The following commit has been merged into the sched/urgent branch of tip: Commit-ID: 55294004b122c997591d9de8446f5a4c60402805 Gitweb: https://git.kernel.org/tip/55294004b122c997591d9de8446f5a4c60402805 Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx> AuthorDate: Tue, 28 Jan 2025 15:39:49 +01:00 Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx> CommitterDate: Sat, 08 Feb 2025 15:43:12 +01:00 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") Reported-by: Mike Galbraith <efault@xxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx Link: https://lkml.kernel.org/r/20250128143949.GD7145@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx --- 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 f4e4d3e..d0b10e1 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -3782,6 +3782,7 @@ 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); @@ -3808,10 +3809,11 @@ 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
