linux-kernel - Re: [PATCH V5 3/6] perf, x86: large PEBS interrupt threshold

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Date:	Mon, 30 Mar 2015 15:54:10 +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 3/6] perf, x86: large PEBS interrupt threshold

On Mon, Feb 23, 2015 at 09:25:53AM -0500, Kan Liang wrote:

> +/*
> + * Flags PEBS can handle without an PMI.
> + *
> + * TID can only be handled by flushing at context switch.
> + */
> +#define PEBS_FREERUNNING_FLAGS	\
> +	(PERF_SAMPLE_IP | PERF_SAMPLE_TID | PERF_SAMPLE_ADDR | \
> +	PERF_SAMPLE_ID | PERF_SAMPLE_CPU | PERF_SAMPLE_STREAM_ID | \
> +	PERF_SAMPLE_DATA_SRC | PERF_SAMPLE_IDENTIFIER | \
> +	PERF_SAMPLE_TRANSACTION)
> +

> @@ -683,6 +700,21 @@ void intel_pmu_pebs_enable(struct perf_event *event)
>  	else if (event->hw.flags & PERF_X86_EVENT_PEBS_ST)
>  		cpuc->pebs_enabled |= 1ULL << 63;
>  
> +	/*
> +	 * When the event is constrained enough we can use a larger
> +	 * threshold and run the event with less frequent PMI.
> +	 */
> +	if (0 && /* disable this temporarily */
> +	    (hwc->flags & PERF_X86_EVENT_AUTO_RELOAD) &&
> +	    !(event->attr.sample_type & ~PEBS_FREERUNNING_FLAGS)) {

Why not do the FREERUNNING flags thing in intel_pmu_hw_config() when you
set AUTO_RELOAD and make AUTO_RELOAD conditional on it.

That way you only have a single test here. AUTO_RELOAD implies the
sample_type conforms.
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