Re: [RFC 1/2] perf/core: Enable sched_task callbacks if PMU has it

[Liang, Kan]

On 11/5/2020 7:53 PM, Namhyung Kim wrote:
> On Fri, Nov 6, 2020 at 4:01 AM Liang, Kan<kan.liang@xxxxxxxxxxxxxxx>  wrote:
> > On 11/5/2020 10:45 AM, Namhyung Kim wrote:
> > > Hello,
> > >
> > > On Thu, Nov 5, 2020 at 11:47 PM Liang, Kan<kan.liang@xxxxxxxxxxxxxxx>  wrote:
> > > > On 11/2/2020 9:52 AM, Namhyung Kim wrote:
> > > > > If an event associated with a PMU which has a sched_task callback,
> > > > > it should be called regardless of cpu/task context.  For example,
> > > >
> > > > I don't think it's necessary. We should call it when we have to.
> > > > Otherwise, it just waste cycles.
> > > > Shouldn't the patch 2 be enough?
> > > I'm not sure, without this patch __perf_event_task_sched_in/out
> > > cannot be called for per-cpu events (w/o cgroups)  IMHO.
> > > And I could not find any other place to check the
> > > perf_sched_cb_usages.
> > Yes, it should a bug for large PEBS, and it should has always been there
> > since the large PEBS was introduced. I just tried some older kernels
> > (before recent change). Large PEBS is not flushed with per-cpu events.
> >
> > But from your description, it looks like the issue is only found after
> > recent change. Could you please double check if the issue can also be
> > reproduced before the recent change?
> Yep, actually Gabriel reported this problem on v4.4 kernel.

Thanks for the confirm.

So large PEBS never works with per-cpu events. :(
I will send a new patch set to address the issue.

Thanks,
Kan