From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Vincent Guittot <vincent.guittot@linaro.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 7.0 1040/1146] sched/fair: Fix wakeup_preempt_fair() for not waking up task
Date: Wed, 20 May 2026 18:21:30 +0200
Message-ID: <20260520162211.767879912@linuxfoundation.org>
In-Reply-To: <20260520162148.390695140@linuxfoundation.org>
References: <20260520162148.390695140@linuxfoundation.org>
User-Agent: quilt/0.69
Precedence: bulk
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit

7.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Vincent Guittot <vincent.guittot@linaro.org>

[ Upstream commit 9f6d929ee2c6f0266edb564bcd2bd47fd6e884a8 ]

Make sure to only call pick_next_entity() on an non-empty cfs_rq.

The assumption that p is always enqueued and not delayed, is only true for
wakeup. If p was moved while delayed, pick_next_entity() will dequeue it and
the cfs might become empty. Test if there are still queued tasks before trying
again to determine if p could be the next one to be picked.

There are at least 2 cases:

When cfs becomes idle, it tries to pull tasks but if those pulled tasks are
delayed, they will be dequeued when attached to cfs. attach_tasks() ->
attach_task() -> wakeup_preempt(rq, p, 0);

A misfit task running on cfs A triggers a load balance to be pulled on a better
cpu, the load balance on cfs B starts an active load balance to pulled the
running misfit task. If there is a delayed dequeue task on cfs A, it can be
pulled instead of the previously running misfit task. attach_one_task() ->
attach_task() -> wakeup_preempt(rq, p, 0);

Fixes: ac8e69e69363 ("sched/fair: Fix wakeup_preempt_fair() vs delayed dequeue")
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260503104503.1732682-1-vincent.guittot@linaro.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/sched/fair.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 87200a22b3169..3bce48ad0bc5a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8922,9 +8922,10 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f
 
 	/*
 	 * Because p is enqueued, nse being null can only mean that we
-	 * dequeued a delayed task.
+	 * dequeued a delayed task. If there are still entities queued in
+	 * cfs, check if the next one will be p.
 	 */
-	if (!nse)
+	if (!nse && cfs_rq->nr_queued)
 		goto pick;
 
 	if (sched_feat(RUN_TO_PARITY))
-- 
2.53.0