linux-kernel - Re: [PATCH v6 2/3]: perf/core: use context tstamp

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Date:   Thu, 3 Aug 2017 17:00:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Dmitri Prokhorov <Dmitry.Prohorov@...el.com>,
        Valery Cherepennikov <valery.cherepennikov@...el.com>,
        Mark Rutland <mark.rutland@....com>,
        Stephane Eranian <eranian@...gle.com>,
        David Carrillo-Cisneros <davidcc@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 2/3]: perf/core: use context tstamp_data for skipped
 events on mux interrupt

On Wed, Aug 02, 2017 at 11:15:39AM +0300, Alexey Budankov wrote:
> @@ -772,6 +780,10 @@ struct perf_event_context {
>  	 */
>  	u64				time;
>  	u64				timestamp;
> +	/*
> +	 * Context cache for filtered out events;
> +	 */
> +	struct perf_event_tstamp	tstamp_data;
>  
>  	/*
>  	 * These fields let us detect when two contexts have both


> @@ -1379,6 +1379,9 @@ static void update_context_time(struct perf_event_context *ctx)
>  
>  	ctx->time += now - ctx->timestamp;
>  	ctx->timestamp = now;
> +
> +	ctx->tstamp_data.running += ctx->time - ctx->tstamp_data.stopped;
> +	ctx->tstamp_data.stopped = ctx->time;
>  }
>  
>  static u64 perf_event_time(struct perf_event *event)

It appears to me we have some redundancy here.


> @@ -1968,9 +1971,13 @@ event_sched_out(struct perf_event *event,
>  	 */
>  	if (event->state == PERF_EVENT_STATE_INACTIVE &&
>  	    !event_filter_match(event)) {
> +		delta = tstamp - event->tstamp->stopped;
> +		event->tstamp->running += delta;
> +		event->tstamp->stopped = tstamp;
> +		if (event->tstamp != &event->tstamp_data) {
> +			event->tstamp_data = *event->tstamp;

This,

> +			event->tstamp = &event->tstamp_data;
> +		}
>  	}
>  
>  	if (event->state != PERF_EVENT_STATE_ACTIVE)


> @@ -3239,8 +3246,11 @@ ctx_pinned_sched_in(struct perf_event *event, void *data)
>  
>  	if (event->state <= PERF_EVENT_STATE_OFF)
>  		return 0;
> -	if (!event_filter_match(event))
> +	if (!event_filter_match(event)) {
> +		if (event->tstamp != &params->ctx->tstamp_data)
> +			event->tstamp = &params->ctx->tstamp_data;

this and

>  		return 0;
> +	}
>  
>  	/* may need to reset tstamp_enabled */
>  	if (is_cgroup_event(event))
> @@ -3273,8 +3283,11 @@ ctx_flexible_sched_in(struct perf_event *event, void *data)
>  	 * Listen to the 'cpu' scheduling filter constraint
>  	 * of events:
>  	 */
> -	if (!event_filter_match(event))
> +	if (!event_filter_match(event)) {
> +		if (event->tstamp != &params->ctx->tstamp_data)
> +			event->tstamp = &params->ctx->tstamp_data;

this..

>  		return 0;
> +	}
>  
>  	/* may need to reset tstamp_enabled */
>  	if (is_cgroup_event(event))


Are the magic spots, right? And I'm not convinced its right.

Suppose I have two events in my context, and I created them 1 minute
apart. Then their respective tstamp_enabled are 1 minute apart as well.
But the above doesn't seem to preserve that difference.

A similar argument can be made for running I think. That is a per event
value and cannot be passed along to the ctx and back.