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

[Davidlohr Bueso <dave@stgolabs.net>]

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@linutronix.de>
>Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

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.