linux-kernel - Re: [PATCH 12/19] perf: Ignore event state for group validation

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Message-ID: <20250826130329.GX4067720@noisy.programming.kicks-ass.net>
Date: Tue, 26 Aug 2025 15:03:29 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Robin Murphy <robin.murphy@....com>
Cc: mingo@...hat.com, will@...nel.org, mark.rutland@....com,
	acme@...nel.org, namhyung@...nel.org,
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Subject: Re: [PATCH 12/19] perf: Ignore event state for group validation

On Wed, Aug 13, 2025 at 06:01:04PM +0100, Robin Murphy wrote:
> It may have been different long ago, but today it seems wrong for these
> drivers to skip counting disabled sibling events in group validation,
> given that perf_event_enable() could make them schedulable again, and
> thus increase the effective size of the group later. Conversely, if a
> sibling event is truly dead then it stands to reason that the whole
> group is dead, so it's not worth going to any special effort to try to
> squeeze in a new event that's never going to run anyway. Thus, we can
> simply remove all these checks.

So currently you can do sort of a manual event rotation inside an
over-sized group and have it work.

I'm not sure if anybody actually does this, but its possible.

Eg. on a PMU that supports only 4 counters, create a group of 5 and
periodically cycle which of the 5 events is off.

So I'm not against changing this, but changing stuff like this always
makes me a little fearful -- it wouldn't be the first time that when it
finally trickles down to some 'enterprise' user in 5 years someone comes
and finally says, oh hey, you broke my shit :-(