From: Peter Zijlstra Date: Tue, 28 Jan 2025 14:39:49 +0000 (+0100) Subject: sched/fair: Adhere to place_entity() constraints X-Git-Tag: v6.16-rc1~197^2~5 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=c70fc32f44431bb30f9025ce753ba8be25acbba3;p=thirdparty%2Fkernel%2Flinux.git 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) Reported-by: Mike Galbraith Signed-off-by: "Peter Zijlstra (Intel)" Signed-off-by: Mike Galbraith Signed-off-by: Peter Zijlstra (Intel) Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/c216eb4ef0e0e0029c600aefc69d56681cee5581.camel@gmx.de --- diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 5e1bd9e8464cd..eb5a2572b4f8b 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