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

[Davidlohr Bueso]

On Wed, 19 Oct 2016, Sebastian Andrzej Siewior wrote:

> 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@xxxxxxxxxxxxx>
> Acked-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

Thanks.

> > --- 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.

Yeah, but I prefer having it this way - separates the init from the actual
work (although no big deal here). The extra load happens ncpu times, so
also no big deal.