Re: [PATCH 17/24] sched/fair: Implement delayed dequeue

Peter Zijlstra
2024-11-06T14:38:31Z

On Wed, Nov 06, 2024 at 03:14:20PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 06, 2024 at 02:53:46PM +0100, Peter Zijlstra wrote:
>
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 54d82c21fc8e..b083c6385e88 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3774,28 +3774,38 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
> > */
> > static int ttwu_runnable(struct task_struct *p, int wake_flags)
> > {
> > + CLASS(__task_rq_lock, rq_guard)(p);
> > + struct rq *rq = rq_guard.rq;
> >
> > + if (!task_on_rq_queued(p))
> > + return 0;
> > +
> > + update_rq_clock(rq);
> > + if (p->se.sched_delayed) {
> > + int queue_flags = ENQUEUE_DELAYED | ENQUEUE_NOCLOCK;
> > +
> > + /*
> > + * Since sched_delayed means we cannot be current anywhere,
> > + * dequeue it here and have it fall through to the
> > + * select_task_rq() case further along the ttwu() path.
> > + */
> > + if (rq->nr_running > 1 && p->nr_cpus_allowed > 1) {
> > + dequeue_task(rq, p, DEQUEUE_SLEEP | queue_flags);
> > + return 0;
> > }
> > +
> > + enqueue_task(rq, p, queue_flags);
>
> And then I wondered... this means that !task_on_cpu() is true for
> sched_delayed, and thus we can move this in the below branch.
>
> But also, we can probably dequeue every such task, not only
> sched_delayed ones.
>
> > }
> > + if (!task_on_cpu(rq, p)) {
> > + /*
> > + * When on_rq && !on_cpu the task is preempted, see if
> > + * it should preempt the task that is current now.
> > + */
> > + wakeup_preempt(rq, p, wake_flags);
> > + }
> > + ttwu_do_wakeup(p);
> >
> > + return 1;
> > }
>
>
> Yielding something like this on top... which boots. But since I forgot
> to make it a feature, I can't actually tell at this point.. *sigh*

It dies real fast, so clearly I'm missing something. Oh well.