linux-kernel - [PATCH 10/24] sched/uclamg: Handle delayed dequeue

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Message-Id: <20240727105029.315205425@infradead.org>
Date: Sat, 27 Jul 2024 12:27:42 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: mingo@...hat.com,
 peterz@...radead.org,
 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
Cc: kprateek.nayak@....com,
 wuyun.abel@...edance.com,
 youssefesmat@...omium.org,
 tglx@...utronix.de,
 efault@....de,
 Luis Machado <luis.machado@....com>,
 Hongyan Xia <hongyan.xia2@....com>
Subject: [PATCH 10/24] sched/uclamg: Handle delayed dequeue

Delayed dequeue has tasks sit around on the runqueue that are not
actually runnable -- specifically, they will be dequeued the moment
they get picked.

One side-effect is that such a task can get migrated, which leads to a
'nested' dequeue_task() scenario that messes up uclamp if we don't
take care.

Notably, dequeue_task(DEQUEUE_SLEEP) can 'fail' and keep the task on
the runqueue. This however will have removed the task from uclamp --
per uclamp_rq_dec() in dequeue_task(). So far so good.

However, if at that point the task gets migrated -- or nice adjusted
or any of a myriad of operations that does a dequeue-enqueue cycle --
we'll pass through dequeue_task()/enqueue_task() again. Without
modification this will lead to a double decrement for uclamp, which is
wrong.

Reported-by: Luis Machado <luis.machado@....com>
Reported-by: Hongyan Xia <hongyan.xia2@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/sched/core.c |   16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1676,6 +1676,9 @@ static inline void uclamp_rq_inc(struct
 	if (unlikely(!p->sched_class->uclamp_enabled))
 		return;
 
+	if (p->se.sched_delayed)
+		return;
+
 	for_each_clamp_id(clamp_id)
 		uclamp_rq_inc_id(rq, p, clamp_id);
 
@@ -1700,6 +1703,9 @@ static inline void uclamp_rq_dec(struct
 	if (unlikely(!p->sched_class->uclamp_enabled))
 		return;
 
+	if (p->se.sched_delayed)
+		return;
+
 	for_each_clamp_id(clamp_id)
 		uclamp_rq_dec_id(rq, p, clamp_id);
 }
@@ -1979,8 +1985,12 @@ void enqueue_task(struct rq *rq, struct
 		psi_enqueue(p, (flags & ENQUEUE_WAKEUP) && !(flags & ENQUEUE_MIGRATED));
 	}
 
-	uclamp_rq_inc(rq, p);
 	p->sched_class->enqueue_task(rq, p, flags);
+	/*
+	 * Must be after ->enqueue_task() because ENQUEUE_DELAYED can clear
+	 * ->sched_delayed.
+	 */
+	uclamp_rq_inc(rq, p);
 
 	if (sched_core_enabled(rq))
 		sched_core_enqueue(rq, p);
@@ -2002,6 +2012,10 @@ inline bool dequeue_task(struct rq *rq,
 		psi_dequeue(p, flags & DEQUEUE_SLEEP);
 	}
 
+	/*
+	 * Must be before ->dequeue_task() because ->dequeue_task() can 'fail'
+	 * and mark the task ->sched_delayed.
+	 */
 	uclamp_rq_dec(rq, p);
 	return p->sched_class->dequeue_task(rq, p, flags);
 }