Re: [PATCH v2] sched/fair: Fix wakeup_preempt_fair for not waking up task

[Peter Zijlstra <peterz@infradead.org>]

On Sun, May 03, 2026 at 12:45:03PM +0200, Vincent Guittot wrote:
> The assumption that p is always enqueued and not delayed, is only true for
> wakeup. If p was moved while delayed, pick_next_entity will dequeue it
> and the cfs might become empty. Test if there are still queued tasks
> before trying again to determine if p could be the next one to be picked.
> 
> There are at least 2 cases:
> 
> When cfs becomes idle, it tries to pull tasks but if those pulled tasks are
> delayed, they will be dequeued when attached to cfs.
>   attach_tasks() -> attach_task() -> wakeup_preempt(rq, p, 0);
> 
> A misfit task running on cfs A triggers a load balance to be pulled on a
> better cpu, the load balance on cfs B starts an active load balance to
> pulled the running misfit task. If there is a delayed dequeue task on cfs A
> , it can be pulled instead of the previously running misfit task.
>   attach_one_task() -> attach_task() -> wakeup_preempt(rq, p, 0);
> 
> Fixes: ac8e69e69363 ("sched/fair: Fix wakeup_preempt_fair() vs delayed dequeue")
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
> 
> Change since v1:
> - Use cfs_rq->nr_queued instead of WF_TTWU flag to cover all cases instead
>   of just wakeup.
> 
> I'm preparing patches to cover those useless newly idle and active load
> balance task migration.
> 
>  kernel/sched/fair.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 728965851842..24087b0f25f6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9145,9 +9145,10 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f
>  
>  	/*
>  	 * Because p is enqueued, nse being null can only mean that we
> -	 * dequeued a delayed task.
> +	 * dequeued a delayed task. If there are still entities queued in
> +	 * cfs, check if the next one will be p.
>  	 */
> -	if (!nse)
> +	if (!nse && cfs_rq->nr_queued)
>  		goto pick;
>  
>  	if (sched_feat(RUN_TO_PARITY))

Right, that works. Thanks!