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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Fri, 26 Mar 2021 16:37:03 +0100
Message-ID: <CAKfTPtB0NEQhuphhYXDZTQvgmLxoLFvbT17gcz6aTrdMUf-_Vg@mail.gmail.com>
Subject: Re: [PATCH 9/9] sched,fair: Alternative sched_slice()
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@kernel.org>, Mel Gorman <mgorman@suse.de>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Josh Don <joshdon@google.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, greg@kroah.com
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On Fri, 26 Mar 2021 at 11:43, Peter Zijlstra <peterz@infradead.org> wrote:
>
> The current sched_slice() seems to have issues; there's two possible
> things that could be improved:
>
>  - the 'nr_running' used for __sched_period() is daft when cgroups are
>    considered. Using the RQ wide h_nr_running seems like a much more
>    consistent number.
>
>  - (esp) cgroups can slice it real fine, which makes for easy
>    over-scheduling, ensure min_gran is what the name says.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/sched/fair.c     |   15 ++++++++++++++-
>  kernel/sched/features.h |    3 +++
>  2 files changed, 17 insertions(+), 1 deletion(-)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -680,7 +680,16 @@ static u64 __sched_period(unsigned long
>   */
>  static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> -       u64 slice = __sched_period(cfs_rq->nr_running + !se->on_rq);
> +       unsigned int nr_running = cfs_rq->nr_running;
> +       u64 slice;
> +
> +       if (sched_feat(ALT_PERIOD))
> +               nr_running = rq_of(cfs_rq)->cfs.h_nr_running;
> +
> +       slice = __sched_period(nr_running + !se->on_rq);
> +
> +       if (sched_feat(BASE_SLICE))
> +               slice -= sysctl_sched_min_granularity;
>
>         for_each_sched_entity(se) {
>                 struct load_weight *load;
> @@ -697,6 +706,10 @@ static u64 sched_slice(struct cfs_rq *cf
>                 }
>                 slice = __calc_delta(slice, se->load.weight, load);
>         }
> +
> +       if (sched_feat(BASE_SLICE))
> +               slice += sysctl_sched_min_granularity;

Why not only doing a max of slice and sysctl_sched_min_granularity
instead of scaling only the part above sysctl_sched_min_granularity ?

With your change, cases where the slices would have been in a good
range already, will be modified as well

> +
>         return slice;
>  }
>
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -90,3 +90,6 @@ SCHED_FEAT(WA_BIAS, true)
>   */
>  SCHED_FEAT(UTIL_EST, true)
>  SCHED_FEAT(UTIL_EST_FASTUP, true)
> +
> +SCHED_FEAT(ALT_PERIOD, true)
> +SCHED_FEAT(BASE_SLICE, true)
>
>