Re: [PATCH 1/2] perf tool AMD: Use non-precise cycles as default event on certain Zen2 processors

[Ravi Bangoria]

>>>>>> By default, Perf uses precise cycles event when no explicit event is
>>>>>> specified by user. Precise cycles event is forwarded to ibs_op// pmu
>>>>>> on AMD. However, IBS has hw issue on certain Zen2 processors where
>>>>>> it might raise NMI without sample_valid bit set, which causes Unknown
>>>>>> NMI warnings. (Erratum #1215: IBS (Instruction Based Sampling) Counter
>>>>>> Valid Value May be Incorrect After Exit From Core C6 (CC6) State.) So,
>>>>>> use non-precise cycles as default event on affected processors.
>>>>>
>>>>> It seems like a kernel issue, do we have a kernel patch not to forward
>>>>> precise cycles or instructions events to IBS on the affected CPUs?
>>>>
>>>> I'm not sure how it's a kernel issue. User can use ibs_op// pmu directly
>>>> and might hit the hw bug.
>>>
>>> Sorry for the late reply.  I know it's the user's fault when using ibs_op//
>>> directly but I think the kernel should not forward cycles:pp to IBS.
>>
>> For all AMD zen uarchs, cycles:p is same as ibs_op// (man perf-list has
>> more detail). It's confusing if ibs_op// works but cycles:p fails on the
>> same machine.
> 
> Make sense.  Probably perf tools can provide a more detailed message
> about the situation.

+1

>> No strong opinion though. I can prepare a kernel patch, if you want.
> 
> I have a report that these unknown NMI messages caused some
> performance issues.  So maybe we need to be safe by not using
> IBS automatically.  I've also posted a patch to rate-limit the message
> itself.

Sure. Will prepare and post the patch.

Thanks,
Ravi