linux-kernel - Re: [PATCH] perf/bench-futex: Avoid worker cacheline bouncing

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Date:   Wed, 19 Oct 2016 20:13:08 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH] perf/bench-futex: Avoid worker cacheline bouncing

On 2016-10-19 10:59:33 [-0700], Davidlohr Bueso wrote:
> Sebastian noted that overhead for worker thread ops (throughput)
> accounting was producing 'perf' to appear in the profiles, consuming
> a non-trivial (ie 13%) amount of CPU. This is due to cacheline
> bouncing due to the increment of w->ops. We can easily fix this by
> just working on a local copy and updating the actual worker once
> done running, and ready to show the program summary. There is no
> danger of the worker being concurrent, so we can trust that no stale
> value is being seen by another thread.
> 
> Reported-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Acked-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

> --- a/tools/perf/bench/futex-hash.c
> +++ b/tools/perf/bench/futex-hash.c
> @@ -63,8 +63,9 @@ static const char * const bench_futex_hash_usage[] = {
> static void *workerfn(void *arg)
> {
> 	int ret;
> -	unsigned int i;
> 	struct worker *w = (struct worker *) arg;
> +	unsigned int i;
> +	unsigned long ops = w->ops; /* avoid cacheline bouncing */

we start at 0 so there is probably no need to init it with w->ops.

Sebastian