linux-kernel - Re: [PATCH 16/19] perf: Introduce positive capability for sampling

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Message-ID: <20250827080630.GC745921@e132581.arm.com>
Date: Wed, 27 Aug 2025 09:06:30 +0100
From: Leo Yan <leo.yan@....com>
To: Robin Murphy <robin.murphy@....com>
Cc: peterz@...radead.org, mingo@...hat.com, will@...nel.org,
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Subject: Re: [PATCH 16/19] perf: Introduce positive capability for sampling

On Tue, Aug 26, 2025 at 04:53:51PM +0100, Robin Murphy wrote:

[...]

> > Genearlly, AUX events generate interrupts based on AUX ring buffer
> > watermark but not the period. Seems to me, it is correct to set the
> > PERF_PMU_CAP_SAMPLING flag for them.
> 
> This cap is given to drivers which handle event->attr.sample_period and call
> perf_event_overflow() - or in a few rare cases, perf_output_sample()
> directly - to do something meaningful with it, since the intent is to convey
> "I properly handle events for which is_sampling_event() is true". My
> understanding is that aux events are something else entirely, but I'm happy
> to be corrected.

If the discussion is based only on this patch, my understanding is
that the PERF_PMU_CAP_SAMPLING flag replaces the
PERF_PMU_CAP_NO_INTERRUPT flag for checking whether a PMU event needs
to be re-enabled in perf_adjust_freq_unthr_context().

AUX events can trigger a large number of interrupts under certain
conditions (e.g., if we set a very small watermark). This is why I
conclude that we need to set the PERF_PMU_CAP_SAMPLING flag to ensure
that AUX events are re-enabled properly after throttling (see
perf_adjust_freq_unthr_events()).

> Otherwise, perhaps this suggests it deserves to be named a little more
> specifically for clarity, maybe PERF_CAP_SAMPLING_EVENTS?

Seems to me, the naming is not critical. If without setting the
PERF_PMU_CAP_SAMPLING flag, AUX events might lose chance to be
re-enabled after throttling.

Thanks,
Leo