[PATCH 1/2] sched/fair: Do not let idle entities preempt others

[Abel Wu]

A task with SCHED_IDLE policy doesn't preempt others by definition, and
the semantics are intended to be preserved when extending to cgroups
introduced in commit 304000390f88 ("sched: Cgroup SCHED_IDLE support").

But current implementation allows idle entities to preempt each other
on wakeup, which seems not behave as expected especially after
commit faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy")
so fix this by explicitly skip wakeup preemption for idle entities.

Fixes: 304000390f88 ("sched: Cgroup SCHED_IDLE support")
Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1c0ef435a7aa..4340178f29b7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8778,12 +8778,15 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
 	pse_is_idle = se_is_idle(pse);
 
 	/*
-	 * Preempt an idle entity in favor of a non-idle entity (and don't preempt
-	 * in the inverse case).
+	 * Preempt an idle entity in favor of a non-idle entity.
 	 */
 	if (cse_is_idle && !pse_is_idle)
 		goto preempt;
-	if (cse_is_idle != pse_is_idle)
+
+	/*
+	 * IDLE entities do not preempt others.
+	 */
+	if (unlikely(pse_is_idle))
 		return;
 
 	/*
-- 
2.37.3