linux-kernel - Re: [RESEND PATCH V2] perf/x86/intel: Avoid unnecessary PEBS

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Date:   Fri, 17 Jan 2020 09:54:12 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     acme@...hat.com, mingo@...nel.org, linux-kernel@...r.kernel.org,
        ak@...ux.intel.com, eranian@...gle.com
Subject: Re: [RESEND PATCH V2] perf/x86/intel: Avoid unnecessary PEBS_ENABLE
 MSR access in PMI

On Thu, Jan 16, 2020 at 10:21:12AM -0800, kan.liang@...ux.intel.com wrote:

> A PMI may land after cpuc->enabled=0 in x86_pmu_disable() and
> PMI throttle may be triggered for the PMI. For this rare case,
> intel_pmu_pebs_disable() will not touch PEBS_ENABLE MSR. The patch
> explicitly disable the PEBS for this case.

intel_pmu_handle_irq()
  pmu_enabled = cpuc->enabled;
  cpuc->enabled = 0;
  __intel_pmu_disable_all();

  ...
    x86_pmu_stop()
      intel_pmu_disable_event()
        intel_pmu_pebs_disable()
	  if (cpuc->enabled) // FALSE!!!

  cpuc->enabled = pmu_enabled;
  if (pmu_enabled)
    __intel_pmu_enable_all();

> @@ -2620,6 +2627,15 @@ static int handle_pmi_common(struct pt_regs *regs, u64 status)
>  		handled++;
>  		x86_pmu.drain_pebs(regs);
>  		status &= x86_pmu.intel_ctrl | GLOBAL_STATUS_TRACE_TOPAPMI;
> +
> +		/*
> +		 * PMI may land after cpuc->enabled=0 in x86_pmu_disable() and
> +		 * PMI throttle may be triggered for the PMI.
> +		 * For this rare case, intel_pmu_pebs_disable() will not touch
> +		 * MSR_IA32_PEBS_ENABLE. Explicitly disable the PEBS here.
> +		 */
> +		if (unlikely(!cpuc->enabled && !cpuc->pebs_enabled))
> +			wrmsrl(MSR_IA32_PEBS_ENABLE, 0);
>  	}

How does that make sense? AFAICT this is all still completely broken.

Please be more careful.