[PATCH] sched/fair: Make SCHED_IDLE se be preempted in strict hierarchy

[Tianchen Ding <dtcccc@linux.alibaba.com>]

Consider the following cgroup:
                       root
                        |
             ------------------------
             |                      |
       normal_cgroup            idle_cgroup
             |                      |
   SCHED_IDLE task_A           SCHED_NORMAL task_B

According to the cgroup hierarchy, A should preempt B. But current
check_preempt_wakeup_fair() treats cgroup se and task separately, so B
will preempt A unexpectedly.
Unify the wakeup logic by {p}se_is_idle only.

Also fix a bug about se_is_idle() definition when
!CONFIG_FAIR_GROUP_SCHED.

Fixes: 304000390f88 ("sched: Cgroup SCHED_IDLE support")
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
---
 kernel/sched/fair.c | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 41b58387023d..c91cfaa7d9ee 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -511,7 +511,7 @@ static int cfs_rq_is_idle(struct cfs_rq *cfs_rq)
 
 static int se_is_idle(struct sched_entity *se)
 {
-	return 0;
+	return task_has_idle_policy(task_of(se));
 }
 
 #endif	/* CONFIG_FAIR_GROUP_SCHED */
@@ -8382,16 +8382,7 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
 	if (test_tsk_need_resched(curr))
 		return;
 
-	/* Idle tasks are by definition preempted by non-idle tasks. */
-	if (unlikely(task_has_idle_policy(curr)) &&
-	    likely(!task_has_idle_policy(p)))
-		goto preempt;
-
-	/*
-	 * Batch and idle tasks do not preempt non-idle tasks (their preemption
-	 * is driven by the tick):
-	 */
-	if (unlikely(p->policy != SCHED_NORMAL) || !sched_feat(WAKEUP_PREEMPTION))
+	if (!sched_feat(WAKEUP_PREEMPTION))
 		return;
 
 	find_matching_se(&se, &pse);
@@ -8408,6 +8399,12 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
 		goto preempt;
 	if (cse_is_idle != pse_is_idle)
 		return;
+	/*
+	 * Batch tasks do not preempt non-idle tasks (their preemption
+	 * is driven by the tick):
+	 */
+	if (unlikely(pse == &p->se && p->policy == SCHED_BATCH))
+		return;
 
 	cfs_rq = cfs_rq_of(se);
 	update_curr(cfs_rq);
-- 
2.39.3