Re: [PATCH V6 3/3] perf/x86/intel: Support PEBS counters snapshotting

[Liang, Kan]

On 2024-12-18 11:32 a.m., Peter Zijlstra wrote:
> On Wed, Dec 18, 2024 at 07:16:43AM -0800, kan.liang@xxxxxxxxxxxxxxx wrote:
> 
>> To prevent the case that a PEBS record value might be in the past
>> relative to what is already in the event, perf always stops the PMU and
>> drains the PEBS buffer before updating the corresponding event->count.
> 
> Like I wrote here:
> 
>   https://lkml.kernel.org/r/20241218082404.GI11133@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> 
> I don't think this is sufficient.


I replied with an explanation this morning in the old V5 thread. I'm not
sure if you got a chance to look at it.
https://lore.kernel.org/all/5a4ab06e-8628-4e1d-addb-2af920deffad@xxxxxxxxxxxxxxx/

There will be a drain_pebs() right before handling A-overflow-PMI.

  B-assist			A=1
  C				A=2
  B-assist			A=3
  		<- drain_pebs()
  A-overflow-PMI		A=4
  C-assist-PMI (DS buffer)	A=5

So the A-overflow-PMI will
   - Process the DS. adjust A->count to 3
   - adjust A->count to 4

Is it sufficient?
If not, could you please share more details?

Thanks,
Kan