linux-kernel - Re: [PATCH V5 5/6] perf, x86: drain PEBS buffer during context switch

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Message-ID: <20150330135045.GW23123@twins.programming.kicks-ass.net>
Date:	Mon, 30 Mar 2015 15:50:45 +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 5/6] perf, x86: drain PEBS buffer during context switch

On Mon, Feb 23, 2015 at 09:25:55AM -0500, Kan Liang wrote:
> From: Yan, Zheng <zheng.z.yan@...el.com>
> 
> Flush the PEBS buffer during context switch if PEBS interrupt threshold
> is larger than one. This allows perf to supply TID for sample outputs.
> 
> Signed-off-by: Yan, Zheng <zheng.z.yan@...el.com>
> Signed-off-by: Kan Liang <kan.liang@...el.com>
> ---
>  arch/x86/kernel/cpu/perf_event.h           |  3 +++
>  arch/x86/kernel/cpu/perf_event_intel.c     | 11 +++++++++-
>  arch/x86/kernel/cpu/perf_event_intel_ds.c  | 33 ++++++++++++++++++++++++++++--
>  arch/x86/kernel/cpu/perf_event_intel_lbr.c |  3 ---
>  4 files changed, 44 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event.h b/arch/x86/kernel/cpu/perf_event.h
> index bc4ae3b..b4f6431 100644
> --- a/arch/x86/kernel/cpu/perf_event.h
> +++ b/arch/x86/kernel/cpu/perf_event.h
> @@ -151,6 +151,7 @@ struct cpu_hw_events {
>  	 */
>  	struct debug_store	*ds;
>  	u64			pebs_enabled;
> +	bool			pebs_sched_cb_enabled;
>  
>  	/*
>  	 * Intel LBR bits

Why do we need that extra state? I would've expected to see a inc/dec
for every AUTO_RELOAD that gets added/removed.

> @@ -704,13 +717,20 @@ void intel_pmu_pebs_enable(struct perf_event *event)
>  	 * 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) &&
> +	if ((hwc->flags & PERF_X86_EVENT_AUTO_RELOAD) &&
>  	    !(event->attr.sample_type & ~PEBS_FREERUNNING_FLAGS)) {
>  		threshold = ds->pebs_absolute_maximum -
>  			x86_pmu.max_pebs_events * x86_pmu.pebs_record_size;
> +		if (first_pebs) {
> +			perf_sched_cb_inc(event->ctx->pmu);
> +			cpuc->pebs_sched_cb_enabled = true;
> +		}
>  	} else {
>  		threshold = ds->pebs_buffer_base + x86_pmu.pebs_record_size;
> +		if (cpuc->pebs_sched_cb_enabled) {
> +			perf_sched_cb_dec(event->ctx->pmu);
> +			cpuc->pebs_sched_cb_enabled = false;
> +		}
>  	}
>  	if (first_pebs || ds->pebs_interrupt_threshold > threshold)
>  		ds->pebs_interrupt_threshold = threshold;

I'm confused, why do you do sched_cb_dec for every event that wasn't
AUTO_RELOAD?
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