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SubjectRe: [PATCH v3 10/10] perf/cgroup: Do not switch system-wide events in cgroup switch
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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@linux.intel.com>
>>
>> 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.

In perf_cgroup_switch(), when the cgroup is scheduled out, current
implementation schedule out everything including !cgroup. I think it
definitely breaks the semantics of !cgroup aka system-wide event.

The patch itself doesn't touch the merge sort in visit_groups_merge().
The perf_cgroup_skip_switch() just skips the !cgroup in schedule_in().
Because !cgroup wasn't scheduled out. We don't want to schedule !cgroup
in again.
The cgroup events must be after !cgroup events, since !cgroup never be
switched.

Thanks,
Kan

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