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

[Peter Zijlstra]

On Thu, Nov 14, 2019 at 08:46:51AM -0500, Liang, Kan wrote:
> 
> 
> On 11/14/2019 5:43 AM, Peter Zijlstra wrote:
> > 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.
> 
> 
> No, the patch intends to avoid switch !cgroup during cgroup context switch.

Which is wrong.