[PATCH v2] sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy

[Tianchen Ding]

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 {c,p}se_is_idle only. This makes SCHED_IDLE of
a task a relative policy that is effective only within its own cgroup,
similar to the behavior of NICE.

Also fix se_is_idle() definition when !CONFIG_FAIR_GROUP_SCHED.

Fixes: 304000390f88 ("sched: Cgroup SCHED_IDLE support")
Signed-off-by: Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx>
Reviewed-by: Josh Don <joshdon@xxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
---
v2:
Use entity_is_task() to check whether pse is a task.
Improve comments and commit log.

v1: https://lore.kernel.org/all/20240624073900.10343-1-dtcccc@xxxxxxxxxxxxxxxxx/
---
 kernel/sched/fair.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 41b58387023d..f0b038de99ce 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);
@@ -8401,7 +8392,7 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
 	pse_is_idle = se_is_idle(pse);
 
 	/*
-	 * Preempt an idle group in favor of a non-idle group (and don't preempt
+	 * Preempt an idle entity in favor of a non-idle entity (and don't preempt
 	 * in the inverse case).
 	 */
 	if (cse_is_idle && !pse_is_idle)
@@ -8409,6 +8400,15 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
 	if (cse_is_idle != pse_is_idle)
 		return;
 
+	/*
+	 * Batch tasks do not preempt non-idle tasks (their preemption
+	 * is driven by the tick).
+	 * We've done the check about "only one of the entities is idle",
+	 * so cse must be non-idle if p is a batch task.
+	 */
+	if (unlikely(entity_is_task(pse) && p->policy == SCHED_BATCH))
+		return;
+
 	cfs_rq = cfs_rq_of(se);
 	update_curr(cfs_rq);
 
-- 
2.39.3