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

[Peter Zijlstra]

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?