linux-kernel - Re: [RFC][PATCH 08/10] sched/fair: Implement delayed dequeue

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Date: Fri, 26 Apr 2024 12:56:07 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Mike Galbraith <efault@....de>
Cc: K Prateek Nayak <kprateek.nayak@....com>, mingo@...hat.com,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
	linux-kernel@...r.kernel.org, wuyun.abel@...edance.com,
	tglx@...utronix.de, Chen Yu <yu.c.chen@...el.com>,
	Oliver Sang <oliver.sang@...el.com>
Subject: Re: [RFC][PATCH 08/10] sched/fair: Implement delayed dequeue

On Thu, Apr 25, 2024 at 01:28:55PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 18, 2024 at 06:24:59PM +0200, Mike Galbraith wrote:
> > The root cause seems to be doing the delay dequeue business on
> > exiting tasks. 
> 
> > ---
> >  kernel/sched/fair.c |    5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5374,6 +5374,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
> >  		update_curr(cfs_rq);
> > 
> >  		if (sched_feat(DELAY_DEQUEUE) && sleep &&
> > +		    !(entity_is_task(se) && (task_of(se)->flags & PF_EXITING)) &&
> >  		    !entity_eligible(cfs_rq, se)) {
> >  			if (cfs_rq->next == se)
> >  				cfs_rq->next = NULL;
> 
> So I think this can be easier done in dequeue_task_fair(), where we
> still know this is a task.
> 
> Perhaps something like (I'll test later):
> 
> 	if (p->flags & PF_EXITING)
> 		flags &= ~DEQUEUE_SLEEP;
> 
> But now I need to go think about the case of removing a cgroup...
> *urgh*.

I ended up with the below instead; lemme go run this unixbench spawn on it.

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 95666034e76c..b5918fa9a0f0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8429,7 +8431,20 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
 
 static void task_dead_fair(struct task_struct *p)
 {
-	remove_entity_load_avg(&p->se);
+	struct sched_entity *se = &p->se;
+
+	if (p->se.sched_delayed) {
+		struct rq_flags rf;
+		struct rq *rq;
+
+		rq = task_rq_lock(p, &rf);
+		update_rq_clock(rq);
+		if (se->sched_delayed)
+			dequeue_entities(rq, se, DEQUEUE_SLEEP | DEQUEUE_DELAYED);
+		task_rq_unlock(rq, p, &rf);
+	}
+
+	remove_entity_load_avg(se);
 }
 
 /*
@@ -13089,28 +13104,34 @@ void online_fair_sched_group(struct task_group *tg)
 
 void unregister_fair_sched_group(struct task_group *tg)
 {
-	unsigned long flags;
-	struct rq *rq;
 	int cpu;
 
 	destroy_cfs_bandwidth(tg_cfs_bandwidth(tg));
 
 	for_each_possible_cpu(cpu) {
-		if (tg->se[cpu])
-			remove_entity_load_avg(tg->se[cpu]);
+		struct cfs_rq *cfs_rq = tg->cfs_rq[cpu];
+		struct sched_entity *se = tg->se[cpu];
+		struct rq *rq = cpu_rq(cpu);
+
+		if (se) {
+			if (se->sched_delayed) {
+				guard(rq_lock_irqsave)(rq);
+				update_rq_clock(rq);
+				if (se->sched_delayed)
+					dequeue_entities(rq, se, DEQUEUE_SLEEP | DEQUEUE_DELAYED);
+				list_del_leaf_cfs_rq(cfs_rq);
+			}
+			remove_entity_load_avg(se);
+		}
 
 		/*
 		 * Only empty task groups can be destroyed; so we can speculatively
 		 * check on_list without danger of it being re-added.
 		 */
-		if (!tg->cfs_rq[cpu]->on_list)
-			continue;
-
-		rq = cpu_rq(cpu);
-
-		raw_spin_rq_lock_irqsave(rq, flags);
-		list_del_leaf_cfs_rq(tg->cfs_rq[cpu]);
-		raw_spin_rq_unlock_irqrestore(rq, flags);
+		if (cfs_rq->on_list) {
+			guard(rq_lock_irqsave)(rq);
+			list_del_leaf_cfs_rq(cfs_rq);
+		}
 	}
 }