Re: [PATCH 1/3] perf/core: Flush PMU internal buffers for per-CPU events

[Namhyung Kim]

Hello,

On Thu, Nov 12, 2020 at 4:54 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>
>
>
> On 11/11/2020 11:25 AM, Peter Zijlstra wrote:
> > On Mon, Nov 09, 2020 at 09:49:31AM -0500, Liang, Kan wrote:
> >
> >> - When the large PEBS was introduced (9c964efa4330), the sched_task() should
> >> be invoked to flush the PEBS buffer in each context switch. However, The
> >> perf_sched_events in account_event() is not updated accordingly. The
> >> perf_event_task_sched_* never be invoked for a pure per-CPU context. Only
> >> per-task event works.
> >>     At that time, the perf_pmu_sched_task() is outside of
> >> perf_event_context_sched_in/out. It means that perf has to double
> >> perf_pmu_disable() for per-task event.
> >
> >> - The patch 1 tries to fix broken per-CPU events. The CPU context cannot be
> >> retrieved from the task->perf_event_ctxp. So it has to be tracked in the
> >> sched_cb_list. Yes, the code is very similar to the original codes, but it
> >> is actually the new code for per-CPU events. The optimization for per-task
> >> events is still kept.
> >>    For the case, which has both a CPU context and a task context, yes, the
> >> __perf_pmu_sched_task() in this patch is not invoked. Because the
> >> sched_task() only need to be invoked once in a context switch. The
> >> sched_task() will be eventually invoked in the task context.
> >
> > The thing is; your first two patches rely on PERF_ATTACH_SCHED_CB and
> > only set that for large pebs. Are you sure the other users (Intel LBR
> > and PowerPC BHRB) don't need it?
>
> I didn't set it for LBR, because the perf_sched_events is always enabled
> for LBR. But, yes, we should explicitly set the PERF_ATTACH_SCHED_CB
> for LBR.
>
>         if (has_branch_stack(event))
>                 inc = true;
>
> >
> > If they indeed do not require the pmu::sched_task() callback for CPU
> > events, then I still think the whole perf_sched_cb_{inc,dec}() interface
>
> No, LBR requires the pmu::sched_task() callback for CPU events.
>
> Now, The LBR registers have to be reset in sched in even for CPU events.
>
> To fix the shorter LBR callstack issue for CPU events, we also need to
> save/restore LBRs in pmu::sched_task().
> https://lore.kernel.org/lkml/1578495789-95006-4-git-send-email-kan.liang@xxxxxxxxxxxxxxx/
>
> > is confusing at best.
> >
> > Can't we do something like this instead?
> >
> I think the below patch may have two issues.
> - PERF_ATTACH_SCHED_CB is required for LBR (maybe PowerPC BHRB as well) now.
> - We may disable the large PEBS later if not all PEBS events support
> large PEBS. The PMU need a way to notify the generic code to decrease
> the nr_sched_task.

Any updates on this?  I've reviewed and tested Kan's patches
and they all look good.

Maybe we can talk to PPC folks to confirm the BHRB case?

Thanks,
Namhyung