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

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Message-ID: <CAM9d7cg8kYMyPHQK_rhEiYQaSddqqt93=pLVNKJm8Y6F=if9ow@mail.gmail.com>
Date:   Tue, 24 Nov 2020 13:51:15 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Ian Rogers <irogers@...gle.com>,
        Gabriel Marin <gmx@...gle.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, Andi Kleen <ak@...ux.intel.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 1/3] perf/core: Flush PMU internal buffers for per-CPU events

Hello,

On Mon, Nov 23, 2020 at 8:00 PM Michael Ellerman <mpe@...erman.id.au> wrote:
>
> Namhyung Kim <namhyung@...nel.org> writes:
> > Hi Peter and Kan,
> >
> > (Adding PPC folks)
> >
> > On Tue, Nov 17, 2020 at 2:01 PM Namhyung Kim <namhyung@...nel.org> wrote:
> >>
> >> Hello,
> >>
> >> On Thu, Nov 12, 2020 at 4:54 AM Liang, Kan <kan.liang@...ux.intel.com> 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@linux.intel.com/
> >> >
> >> > > 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.
>
> Sorry I've read the whole thread, but I'm still not entirely sure I
> understand the question.

Thanks for your time and sorry about not being clear enough.

We found per-cpu events are not calling pmu::sched_task()
on context switches.  So PERF_ATTACH_SCHED_CB was
added to indicate the core logic that it needs to invoke the
callback.

The patch 3/3 added the flag to PPC (for BHRB) with other
changes (I think it should be split like in the patch 2/3) and
want to get ACKs from the PPC folks.

Thanks,
Namhyung