == MAX_INTERRUPTS) {
hwc->interrupts = 0;
perf_log_throttle(event, 1);
event->pmu->start(event, 0);
if (!event->attr.freq || !event->attr.sample_freq)
event->pmu->start(event, 0);
}
if (!event->attr.freq || !event->attr.sample_freq)