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

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Message-ID: <20241106143822.GA33184@noisy.programming.kicks-ass.net>
Date: Wed, 6 Nov 2024 15:38:22 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Mike Galbraith <efault@....de>
Cc: Phil Auld <pauld@...hat.com>, mingo@...hat.com, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, linux-kernel@...r.kernel.org,
	kprateek.nayak@....com, wuyun.abel@...edance.com,
	youssefesmat@...omium.org, tglx@...utronix.de
Subject: Re: [PATCH 17/24] sched/fair: Implement delayed dequeue

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.