Re: [PATCH] Revert "sched/core: Tweak wait_task_inactive() to force dequeue sched_delayed tasks"

[John Stultz]

On Thu, Sep 25, 2025 at 6:33 AM Matt Fleming <matt@xxxxxxxxxxxxxxxx> wrote:
>
> From: Matt Fleming <mfleming@xxxxxxxxxxxxxx>
>
> This reverts commit b7ca5743a2604156d6083b88cefacef983f3a3a6.
>
> If we dequeue a task (task B) that was sched delayed then that task is
> definitely no longer on the rq and not tracked in the rbtree.
> Unfortunately, task_on_rq_queued(B) will still return true because
> dequeue_task() doesn't update p->on_rq.

Hey!
  Sorry again my patch has been causing you trouble. Thanks for your
persistence in chasing this down!

It's confusing as this patch uses the similar logic as logic
pick_next_entity() uses when a sched_delayed task is picked to run, as
well as elsewhere in __sched_setscheduler() and in sched_ext, so I'd
fret that similar

And my impression was that dequeue_task() on a sched_delayed task
would update p->on_rq via calling __block_task() at the end of
dequeue_entities().

However, there are two spots where we might exit dequeue_entities()
early when cfs_rq_throttled(rq), so maybe that's what's catching us
here?

Peter: Those cfs_rq_throttled() exits in dequeue_entities() seem a
little odd, as the desired dequeue didn't really complete, but
dequeue_task_fair() will still return true indicating success - not
that too many places are checking the dequeue_task return. Is that
right?

thanks
-john