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

Madhavan Srinivasan
2020-11-24T05:43:49Z

On 11/24/20 10:21 AM, Namhyung Kim wrote:
> 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.
>
> 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.

Sorry for delay.

I guess first it will be better to split the ppc change to a separate patch,

secondly, we are missing the changes needed in the power_pmu_bhrb_disable()

where perf_sched_cb_dec() needs the "state" to be included.

Maddy

>
> Thanks,
> Namhyung