Re: SCX_ENQ_IMMED potentially leaving dispatched tasks lingering on local DSQs

[Tejun Heo]

Hello, Kuba.

On Wed, Apr 22, 2026 at 01:21:27PM +0000, Kuba Piecuch wrote:
> diff --git i/kernel/sched/ext.c w/kernel/sched/ext.c
> index d66fea57ee69..480627fdc203 100644
> --- i/kernel/sched/ext.c
> +++ w/kernel/sched/ext.c
> @@ -3079,8 +3079,11 @@ do_pick_task_scx(struct rq *rq, struct rq_flags *rf, bool force_scx)
>          * If @force_scx is true, always try to pick a SCHED_EXT task,
>          * regardless of any higher-priority sched classes activity.
>          */
> -       if (!force_scx && rq_modified_above(rq, &ext_sched_class))
> +       if (!force_scx && rq_modified_above(rq, &ext_sched_class)) {
> +               if (rq->scx.nr_immed)
> +                       schedule_reenq_local(rq, 0);
>                 return RETRY_TASK;
> +       }
> 
>         keep_prev = rq->scx.flags & SCX_RQ_BAL_KEEP;
>         if (unlikely(keep_prev &&

Ah, good catch.

> ...but I think this only fixes the case where the RT task wakes up on the CPU
> that is doing the dispatch. The other case is one where the RT task wakes up
> on the remote CPU (the one the second task was dispatched to) after insertion
> of the second task, assuming the remote CPU is initially idle.
> 
> To fix both cases, one potential solution that comes to mind is bumping
> rq->next_class to &ext_sched_class when inserting a task into rq->scx.local_dsq.
> Perhaps we should call wakeup_preempt() in dispatch_to_local_dsq()?

I think what's missing is wakeup_preempt() call in
move_remote_task_to_local_dsq(). This is SCX's version of move_queued_task()
and we're missing wakeup_preempt() call after activate_task().

Thanks.

-- 
tejun