linux-kernel - Re: [PATCH v2 1/5] perf: Allow periodic events to alternate between two sample periods

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Message-ID: <20250310105551.GB9682@e132581.arm.com>
Date: Mon, 10 Mar 2025 10:55:51 +0000
From: Leo Yan <leo.yan@....com>
To: Mark Barnett <mark.barnett@....com>
Cc: peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
	namhyung@...nel.org, irogers@...gle.com, ben.gainey@....com,
	deepak.surti@....com, ak@...ux.intel.com, will@...nel.org,
	james.clark@....com, mark.rutland@....com,
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
	adrian.hunter@...el.com, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/5] perf: Allow periodic events to alternate between
 two sample periods

On Fri, Mar 07, 2025 at 08:28:13PM +0000, Mark Barnett wrote:
> On 1/21/25 13:01, Leo Yan wrote:
> > >   	local64_set(&hwc->period_left, hwc->sample_period);
> > > +	if (attr->alt_sample_period) {
> > > +		hwc->sample_period = attr->alt_sample_period;
> > > +		hwc->using_alt_sample_period = true;
> > > +	}
> > 
> > My understanding it sets a short sample window for the first period.
> > Would it initialize the `hwc->period_left` with the updated sample
> > period?
> > 
> 
> It sets the long period first: hwc->period_left is used to program the PMU
> when setting up the event, and hwc->sample_period is queued up as the next
> period to switch to.

Makes sense to me.  Thanks for explanation.

Leo