linux-kernel - Re: [PATCH 2/2] perf/x86/intel: Support PEBS output to PT

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Date:   Wed, 8 May 2019 11:34:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Arnaldo Carvalho de Melo <acme@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        jolsa@...hat.com, adrian.hunter@...el.com
Subject: Re: [PATCH 2/2] perf/x86/intel: Support PEBS output to PT

On Thu, May 02, 2019 at 01:50:22PM +0300, Alexander Shishkin wrote:

> The output setting is per-CPU, so all PEBS events must be either writing
> to PT or to the DS area, so in order to not mess up the event scheduling,
> we fall back to the latter in case both types of events are scheduled in.

> +static void intel_pmu_pebs_via_pt_disable(struct perf_event *event)
> +{
> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> +
> +	if (!(event->hw.flags & PERF_X86_EVENT_PEBS_VIA_PT))
> +		return;
> +
> +	if (!(cpuc->pebs_enabled & ~PEBS_VIA_PT_MASK))
> +		cpuc->pebs_enabled &= ~PEBS_VIA_PT_MASK;
> +}
> +
> +static void intel_pmu_pebs_via_pt_enable(struct perf_event *event)
> +{
> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> +	struct hw_perf_event *hwc = &event->hw;
> +	struct debug_store *ds = cpuc->ds;
> +
> +	if (!(event->hw.flags & PERF_X86_EVENT_PEBS_VIA_PT))
> +		return;
> +
> +	/*
> +	 * In case there's a mix of PEBS->PT and PEBS->DS, fall back
> +	 * to DS.
> +	 */
> +	if (cpuc->n_pebs != cpuc->n_pebs_via_pt) {
> +		/* PEBS-to-DS events present, fall back to DS */
> +		intel_pmu_pebs_via_pt_disable(event);
> +		return;
> +	}
> +
> +	if (!(event->hw.flags & PERF_X86_EVENT_LARGE_PEBS))
> +		cpuc->pebs_enabled |= PEBS_PMI_AFTER_EACH_RECORD;
> +
> +	cpuc->pebs_enabled |= PEBS_OUTPUT_PT;
> +
> +	wrmsrl(MSR_RELOAD_PMC0 + hwc->idx, ds->pebs_event_reset[hwc->idx]);
> +}
> +
>  void intel_pmu_pebs_enable(struct perf_event *event)
>  {
>  	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> @@ -1100,6 +1146,8 @@ void intel_pmu_pebs_enable(struct perf_event *event)
>  	} else {
>  		ds->pebs_event_reset[hwc->idx] = 0;
>  	}
> +
> +	intel_pmu_pebs_via_pt_enable(event);
>  }

I think that doesn't even do what it says on the tin. Suppose you first
schedule that PEBS-via-PT event and then the normal one, nothing then
cancels the PT link.

Like I wrote in that prevoius email; I really don't like this. I think
silently falling back to another output method is wrong.

Ideally we create schedulig conflicts and cause the PT and DS events to
round robin.