Re: [PATCH V2] perf/x86/intel: Apply mid ACK for small core

[Peter Zijlstra]

On Tue, Aug 03, 2021 at 11:20:20AM -0400, Liang, Kan wrote:
> 
> 
> On 8/3/2021 10:55 AM, Peter Zijlstra wrote:
> > On Tue, Aug 03, 2021 at 06:25:28AM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> > > From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > > 
> > > A warning as below may be occasionally triggered in an ADL machine when
> > > these conditions occur,
> > > - Two perf record commands run one by one. Both record a PEBS event.
> > > - Both runs on small cores.
> > > - They have different adaptive PEBS configuration (PEBS_DATA_CFG).
> > > 
> > > [  673.663291] WARNING: CPU: 4 PID: 9874 at
> > > arch/x86/events/intel/ds.c:1743
> > > setup_pebs_adaptive_sample_data+0x55e/0x5b0
> > > [  673.663348] RIP: 0010:setup_pebs_adaptive_sample_data+0x55e/0x5b0
> > > [  673.663357] Call Trace:
> > > [  673.663357]  <NMI>
> > > [  673.663357]  intel_pmu_drain_pebs_icl+0x48b/0x810
> > > [  673.663360]  perf_event_nmi_handler+0x41/0x80
> > > [  673.663368]  </NMI>
> > > [  673.663370]  __perf_event_task_sched_in+0x2c2/0x3a0
> > > 
> > > Different from the big core, the small core requires the ACK right
> > > before re-enabling counters in the NMI handler, otherwise a stale PEBS
> > > record may be dumped into the later NMI handler, which trigger the
> > > warning.
> > > 
> > > Add a new mid_ack flag to track the case. Add all PMI handler bits in
> > > the struct x86_hybrid_pmu to track the bits for different types of PMUs.
> > > Apply mid ACK for the small cores on an Alder Lake machine.
> > 
> > Why do we need a new option? Why isn't early (as in not late) good
> > enough?
> > 
> 
> The early ACK can fix this issue, however it triggers a spurious NMI during
> the stress test. I'm told to do the ACK right before re-enabling counters
> for small cores. That indeed fixes all the issues.

Any chance that would also work for the chips that now use late_ack?

I'm just (desperately) trying to minimize the amount of quirks here ;-)