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

[Namhyung Kim]

Hi Peter and Kan,

(Adding PPC folks)

On Tue, Nov 17, 2020 at 2:01 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> 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?

Can we move this forward?  I saw patch 3/3 also adds PERF_ATTACH_SCHED_CB
for PowerPC too.  But it'd be nice if ppc folks can confirm the change.

Thanks,
Namhyung