The following commit has been merged into the sched/core branch of tip:
Commit-ID: c70fc32f44431bb30f9025ce753ba8be25acbba3 Gitweb: https://git.kernel.org/tip/c70fc32f44431bb30f9025ce753ba8be25acbba3 Author: Peter Zijlstra peterz@infradead.org AuthorDate: Tue, 28 Jan 2025 15:39:49 +01:00 Committer: Peter Zijlstra peterz@infradead.org CommitterDate: Wed, 16 Apr 2025 21:09:12 +02:00
sched/fair: Adhere to place_entity() constraints
Mike reports that commit 6d71a9c61604 ("sched/fair: Fix EEVDF entity placement bug causing scheduling lag") relies on commit 4423af84b297 ("sched/fair: optimize the PLACE_LAG when se->vlag is zero") to not trip a WARN in place_entity().
What happens is that the lag of the very last entity is 0 per definition -- the average of one element matches the value of that element. Therefore place_entity() will match the condition skipping the lag adjustment:
if (sched_feat(PLACE_LAG) && cfs_rq->nr_queued && se->vlag) {
Without the 'se->vlag' condition -- it will attempt to adjust the zero lag even though we're inserting into an empty tree.
Notably, we should have failed the 'cfs_rq->nr_queued' condition, but don't because they didn't get updated.
Additionally, move update_load_add() after placement() as is consistent with other place_entity() users -- this change is non-functional, place_entity() does not use cfs_rq->load.
Fixes: 6d71a9c61604 ("sched/fair: Fix EEVDF entity placement bug causing scheduling lag") Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reported-by: Mike Galbraith efault@gmx.de Signed-off-by: "Peter Zijlstra (Intel)" peterz@infradead.org Signed-off-by: Mike Galbraith efault@gmx.de Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/c216eb4ef0e0e0029c600aefc69d56681cee5581.camel@gmx... --- kernel/sched/fair.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 5e1bd9e..eb5a257 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -3795,6 +3795,7 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, update_entity_lag(cfs_rq, se); se->deadline -= se->vruntime; se->rel_deadline = 1; + cfs_rq->nr_queued--; if (!curr) __dequeue_entity(cfs_rq, se); update_load_sub(&cfs_rq->load, se->load.weight); @@ -3821,10 +3822,11 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
enqueue_load_avg(cfs_rq, se); if (se->on_rq) { - update_load_add(&cfs_rq->load, se->load.weight); place_entity(cfs_rq, se, 0); + update_load_add(&cfs_rq->load, se->load.weight); if (!curr) __enqueue_entity(cfs_rq, se); + cfs_rq->nr_queued++;
/* * The entity's vruntime has been adjusted, so let's check