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

Michael Ellerman
2020-11-25T08:12:45Z

Namhyung Kim <namhyung@kernel.org> writes:
> Hello,
>
> On Mon, Nov 23, 2020 at 8:00 PM Michael Ellerman <mpe@ellerman.id.au> wrote:
>>
>> Namhyung Kim <namhyung@kernel.org> writes:
>> > Hi Peter and Kan,
>> >
>> > (Adding PPC folks)
>> >
>> > On Tue, Nov 17, 2020 at 2:01 PM Namhyung Kim <namhyung@kernel.org> wrote:
>> >>
>> >> Hello,
>> >>
>> >> On Thu, Nov 12, 2020 at 4:54 AM Liang, Kan <kan.liang@linux.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.

OK. TBH I've never thought of using branch stack with a per-cpu event,
but I guess you can do it.

I think the same logic applies as LBR, we need to read the BHRB entries
in the context of the task that they were recorded for.

> 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.

If you post a new version with Maddy's comments addressed then he or I
can ack it.

cheers