Re: [PATCH v3 10/10] perf/cgroup: Do not switch system-wide events in cgroup switch

[Peter Zijlstra]

On Wed, Nov 13, 2019 at 04:30:42PM -0800, Ian Rogers wrote:
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> 
> When counting system-wide events and cgroup events simultaneously, the
> system-wide events are always scheduled out then back in during cgroup
> switches, bringing extra overhead and possibly missing events. Switching
> out system wide flexible events may be necessary if the scheduled in
> task's cgroups have pinned events that need to be scheduled in at a higher
> priority than the system wide flexible events.

I'm thinking this patch is actively broken. groups->index 'group' wide
and therefore across cpu/cgroup boundaries.

There is no !cgroup to cgroup hierarchy as this patch seems to assume,
specifically look at how the merge sort in visit_groups_merge() allows
cgroup events to be picked before !cgroup events.