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 :-(