Re: [RFC 1/2] perf/core: Enable sched_task callbacks if PMU has it

Liang, Kan
2020-11-06T16:12:00Z

On 11/5/2020 7:53 PM, Namhyung Kim wrote:
> On Fri, Nov 6, 2020 at 4:01 AM Liang, Kan<kan.liang@linux.intel.com> wrote:
>>
>>
>> On 11/5/2020 10:45 AM, Namhyung Kim wrote:
>>> Hello,
>>>
>>> On Thu, Nov 5, 2020 at 11:47 PM Liang, Kan<kan.liang@linux.intel.com> wrote:
>>>>
>>>>
>>>> On 11/2/2020 9:52 AM, Namhyung Kim wrote:
>>>>> If an event associated with a PMU which has a sched_task callback,
>>>>> it should be called regardless of cpu/task context. For example,
>>>>
>>>> I don't think it's necessary. We should call it when we have to.
>>>> Otherwise, it just waste cycles.
>>>> Shouldn't the patch 2 be enough?
>>> I'm not sure, without this patch __perf_event_task_sched_in/out
>>> cannot be called for per-cpu events (w/o cgroups) IMHO.
>>> And I could not find any other place to check the
>>> perf_sched_cb_usages.
>>>
>> Yes, it should a bug for large PEBS, and it should has always been there
>> since the large PEBS was introduced. I just tried some older kernels
>> (before recent change). Large PEBS is not flushed with per-cpu events.
>>
>> But from your description, it looks like the issue is only found after
>> recent change. Could you please double check if the issue can also be
>> reproduced before the recent change?
> Yep, actually Gabriel reported this problem on v4.4 kernel.

Thanks for the confirm.

So large PEBS never works with per-cpu events. :(
I will send a new patch set to address the issue.

Thanks,
Kan