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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Fri, 5 Jul 2019 16:51:45 +0200
Message-ID: <CAKfTPtDsh-VxTrwuhb88fi-L4j0ODnNOqhoQ=ZC6E8FVV7Kmkw@mail.gmail.com>
Subject: Re: [PATCH 07/10] sched,cfs: fix zero length timeslice calculation
To:     Rik van Riel <riel@surriel.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Kernel Team <kernel-team@fb.com>, Paul Turner <pjt@google.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>
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On Fri, 28 Jun 2019 at 22:49, Rik van Riel <riel@surriel.com> wrote:
>
> The way the time slice length is currently calculated, not only do high
> priority tasks get longer time slices than low priority tasks, but due
> to fixed point math, low priority tasks could end up with a zero length
> time slice. This can lead to cache thrashing and other inefficiencies.
>
> Simplify the logic a little bit, and cap the minimum time slice length
> to sysctl_sched_min_granularity.
>
> Tasks that end up getting a time slice length too long for their relative
> priority will simply end up having their vruntime advanced much faster than
> other tasks, resulting in them receiving time slices less frequently.
>
> Signed-off-by: Rik van Riel <riel@surriel.com>
> ---
>  kernel/sched/fair.c | 25 ++++++++-----------------
>  1 file changed, 8 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d48bff5118fc..8da2823401ca 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -671,22 +671,6 @@ static inline u64 calc_delta_fair(u64 delta, struct sched_entity *se)
>         return delta;
>  }
>
> -/*
> - * The idea is to set a period in which each task runs once.
> - *
> - * When there are too many tasks (sched_nr_latency) we have to stretch
> - * this period because otherwise the slices get too small.
> - *
> - * p = (nr <= nl) ? l : l*nr/nl
> - */
> -static u64 __sched_period(unsigned long nr_running)
> -{
> -       if (unlikely(nr_running > sched_nr_latency))
> -               return nr_running * sysctl_sched_min_granularity;
> -       else
> -               return sysctl_sched_latency;
> -}
> -
>  /*
>   * We calculate the wall-time slice from the period by taking a part
>   * proportional to the weight.
> @@ -695,7 +679,7 @@ static u64 __sched_period(unsigned long nr_running)
>   */
>  static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> -       u64 slice = __sched_period(cfs_rq->nr_running + !se->on_rq);
> +       u64 slice = sysctl_sched_latency;

Is the change above and the remove of __sched_period() really needed
for fixing the null time slice problem ?
This change impacts how tasks will preempt each other and as a result
the throughput. It should have it dedicated patch so we can evaluate
its impact

>
>         for_each_sched_entity(se) {
>                 struct load_weight *load;
> @@ -712,6 +696,13 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
>                 }
>                 slice = __calc_delta(slice, se->load.weight, load);
>         }
> +
> +       /*
> +        * To avoid cache thrashing, run at least sysctl_sched_min_granularity.
> +        * The vruntime of a low priority task advances faster; those tasks
> +        * will simply get time slices less frequently.
> +        */
> +       slice = max_t(u64, slice, sysctl_sched_min_granularity);
>         return slice;
>  }
>
> --
> 2.20.1
>