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

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Message-ID: <20241106141420.GZ33184@noisy.programming.kicks-ass.net>
Date: Wed, 6 Nov 2024 15:14:20 +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 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*

Anyway, more toys to poke at I suppose.


diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b083c6385e88..69b19ba77598 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3781,28 +3781,32 @@ static int ttwu_runnable(struct task_struct *p, int wake_flags)
 		return 0;
 
 	update_rq_clock(rq);
-	if (p->se.sched_delayed) {
-		int queue_flags = ENQUEUE_DELAYED | ENQUEUE_NOCLOCK;
+	if (!task_on_cpu(rq, p)) {
+		int queue_flags = DEQUEUE_NOCLOCK;
+
+		if (p->se.sched_delayed)
+			queue_flags |= DEQUEUE_DELAYED;
 
 		/*
-		 * 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.
+		 * Since we're not current anywhere *AND* hold pi_lock, 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);
-	}
-	if (!task_on_cpu(rq, p)) {
+		if (p->se.sched_delayed)
+			enqueue_task(rq, p, queue_flags);
+
 		/*
 		 * 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);
 	}
+	SCHED_WARN_ON(p->se.sched_delayed);
 	ttwu_do_wakeup(p);
 
 	return 1;