[PATCH] sched/fair: Fix flat hierarchy

[Vincent Guittot]

When a fair task is enqueued, we must update curr and more precisely
its vruntime before placing the enqueued task so avg vruntime will take
into account the last exec phase.

Example:
TA is an always running task in cgroup G0.
TB is a short running task (cyclictest) in cgroup G1.
The lag of TB always increases up the clamp limit because TB is placed
before TA(curr) is updated (since the last tick). When curr(TA) is
finally updated, its last exec phase provide positive lag to TB

Because TA and TB don't belong to the same group, enqueue_hierarchy()
will not update TA's entity when updating curr but only G0's entity at
root level.

The same applies when dequeuing.

Fixes: 85570f10a4c6 ("sched/eevdf: Move to a single runqueue")
Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
---

I overlooked the cgroup part when I reviewed the update_entity_lag() in
flat hierarchy.

 kernel/sched/fair.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index dcf860c59a14..649b4f7505a1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7983,6 +7983,9 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	if (!p->se.sched_delayed || (flags & ENQUEUE_DELAYED))
 		util_est_enqueue(cfs_rq, p);
 
+	if (cfs_rq->curr)
+		update_curr(cfs_rq_of(cfs_rq->curr));
+
 	if (flags & ENQUEUE_DELAYED) {
 		requeue_delayed_entity(cfs_rq, se);
 		return;
@@ -8103,7 +8106,8 @@ static bool __dequeue_task(struct rq *rq, struct task_struct *p, int flags)
 
 	clear_buddies(cfs_rq, se);
 
-	update_curr(cfs_rq_of(se));
+	if (cfs_rq->curr)
+		update_curr(cfs_rq_of(cfs_rq->curr));
 	update_entity_lag(cfs_rq, se);
 
 	if (flags & DEQUEUE_DELAYED) {
-- 
2.43.0