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

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On 8/3/2021 12:17 PM, Peter Zijlstra wrote:
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?


Let me check and do some tests.

Thanks,
Kan



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