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

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Date: Fri, 26 Apr 2024 11:32:41 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Luis Machado <luis.machado@....com>
Cc: 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, kprateek.nayak@....com,
	wuyun.abel@...edance.com, tglx@...utronix.de, efault@....de,
	nd <nd@....com>, John Stultz <jstultz@...gle.com>
Subject: Re: [RFC][PATCH 08/10] sched/fair: Implement delayed dequeue

On Thu, Apr 25, 2024 at 01:49:49PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 25, 2024 at 12:42:20PM +0200, Peter Zijlstra wrote:
> 
> > > I wonder if the delayed dequeue logic is having an unwanted effect on the calculation of
> > > utilization/load of the runqueue and, as a consequence, we're scheduling things to run on
> > > higher OPP's in the big cores, leading to poor decisions for energy efficiency.
> > 
> > Notably util_est_update() gets delayed. Given we don't actually do an
> > enqueue when a delayed task gets woken, it didn't seem to make sense to
> > update that sooner.
> 
> The PELT runnable values will be inflated because of delayed dequeue.
> cpu_util() uses those in the @boost case, and as such this can indeed
> affect things.
> 
> This can also slightly affect the cgroup case, but since the delay goes
> away as contention goes away, and the cgroup case must already assume
> worst case overlap, this seems limited.
> 
> /me goes ponder things moar.

First order approximation of a fix would be something like the totally
untested below I suppose...

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index cfd1fd188d29..f3f70b5adca0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5391,6 +5391,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 			if (cfs_rq->next == se)
 				cfs_rq->next = NULL;
 			se->sched_delayed = 1;
+			update_load_avg(cfs_rq, se, 0);
 			return false;
 		}
 	}
@@ -6817,6 +6818,7 @@ requeue_delayed_entity(struct sched_entity *se)
 	}
 
 	se->sched_delayed = 0;
+	update_load_avg(qcfs_rq, se, 0);
 }
 
 /*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index d07a3b98f1fb..d16529613123 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -810,6 +810,9 @@ static inline void se_update_runnable(struct sched_entity *se)
 
 static inline long se_runnable(struct sched_entity *se)
 {
+	if (se->sched_delayed)
+		return false;
+
 	if (entity_is_task(se))
 		return !!se->on_rq;
 	else
@@ -823,6 +826,9 @@ static inline void se_update_runnable(struct sched_entity *se) {}
 
 static inline long se_runnable(struct sched_entity *se)
 {
+	if (se->sched_delayed)
+		return false;
+
 	return !!se->on_rq;
 }
 #endif