linux-kernel - Re: [PATCH 4/4] sched/fair: Improve NO_RUN_TO

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Message-ID: <CAKfTPtDyS=vNXvPncZutnydXFvV9WDm1N1y1xibPP-tibjiDrg@mail.gmail.com>
Date: Thu, 19 Jun 2025 14:31:20 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, 
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, 
	mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] sched/fair: Improve NO_RUN_TO_PARITY

On Fri, 13 Jun 2025 at 16:05, Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> The slice protection of RUN_TO_PARITY should also be applied with a
> minimal quantum of time for NO_RUN_TO_PARITY in order to ensure a minimum
> runtime for each task with same slice duration but also to ensure
> periodic switch between threads.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d8345219dfd4..73bde511c53b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -921,15 +921,20 @@ struct sched_entity *__pick_first_entity(struct cfs_rq *cfs_rq)
>   * one in vlag, which isn't used until dequeue.
>   * In case of run to parity, we use the shortest slice of the enqueued
>   * entities.
> + * When run to parity is disable we give a minimum quantum to the
> + * running entity to ensure progress.
>   */
>  static inline void set_protect_slice(struct sched_entity *se)
>  {
> -       u64 min_slice;
> +       u64 quantum;
>
> -       min_slice = cfs_rq_min_slice(cfs_rq_of(se));
> +       if (sched_feat(RUN_TO_PARITY))
> +               quantum = cfs_rq_min_slice(cfs_rq_of(se));
> +       else
> +               quantum = min(se->slice, normalized_sysctl_sched_base_slice);
The above is not correct and needs the fix below on top

-               quantum = min(se->slice, normalized_sysctl_sched_base_slice);
+       quantum = normalized_sysctl_sched_base_slice;

+       quantum = min(se->slice, quantum);

>
> -       if (min_slice != se->slice)
> -               se->vlag = min(se->deadline, se->vruntime + calc_delta_fair(min_slice, se));
> +       if (quantum != se->slice)
> +               se->vlag = min(se->deadline, se->vruntime + calc_delta_fair(quantum, se));
>         else
>                 se->vlag = se->deadline;
>  }
> @@ -981,7 +986,7 @@ static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
>         if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
>                 curr = NULL;
>
> -       if (sched_feat(RUN_TO_PARITY) && curr && protect_slice(curr))
> +       if (curr && protect_slice(curr))
>                 return curr;
>
>         /* Pick the leftmost entity if it's eligible */
> @@ -1215,11 +1220,8 @@ static inline void update_curr_task(struct task_struct *p, s64 delta_exec)
>         cgroup_account_cputime(p, delta_exec);
>  }
>
> -static inline bool did_preempt_short(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> +static inline bool resched_next_quantum(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>  {
> -       if (!sched_feat(PREEMPT_SHORT))
> -               return false;
> -
>         if (protect_slice(curr))
>                 return false;
>
> @@ -1307,7 +1309,7 @@ static void update_curr(struct cfs_rq *cfs_rq)
>         if (cfs_rq->nr_queued == 1)
>                 return;
>
> -       if (resched || did_preempt_short(cfs_rq, curr)) {
> +       if (resched || resched_next_quantum(cfs_rq, curr)) {
>                 resched_curr_lazy(rq);
>                 clear_buddies(cfs_rq, curr);
>         }
> --
> 2.43.0
>