[PATCH v2] sched: ensure a task has a non-normalized vruntime when returning back to cfs

[Byungchul Park]

current code ensures a task has a normalized vruntime when switching off
from fair class, but it does not ensure the task has a non-normalized
vruntime when switching back to the fair class.

this is an example breaking this consistency.

1. a task is in fair class and !queue
2. change its class to rt class (still !queue)
3. change its class to fair class again (still !queue)

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
 kernel/sched/fair.c |   19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ffa70dc..61e80b3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8142,16 +8142,31 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p)
  */
 static void switched_to_fair(struct rq *rq, struct task_struct *p)
 {
-#ifdef CONFIG_FAIR_GROUP_SCHED
 	struct sched_entity *se = &p->se;
+
+#ifdef CONFIG_FAIR_GROUP_SCHED
 	/*
 	 * Since the real-depth could have been changed (only FAIR
 	 * class maintain depth value), reset depth properly.
 	 */
 	se->depth = se->parent ? se->parent->depth + 1 : 0;
 #endif
-	if (!task_on_rq_queued(p))
+
+	if (!task_on_rq_queued(p)) {
+
+		/*
+		 * Ensure the task has a non-normalized vruntime when it is switched
+		 * back to the fair class with !queued, so that enqueue_entity() at
+		 * wake-up time will do the right thing.
+		 *
+		 * If it's queued, then the enqueue_entity(.flags=0) makes the task
+		 * has non-normalized vruntime, if it's !queued, then it still has
+		 * normalized vruntime.
+		 */
+		if (p->state != TASK_RUNNING)
+			se->vruntime += cfs_rq_of(se)->min_vruntime;
 		return;
+	}
 
 	/*
 	 * We were most likely switched from sched_rt, so
-- 
1.7.9.5

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