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SubjectRe: [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@hp-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@kernel.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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