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

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Message-ID: <20170712114829.v2tbvk37xdkkwqme@hirez.programming.kicks-ass.net>
Date:   Wed, 12 Jul 2017 13:48:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        linux-kernel@...r.kernel.org
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:
> 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);
> +			}
> +		}
> +	}
> +
>  	if (unlikely(head - local_read(&rb->wakeup) > rb->watermark))
>  		local_add(rb->watermark, &rb->wakeup);
>  

So this is a very performance sensitive function; not at all happy to
add bits here ... :/