Re: [PATCH v6] perf: Sharing PMU counters across compatible events

[Peter Zijlstra]

On Tue, Nov 05, 2019 at 05:11:08PM +0000, Song Liu wrote:

> > I think we can use one of the event as master. We need to be careful when
> > the master event is removed, but it should be doable. Let me try. 
> 
> Actually, there is a bigger issue when we use one event as the master: what
> shall we do if the master event is not running? Say it is an cgroup event, 
> and the cgroup is not running on this cpu. An extra master (and all these
> array hacks) help us get O(1) complexity in such scenario. 
> 
> Do you have suggestions on how to solve this problem? Maybe we can keep the 
> extra master, and try get rid of the double alloc? 

Right, you have to consider scope when sharing. The master should be the
largest scope event and any slaves should be complete subsets.

Without much thought this seems a fairly straight forward constraint;
that is, given cgroups I'm not immediately seeing how we can violate
that.

Basically, pick the cgroup event nearest to the root as the master.
We have to have logic to re-elect the master anyway for deletion, so
changing it on add shouldn't be different.

(obviously the root-cgroup is cpu-wide and always on, and if you have
two events from disjoint subtrees they have no overlap, so it doesn't
make sense to share anyway)