linux-kernel - Re: [PATCH] sched: Move sched_entity::avg into separate cache line

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Message-ID: <20151208203308.GM11564@kernel.org>
Date:	Tue, 8 Dec 2015 17:33:08 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Jiri Olsa <jolsa@...nel.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	lkml <linux-kernel@...r.kernel.org>,
	Don Zickus <dzickus@...hat.com>, Joe Mario <jmario@...hat.com>
Subject: Re: [PATCH] sched: Move sched_entity::avg into separate cache line

Em Tue, Dec 08, 2015 at 09:23:59PM +0100, Jiri Olsa escreveu:
> From: root <root@...dl380gen9-01.khw.lab.eng.bos.redhat.com>
> 
> hi,
> I tried Joe's and Don's c2c tool and it identified
> a place with cache line contention. There're more
> that poped up, this one was just too obvious ;-)

Would be interesting to see output from that tool and how you have
interpreted it to be for that specific data structure.

- Arnaldo
 
> thanks
> jirka
> 
> 
> ---
> The sched_entity::avg collides with read-mostly sched_entity data.
> 
> The perf c2c tool showed many read HITM accesses across
> many CPUs for sched_entity's cfs_rq and my_q, while having
> at the same time tons of stores for avg.
> 
> After placing sched_entity::avg into separate cache line,
> the perf bench sched pipe showed around 20 seconds speedup.
> 
> NOTE I cut out all perf events except for cycles and
> instructions from following output.
> 
> Before:
>   $ perf stat -r 5 perf bench sched pipe -l 10000000
>   # Running 'sched/pipe' benchmark:
>   # Executed 10000000 pipe operations between two processes
> 
>        Total time: 270.348 [sec]
> 
>         27.034805 usecs/op
>             36989 ops/sec
>    ...
> 
>      245,537,074,035      cycles                    #    1.433 GHz
>      187,264,548,519      instructions              #    0.77  insns per cycle
> 
>        272.653840535 seconds time elapsed           ( +-  1.31% )
> 
> After:
>   $ perf stat -r 5 perf bench sched pipe -l 10000000
>   # Running 'sched/pipe' benchmark:
>   # Executed 10000000 pipe operations between two processes
> 
>        Total time: 251.076 [sec]
> 
>         25.107678 usecs/op
>             39828 ops/sec
>   ...
> 
>      244,573,513,928      cycles                    #    1.572 GHz
>      187,409,641,157      instructions              #    0.76  insns per cycle
> 
>        251.679315188 seconds time elapsed           ( +-  0.31% )
> 
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  include/linux/sched.h | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 3b0de68bce41..80cc1432e6e3 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1268,8 +1268,13 @@ struct sched_entity {
>  #endif
>  
>  #ifdef CONFIG_SMP
> -	/* Per entity load average tracking */
> -	struct sched_avg	avg;
> +	/*
> +	 * Per entity load average tracking.
> +	 *
> +	 * Put into separate cache line so it does not
> +	 * collide with read-mostly values above.
> +	 */
> +	struct sched_avg	avg ____cacheline_aligned_in_smp;
>  #endif
>  };
>  
> -- 
> 2.4.3
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