linux-kernel - Re: [PATCH 08/15] sched,fair: simplify timeslice length code

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Message-ID: <CAKfTPtDxHijR3PCOFfxA-r02rf2hVP4LpB=y-9emHS7znTPxTA@mail.gmail.com>
Date:   Wed, 28 Aug 2019 19:32:05 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>, Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH 08/15] sched,fair: simplify timeslice length code

On Thu, 22 Aug 2019 at 04:18, Rik van Riel <riel@...riel.com> wrote:
>
> The idea behind __sched_period makes sense, but the results do not always.
>
> When a CPU has one high priority task and a large number of low priority
> tasks, __sched_period will return a value larger than sysctl_sched_latency,
> and the one high priority task may end up getting a timeslice all for itself
> that is also much larger than sysctl_sched_latency.

note that unless you enable sched_feat(HRTICK), the sched_slice is
mainly used to decide how fast we preempt running task at tick but a
newly wake up task can preempt it before

>
> The low priority tasks will have their time slices rounded up to
> sysctl_sched_min_granularity, resulting in an even larger scheduling
> latency than targeted by __sched_period.

Will this not break the fairness between a always running task and a
short sleeping one with this changes ?

>
> Simplify the code by simply ripping out __sched_period and always taking
> fractions of sysctl_sched_latency.
>
> If a high priority task ends up getting a "too small" time slice compared
> to low priority tasks, the vruntime scaling ensures that it will simply
> get scheduled more frequently than low priority tasks.

Will you not increase the number of context switch ?

>
> Signed-off-by: Rik van Riel <riel@...riel.com>
> ---
>  kernel/sched/fair.c | 18 +-----------------
>  1 file changed, 1 insertion(+), 17 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8f8c85c6da9b..74ee22c59d13 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -691,22 +691,6 @@ 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.
> @@ -715,7 +699,7 @@ 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;
>
>         for_each_sched_entity(se) {
>                 struct load_weight *load;
> --
> 2.20.1
>