[PATCH 6.12 082/169] sched/fair: Fix pelt clock sync when entering idle

Greg Kroah-Hartman
2026-01-28T15:46:44Z

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")
Closes: https://lore.kernel.org/all/CAG2KctpO6VKS6GN4QWDji0t92_gNBJ7HjjXrE+6H+RwRXt=iLg@mail.gmail.com/
Closes: https://lore.kernel.org/all/8cf19bf0e0054dcfed70e9935029201694f1bb5a.camel@mediatek.com/
Reported-by: Samuel Wu <wusamuel@google.com>
Reported-by: Alex Hoh <Alex.Hoh@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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/sched/fair.c | 6 ------
kernel/sched/idle.c | 6 ++++++
2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1436d6bb86ec4..6efb1dfcd943a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9033,12 +9033,6 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
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;
}

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 53bb9193c537a..624ef809f6715 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -462,6 +462,12 @@ static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool fir
scx_update_idle(rq, true, true);
schedstat_inc(rq->sched_goidle);
next->se.exec_start = rq_clock_task(rq);
+
+ /*
+ * 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);
}

struct task_struct *pick_task_idle(struct rq *rq)
--
2.51.0