linux-kernel - Re: [PATCH 19/24] sched/eevdf: Fixup PELT vs DELAYED

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Message-ID: <20240814072548.GB22686@noisy.programming.kicks-ass.net>
Date: Wed, 14 Aug 2024 09:25:48 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Valentin Schneider <vschneid@...hat.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, linux-kernel@...r.kernel.org,
	kprateek.nayak@....com, wuyun.abel@...edance.com,
	youssefesmat@...omium.org, tglx@...utronix.de, efault@....de
Subject: Re: [PATCH 19/24] sched/eevdf: Fixup PELT vs DELAYED_DEQUEUE

On Wed, Aug 14, 2024 at 12:18:07AM +0200, Peter Zijlstra wrote:
> On Tue, Aug 13, 2024 at 02:43:56PM +0200, Valentin Schneider wrote:
> > On 27/07/24 12:27, Peter Zijlstra wrote:
> > > Note that tasks that are kept on the runqueue to burn off negative
> > > lag, are not in fact runnable anymore, they'll get dequeued the moment
> > > they get picked.
> > >
> > > As such, don't count this time towards runnable.
> > >
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > > ---
> > >  kernel/sched/fair.c  |    2 ++
> > >  kernel/sched/sched.h |    6 ++++++
> > >  2 files changed, 8 insertions(+)
> > >
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -5388,6 +5388,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
> > >                       if (cfs_rq->next == se)
> > >                               cfs_rq->next = NULL;
> > >                       se->sched_delayed = 1;
> > > +			update_load_avg(cfs_rq, se, 0);
> > 
> > Shouldn't this be before setting ->sched_delayed? accumulate_sum() should
> > see the time delta as spent being runnable.
> > 
> > >                       return false;
> > >               }
> > >       }
> > > @@ -6814,6 +6815,7 @@ requeue_delayed_entity(struct sched_enti
> > >       }
> > >
> > >       se->sched_delayed = 0;
> > > +	update_load_avg(cfs_rq, se, 0);
> > 
> > Ditto on the ordering
> 
> Bah, so I remember thinking about it and then I obviously go and do it
> the exact wrong way around eh? Let me double check this tomorrow morning
> with the brain slightly more awake :/

OK, so I went over it again and I ended up with the below diff -- which
assuming I didn't make a giant mess of things *again*, I should go fold
back into various other patches ...

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1b15dbfb1ce5..fa8907f2c716 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5461,14 +5461,10 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 {
 	bool sleep = flags & DEQUEUE_SLEEP;
 
+	update_curr(cfs_rq);
+
 	if (flags & DEQUEUE_DELAYED) {
-		/*
-		 * DEQUEUE_DELAYED is typically called from pick_next_entity()
-		 * at which point we've already done update_curr() and do not
-		 * want to do so again.
-		 */
 		SCHED_WARN_ON(!se->sched_delayed);
-		se->sched_delayed = 0;
 	} else {
 		bool delay = sleep;
 		/*
@@ -5479,14 +5475,13 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 			delay = false;
 
 		SCHED_WARN_ON(delay && se->sched_delayed);
-		update_curr(cfs_rq);
 
 		if (sched_feat(DELAY_DEQUEUE) && delay &&
 		    !entity_eligible(cfs_rq, se)) {
 			if (cfs_rq->next == se)
 				cfs_rq->next = NULL;
-			se->sched_delayed = 1;
 			update_load_avg(cfs_rq, se, 0);
+			se->sched_delayed = 1;
 			return false;
 		}
 	}
@@ -5536,6 +5531,12 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 	if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) != DEQUEUE_SAVE)
 		update_min_vruntime(cfs_rq);
 
+	if (flags & DEQUEUE_DELAYED) {
+		se->sched_delayed = 0;
+		if (sched_feat(DELAY_ZERO) && se->vlag > 0)
+			se->vlag = 0;
+	}
+
 	if (cfs_rq->nr_running == 0)
 		update_idle_cfs_rq_clock_pelt(cfs_rq);
 
@@ -5611,11 +5612,6 @@ pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq)
 	struct sched_entity *se = pick_eevdf(cfs_rq);
 	if (se->sched_delayed) {
 		dequeue_entities(rq, se, DEQUEUE_SLEEP | DEQUEUE_DELAYED);
-		SCHED_WARN_ON(se->sched_delayed);
-		SCHED_WARN_ON(se->on_rq);
-		if (sched_feat(DELAY_ZERO) && se->vlag > 0)
-			se->vlag = 0;
-
 		return NULL;
 	}
 	return se;
@@ -6906,7 +6902,7 @@ requeue_delayed_entity(struct sched_entity *se)
 	struct cfs_rq *cfs_rq = cfs_rq_of(se);
 
 	/*
-	 * se->sched_delayed should imply both: se->on_rq == 1.
+	 * se->sched_delayed should imply: se->on_rq == 1.
 	 * Because a delayed entity is one that is still on
 	 * the runqueue competing until elegibility.
 	 */
@@ -6927,8 +6923,8 @@ requeue_delayed_entity(struct sched_entity *se)
 		}
 	}
 
-	se->sched_delayed = 0;
 	update_load_avg(cfs_rq, se, 0);
+	se->sched_delayed = 0;
 }
 
 /*