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

[Ian Rogers <irogers@google.com>]

On Tue, Aug 26, 2025 at 8:32 AM Robin Murphy <robin.murphy@arm.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. Not all PMUs validate a group will fit on the
counters, but I thought at least Intel's core PMU would validate and
not allow this. Fwiw, the metric code is reliant on this behavior as
by default all events are placed into a weak group:
https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/metricgroup.c?h=perf-tools-next#n631
Weak groups are really just groups that when the perf_event_open fails
retry with the grouping removed. PMUs that don't fail the
perf_event_open are problematic as the reads just report "not counted"
and the metric doesn't work. Sometimes the PMU can't help it due to
errata. There are a bunch of workarounds for those cases carried in
the perf tool, but in general weak groups working is relied upon:
https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/pmu-events/pmu-events.h?h=perf-tools-next#n16

Thanks,
Ian

> > 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 :-(
>
> Eww, I see what you mean... and I guess that's probably lower-overhead
> than actually deleting and recreating the sibling event(s) each time,
> and potentially less bother then wrangling multiple groups for different
> combinations of subsets when one simply must still approximate a complex
> metric that requires more counters than the hardware offers.
>
> I'm also not keen to break anything that wasn't already somewhat broken,
> especially since this patch is only intended as cleanup, so either we
> could just drop it altogether, or perhaps I can wrap the existing
> behaviour in a helper that can at least document this assumption and
> discourage new drivers from copying it. Am I right that only
> PERF_EVENT_STATE_{OFF,ERROR} would matter for this, though, and my
> reasoning for state <= PERF_EVENT_STATE_EXIT should still stand? As for
> the fiddly discrepancy with enable_on_exec between arm_pmu and others
> I'm not really sure what to think...
>
> Thanks,
> Robin.