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

["Liang, Kan" <kan.liang@linux.intel.com>]

On 8/3/2021 10:55 AM, Peter Zijlstra wrote:
> On Tue, Aug 03, 2021 at 06:25:28AM -0700, kan.liang@linux.intel.com wrote:
>> From: Kan Liang <kan.liang@linux.intel.com>
>>
>> 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.

Thanks,
Kan