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

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Message-ID: <aK6_XrA_OaLnoFkr@J2N7QTR9R3>
Date: Wed, 27 Aug 2025 09:18:38 +0100
From: Mark Rutland <mark.rutland@....com>
To: Ian Rogers <irogers@...gle.com>
Cc: Robin Murphy <robin.murphy@....com>,
	Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
	will@...nel.org, acme@...nel.org, namhyung@...nel.org,
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
	adrian.hunter@...el.com, kan.liang@...ux.intel.com,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-alpha@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org, imx@...ts.linux.dev,
	linux-csky@...r.kernel.org, loongarch@...ts.linux.dev,
	linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
	sparclinux@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-rockchip@...ts.infradead.org, dmaengine@...r.kernel.org,
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	dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
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	iommu@...ts.linux.dev, linux-amlogic@...ts.infradead.org,
	linux-cxl@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-riscv@...ts.infradead.org
Subject: Re: [PATCH 12/19] perf: Ignore event state for group validation

On Tue, Aug 26, 2025 at 11:48:48AM -0700, Ian Rogers wrote:
> On Tue, Aug 26, 2025 at 8:32 AM Robin Murphy <robin.murphy@....com> wrote:
> >
> > On 2025-08-26 2:03 pm, Peter Zijlstra wrote:
> > > 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.
> 
> I'm not sure this is true, I thought this would fail in the
> perf_event_open when adding the 5th event and there being insufficient
> counters for the group.

We're talking specifically about cases where the logic in a pmu's
pmu::event_init() callback doesn't count events in specific states, and
hence the 5th even doesn't get rejected when it is initialised.

For example, in arch/x86/events/core.c, validate_group() uses
collect_events(), which has:

	for_each_sibling_event(event, leader) {
		if (!is_x86_event(event) || event->state <= PERF_EVENT_STATE_OFF)
			continue;

		if (collect_event(cpuc, event, max_count, n))
			return -EINVAL;

		n++;
	}

... and so where an event's state is <= PERF_EVENT_STATE_OFF at init
time, that event is not counted to see if it fits into HW counters.

Mark.