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Subject: [PATCH -tip 11/32] sched: Enqueue task into core queue only after vruntime is updated
Date:   Tue, 17 Nov 2020 18:19:41 -0500
Message-Id: <20201117232003.3580179-12-joel@joelfernandes.org>
In-Reply-To: <20201117232003.3580179-1-joel@joelfernandes.org>
References: <20201117232003.3580179-1-joel@joelfernandes.org>
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A waking task may have its vruntime adjusted. However, the code right
now puts it into the core queue without the adjustment. This means the
core queue may have a task with incorrect vruntime, potentially a very
long one. This may cause a task to get artificially boosted during
picking.

Fix it by enqueuing into the core queue only after the class-specific
enqueue callback has been called. This ensures that for CFS tasks, the
updated vruntime value is used when enqueuing the task into the core
rbtree.

Reviewed-by: Vineeth Pillai <viremana@linux.microsoft.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/sched/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 53af817740c0..6aa76de55ef2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1753,9 +1753,6 @@ static inline void init_uclamp(void) { }
 
 static inline void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
 {
-	if (sched_core_enabled(rq))
-		sched_core_enqueue(rq, p);
-
 	if (!(flags & ENQUEUE_NOCLOCK))
 		update_rq_clock(rq);
 
@@ -1766,6 +1763,9 @@ static inline void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
 
 	uclamp_rq_inc(rq, p);
 	p->sched_class->enqueue_task(rq, p, flags);
+
+	if (sched_core_enabled(rq))
+		sched_core_enqueue(rq, p);
 }
 
 static inline void dequeue_task(struct rq *rq, struct task_struct *p, int flags)
-- 
2.29.2.299.gdc1121823c-goog