From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Samuel Wu <wusamuel@google.com>,
	Alex Hoh <Alex.Hoh@mediatek.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>
Subject: [PATCH 6.1 445/481] sched/fair: Fix pelt clock sync when entering idle
Date: Mon, 23 Mar 2026 14:47:07 +0100
Message-ID: <20260323134536.059920879@linuxfoundation.org>
In-Reply-To: <20260323134525.256603107@linuxfoundation.org>
References: <20260323134525.256603107@linuxfoundation.org>
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6.1-stable review patch.  If anyone has any objections, please let me know.

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

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

[ Upstream commit 98c88dc8a1ace642d9021b103b28cba7b51e3abc ]

Samuel and Alex reported regressions of the util_avg of RT rq with
commit 17e3e88ed0b6 ("sched/fair: Fix pelt lost idle time detection").
It happens that fair is updating and syncing the pelt clock with task one
when pick_next_task_fair() fails to pick a task but before the prev
scheduling class got a chance to update its pelt signals.

Move update_idle_rq_clock_pelt() in set_next_task_idle() which is called
after prev class has been called.

Fixes: 17e3e88ed0b6 ("sched/fair: Fix pelt lost idle time detection")
Reported-by: Samuel Wu <wusamuel@google.com>
Closes: https://lore.kernel.org/all/CAG2KctpO6VKS6GN4QWDji0t92_gNBJ7HjjXrE+6H+RwRXt=iLg@mail.gmail.com/
Reported-by: Alex Hoh <Alex.Hoh@mediatek.com>
Closes: https://lore.kernel.org/all/8cf19bf0e0054dcfed70e9935029201694f1bb5a.camel@mediatek.com/
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Samuel Wu <wusamuel@google.com>
Tested-by: Alex Hoh <Alex.Hoh@mediatek.com>
Link: https://patch.msgid.link/20260121163317.505635-1-vincent.guittot@linaro.org
(cherry picked from commit 98c88dc8a1ace642d9021b103b28cba7b51e3abc)
[ wusamuel: Did not include line 'exec_start = rq_clock_task()', which
is not present in 6.1.y but found in mainline ]
Signed-off-by: Samuel Wu <wusamuel@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/sched/fair.c |    6 ------
 kernel/sched/idle.c |    6 ++++++
 2 files changed, 6 insertions(+), 6 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7924,12 +7924,6 @@ idle:
 			goto again;
 	}
 
-	/*
-	 * rq is about to be idle, check if we need to update the
-	 * lost_idle_time of clock_pelt
-	 */
-	update_idle_rq_clock_pelt(rq);
-
 	return NULL;
 }
 
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -456,6 +456,12 @@ static void set_next_task_idle(struct rq
 {
 	update_idle_core(rq);
 	schedstat_inc(rq->sched_goidle);
+
+	/*
+	 * rq is about to be idle, check if we need to update the
+	 * lost_idle_time of clock_pelt
+	 */
+	update_idle_rq_clock_pelt(rq);
 }
 
 #ifdef CONFIG_SMP