linux-kernel - Re: [PATCH 1/4] perf, x86: Avoid checkpointed counters causing excessive TSX aborts v4

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Date:	Tue, 13 Aug 2013 12:29:32 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org, acme@...radead.org,
	jolsa@...hat.com, eranian@...gle.com,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/4] perf, x86: Avoid checkpointed counters causing
 excessive TSX aborts v4

On Thu, Aug 08, 2013 at 06:15:43PM -0700, Andi Kleen wrote:
> +++ b/arch/x86/kernel/cpu/perf_event_intel.c

> @@ -1141,6 +1146,17 @@ static void intel_pmu_enable_event(struct perf_event *event)
>  int intel_pmu_save_and_restart(struct perf_event *event)
>  {
>  	x86_perf_event_update(event);
> +	/*
> +	 * For a checkpointed counter always reset back to 0.  This
> +	 * avoids a situation where the counter overflows, aborts the
> +	 * transaction and is then set back to shortly before the
> +	 * overflow, and overflows and aborts again.
> +	 */
> +	if (unlikely(event_is_checkpointed(event))) {
> +		/* No race with NMIs because the counter should not be armed */
> +		wrmsrl(event->hw.event_base, 0);
> +		local64_set(&event->hw.prev_count, 0);
> +	}

Right, if it wasn't for KVM you could've done a smaller special case
handler for checkpointed events, but as it stands I suppose it makes
sense to use the normal paths.

>  	return x86_perf_event_set_period(event);
>  }
>  
> @@ -1224,6 +1240,15 @@ again:
>  		x86_pmu.drain_pebs(regs);
>  	}
>  
> +	/*
> +	 * To avoid spurious interrupts with perf stat always reset checkpointed
> +	 * counters.
> +	 *
> +	 * XXX move somewhere else.

Like where? Afaict it needs to be here. You could write it prettier I
suppose and I guess we'll eventually need to assume all events can be
checkpointed but I don't see how it could be done elsewhere.

> +	 */
> +	if (cpuc->events[2] && event_is_checkpointed(cpuc->events[2]))
> +		status |= (1ULL << 2);
> +
>  	for_each_set_bit(bit, (unsigned long *)&status, X86_PMC_IDX_MAX) {
>  		struct perf_event *event = cpuc->events[bit];
>  


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