linux-kernel - Re: [PATCH 2/2] kernel/events: Add option to enable counting sideband events in wakeup

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Date:   Wed, 12 Jul 2017 12:48:55 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Vince Weaver <vincent.weaver@...ne.edu>
Subject: Re: [PATCH 2/2] kernel/events: Add option to enable counting
 sideband events in wakeup_events

On Mon, Jun 19, 2017 at 08:01:08PM +0530, Naveen N. Rao wrote:

SNIP

> -	if (!event->attr.watermark) {
> +	if (!event->attr.count_sb_events && !event->attr.watermark) {
>  		int wakeup_events = event->attr.wakeup_events;
>  
>  		if (wakeup_events) {
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 4e7c728569a8..f43a6081141f 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -197,6 +197,19 @@ __perf_output_begin(struct perf_output_handle *handle,
>  	 * none of the data stores below can be lifted up by the compiler.
>  	 */
>  
> +	if (event->attr.count_sb_events && !event->attr.watermark) {
> +		int wakeup_events = event->attr.wakeup_events;
> +
> +		if (wakeup_events) {
> +			int events = local_inc_return(&rb->events);
> +
> +			if (events >= wakeup_events) {
> +				local_sub(wakeup_events, &rb->events);
> +				local_inc(&rb->wakeup);
> +			}
> +		}
> +	}

hum, so there's the same wakeup code in perf_output_sample,
but it's not called for non-sample (sideband) events

it'd be nice to have this wakeup only once in __perf_output_begin,
but it'd change the behaviour for sideband events, which would start to
follow the wakeup_events logic.. and possibly disturb Vince's tests?

jirka