Re: [PATCH] sched/fair: Fix flat hierarchy

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Aug 14, 2026 at 11:26:13AM +0200, Vincent Guittot wrote:

> For the enqueue, the 1st requeue_delayed_entity() which is the main
> path for delayed task, requires the update. Then another one before
> reweight_eevdf(cfs_rq, se, weight, false); in the !curr case.
> 
> But I'm not sure we want to spread this in different places.

Fair enough. I'll test the below and then push to sched/urgent.

---
Subject: sched/fair: Fix flat hierarchy
From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Wed, 12 Aug 2026 14:50:39 +0200

From: Vincent Guittot <vincent.guittot@linaro.org>

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.

This is because update_curr() uses ->h_curr, rather than ->curr, and therefore,
while it is invoked on the root cfs_rq, which contains all the eevdf bits, it
does not do the right thing.

Fixes: 85570f10a4c6 ("sched/eevdf: Move to a single runqueue")
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260812125039.1717249-1-vincent.guittot@linaro.org
---
 kernel/sched/fair.c |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7958,6 +7958,15 @@ static unsigned long enqueue_hierarchy(s
 	return weight;
 }
 
+/* Update curr's vruntime before placing entity or updating lag */
+static inline void update_curr_eevdf(struct cfs_rq *cfs_rq)
+{
+	if (!cfs_rq->curr)
+		return;
+
+	update_curr(cfs_rq_of(cfs_rq->curr));
+}
+
 /*
  * The enqueue_task method is called before nr_running is
  * increased. Here we update the fair scheduling stats and
@@ -7985,6 +7994,8 @@ enqueue_task_fair(struct rq *rq, struct
 	if (!p->se.sched_delayed || (flags & ENQUEUE_DELAYED))
 		util_est_enqueue(cfs_rq, p);
 
+	update_curr_eevdf(cfs_rq);
+
 	if (flags & ENQUEUE_DELAYED) {
 		requeue_delayed_entity(cfs_rq, se);
 		return;
@@ -8105,7 +8116,7 @@ static bool __dequeue_task(struct rq *rq
 
 	clear_buddies(cfs_rq, se);
 
-	update_curr(cfs_rq_of(se));
+	update_curr_eevdf(cfs_rq);
 	update_entity_lag(cfs_rq, se);
 
 	if (flags & DEQUEUE_DELAYED) {