Re: [PATCH] sched/eevdf: Delayed dequeue task can't preempt

[K Prateek Nayak]

Hello Vincent,

On 7/1/2026 10:19 PM, Vincent Guittot wrote:
> Load balancing can migrate delayed dequeue tasks to even the load between
> CPUs.
> 
> sched_balance_rq()
>   -> detach_task()
>     -> deactivate_task(DEQUEUE_NOCLOCK)
>     -> set_task_cpu(dst_cpu)
> 
>   -> attach_task()
>     -> activate_task(ENQUEUE_NOCLOCK)
>     -> wakeup_preempt()
> 
> A delayed task with shorter slice can be dequeued during pick_next_entity()
> but then jump to preempt because eligible.
> 
> Always jump to update for delayed dequeue task even with shorter slice.
> 
> Fixes: ba0d3bf5f97b ("sched/eevdf: Cancel slice protection if short slice task is eligible")
> Reported-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

Thank you for fixing this! Feel free to include:

Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>

> ---
>  kernel/sched/fair.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fc6cd55f9d22..df8c9c2c7918 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9948,6 +9948,13 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f
>  	if (unlikely(!normal_policy(p->policy)))
>  		goto update;
>  
> +	/*
> +	 * Do not preempt for tasks that are sched_delayed as it would violate
> +	 * EEVDF to forcibly queue an ineligible task.
> +	 */
> +	if (pse->sched_delayed)
> +		goto update;
> +

Small question: Are we updating the protect slice just to maintain the
lag bounds with delayed entities?

Otherwise, it might be okay to return early here and keep the current
running for a bit longer and wait until the delayed entity gets a wakeup
which does a wakeup_preempt().

/me goes and checks

My hackbench run says:

  Test:                  vanilla              early_return
   1-groups:         4.75 (0.00 pct)         4.46 ( 6.10 pct)
   2-groups:         4.52 (0.00 pct)         4.57 (-1.10 pct)
   4-groups:         4.67 (0.00 pct)         4.73 (-1.28 pct)
   8-groups:         7.09 (0.00 pct)         6.85 ( 3.38 pct)
  16-groups:        12.07 (0.00 pct)        12.03 ( 0.33 pct)

but there is enough variance in the improved runs that it could all be
noise so your patch should be good as is.

-- 
Thanks and Regards,
Prateek