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    Subject[tip: perf/core] perf/core: Reduce PMU access to adjust sample freq
    The following commit has been merged into the perf/core branch of tip:

    Commit-ID: f38628b06c36222367e26820879789ae59e49f60
    Gitweb: https://git.kernel.org/tip/f38628b06c36222367e26820879789ae59e49f60
    Author: Namhyung Kim <namhyung@kernel.org>
    AuthorDate: Tue, 06 Feb 2024 21:05:45 -08:00
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitterDate: Wed, 10 Apr 2024 06:13:57 +02:00

    perf/core: Reduce PMU access to adjust sample freq

    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
    ---
    kernel/events/core.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/kernel/events/core.c b/kernel/events/core.c
    index 9566cfb..fd94e45 100644
    --- a/kernel/events/core.c
    +++ b/kernel/events/core.c
    @@ -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)
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