linux-kernel - Re: [PATCH V5 1/6] perf, x86: use the PEBS auto reload mechanism when possible

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Date:	Mon, 30 Mar 2015 14:06:07 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kan Liang <kan.liang@...el.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, acme@...radead.org,
	eranian@...gle.com, andi@...stfloor.org
Subject: Re: [PATCH V5 1/6] perf, x86: use the PEBS auto reload mechanism
 when possible

On Mon, Feb 23, 2015 at 09:25:51AM -0500, Kan Liang wrote:
> +++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c
> @@ -673,6 +673,8 @@ void intel_pmu_pebs_enable(struct perf_event *event)
>  	struct hw_perf_event *hwc = &event->hw;
>  
>  	hwc->config &= ~ARCH_PERFMON_EVENTSEL_INT;
> +	if (!event->attr.freq)
> +		hwc->flags |= PERF_X86_EVENT_AUTO_RELOAD;
>  
>  	cpuc->pebs_enabled |= 1ULL << hwc->idx;

Why not in x86_pmu_event_init()? This is not something that will change,
ever, right?

> @@ -680,6 +682,12 @@ void intel_pmu_pebs_enable(struct perf_event *event)
>  		cpuc->pebs_enabled |= 1ULL << (hwc->idx + 32);
>  	else if (event->hw.flags & PERF_X86_EVENT_PEBS_ST)
>  		cpuc->pebs_enabled |= 1ULL << 63;
> +
> +	/* Use auto-reload if possible to save a MSR write in the PMI */
> +	if (hwc->flags & PERF_X86_EVENT_AUTO_RELOAD) {
> +		ds->pebs_event_reset[hwc->idx] =
> +			(u64)-hwc->sample_period & x86_pmu.cntval_mask;
> +	}
>  }
>  
>  void intel_pmu_pebs_disable(struct perf_event *event)
> @@ -698,6 +706,7 @@ void intel_pmu_pebs_disable(struct perf_event *event)
>  		wrmsrl(MSR_IA32_PEBS_ENABLE, cpuc->pebs_enabled);
>  
>  	hwc->config |= ARCH_PERFMON_EVENTSEL_INT;
> +	hwc->flags &= ~PERF_X86_EVENT_AUTO_RELOAD;

Equally, we should not ever have to clear this. You cannot change
attributes after the event is created, if that flag ever gets set, it
stays valid.
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