Re: [PATCH 6/6 v3] sched/eevdf: Speedup short slice task scheduling

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jun 24, 2026 at 05:12:29PM +0200, Vincent Guittot wrote:
> When a task with a shorter slice is enqueued, we protect the running
> task which has a longer slice until it becomes ineligible instead of a
> full slice in order to speedup the switch to other tasks until the task
> with the shortest slice is scheduled. This helps to the task to not wait
> too many full slices before running.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
> ---
>  kernel/sched/fair.c | 52 +++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 50 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f972987618e7..7c541f27a1ed 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -813,6 +813,48 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
>  	return cfs_rq->zero_vruntime;
>  }
>  
> +/*
> + * Compute the vruntime until which the entity remains eligible when it runs
> + * or is about to run on the CPU. We use this value to set vprot to the min
> + * value until which other entities would not be picked anyway.
> + *     \Sum (v_i - v0)*w_i
> + * V = ------------------- + v0
> + *          \Sum w_i
> + *
> + * We want V' for (v_se - v0) == 0. Previous entity has already been enqueued
> + * in the rb tree and next is already dequeued so
> + *
> + *      cfs_rq->sum_w_vruntime
> + * V' = ------------------------- + v0
> + *      cfs_rq->sum_weight + w_se
> +
> + */
> +static u64 eligible_vruntime(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +{
> +	struct sched_entity *curr = cfs_rq->curr;
> +	long weight = cfs_rq->sum_weight;
> +	s64 delta = 0;

'curr' goes unused in this function, did you want:

	if (curr && !curr->on_rq)
		curr = NULL;

> +
> +	if (weight) {
> +		s64 runtime = cfs_rq->sum_w_vruntime;


		if (curr) {
			unsigned long w = avg_vruntime_weight(cfs_rq, curr->load.weight);

			runtime += entity_key(cfs_rq, curr) * w;
			weight += w;
		}

?

> +
> +		weight += avg_vruntime_weight(cfs_rq, se->load.weight);
> +
> +		/* sign flips effective floor / ceiling */
> +		if (runtime < 0)
> +			runtime -= (weight - 1);
> +
> +		delta = div64_long(runtime, weight);
> +	} else {
> +		/*
> +		 * When there is but one element, it is the average.
> +		 */
> +		delta = 0;
> +	}
> +
> +	return cfs_rq->zero_vruntime + delta + 1;
> +}