[PATCH v3 2/2] sched/fair: Fix premature check of WAKEUP_PREEMPTION

[Abel Wu]

The commit 6bc912b71b6f ("sched: SCHED_OTHER vs SCHED_IDLE isolation")
defines the behavior of SCHED_IDLE as following:

 - no SCHED_IDLE buddies
 - never let SCHED_IDLE preempt on wakeup
 - always preempt SCHED_IDLE on wakeup
 - limit SLEEPER fairness for SCHED_IDLE

and the 3rd rule is broken if !WAKEUP_PREEMPTION due to recently merged
commit faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy")

Although WAKEUP_PREEMPTION is mainly there for debug purpose to provide
a way to check whether a performance degrade of certain workload is due
to overscheduling or not, it is still kind of weird that we treat sched-
idle cpus as idle but don't let the non-idle tasks preempt the sched-idle
cpus in debug mode (!WAKEUP_PREEMPTION).

Fix it by strictly following the aforementioned rules.

Fixes: faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy")
Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>
Acked-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---
 kernel/sched/fair.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 19ae870cbeda..a5b58da629ef 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8787,9 +8787,6 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
 	if (test_tsk_need_resched(rq->curr))
 		return;
 
-	if (!sched_feat(WAKEUP_PREEMPTION))
-		return;
-
 	find_matching_se(&se, &pse);
 	WARN_ON_ONCE(!pse);
 
@@ -8808,6 +8805,9 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
 		goto preempt;
 	}
 
+	if (!sched_feat(WAKEUP_PREEMPTION))
+		return;
+
 	/*
 	 * IDLE entities do not preempt others.
 	 */
-- 
2.37.3