Date: Wed, 19 Oct 2016 20:13:08 +0200
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel@vger.kernel.org, Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH] perf/bench-futex: Avoid worker cacheline bouncing
Message-ID: <20161019181308.maacqqzdx4ep5yld@linutronix.de>
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 <20161019175933.GA28074@linux-80c1.suse>
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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@linutronix.de>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.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