[PATCH 6.6 063/105] sched/fair: Fix pelt lost idle time detection

Greg Kroah-Hartman
2025-10-21T19:55:27Z

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

[ Upstream commit 17e3e88ed0b6318fde0d1c14df1a804711cab1b5 ]

The check for some lost idle pelt time should be always done when
pick_next_task_fair() fails to pick a task and not only when we call it
from the fair fast-path.

The case happens when the last running task on rq is a RT or DL task. When
the latter goes to sleep and the /Sum of util_sum of the rq is at the max
value, we don't account the lost of idle time whereas we should.

Fixes: 67692435c411 ("sched: Rework pick_next_task() slow-path")
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/sched/fair.c | 26 +++++++++++++-------------
1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 84d5caf6230f6..58231999d929e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8528,21 +8528,21 @@ done: __maybe_unused;
return p;

idle:
- if (!rf)
- return NULL;
-
- new_tasks = sched_balance_newidle(rq, rf);
+ if (rf) {
+ new_tasks = sched_balance_newidle(rq, rf);

- /*
- * Because sched_balance_newidle() releases (and re-acquires) rq->lock, it is
- * possible for any higher priority task to appear. In that case we
- * must re-start the pick_next_entity() loop.
- */
- if (new_tasks < 0)
- return RETRY_TASK;
+ /*
+ * Because sched_balance_newidle() releases (and re-acquires)
+ * rq->lock, it is possible for any higher priority task to
+ * appear. In that case we must re-start the pick_next_entity()
+ * loop.
+ */
+ if (new_tasks < 0)
+ return RETRY_TASK;

- if (new_tasks > 0)
- goto again;
+ if (new_tasks > 0)
+ goto again;
+ }

/*
* rq is about to be idle, check if we need to update the
--
2.51.0