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

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