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

[Liang, Kan]

On 11/14/2019 8:57 AM, Peter Zijlstra wrote:
> 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.
Why we want to switch !cgroup system-wide event in context switch?

How should current perf handle this case?
For example,
User A: perf stat -e cycles -G cgroup1
User B: perf stat -e instructions -a

There is only one cpuctx for each CPU. So both cycles and instructions 
are tracked in flexible_active list.
When user A left, the cgroup context-switch schedule out everything 
including both cycles and instructions.
It seems that we will never switch the instructions event back for user B.


Thanks,
Kan