From: Vincent Guittot Date: Tue, 31 Mar 2026 16:23:52 +0000 (+0200) Subject: sched/fair: Prevent negative lag increase during delayed dequeue X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=059258b0d424510202b6f2796279dbdbf0c6a83d;p=thirdparty%2Fkernel%2Flinux.git sched/fair: Prevent negative lag increase during delayed dequeue Delayed dequeue feature aims to reduce the negative lag of a dequeued task while sleeping but it can happens that newly enqueued tasks will move backward the avg vruntime and increase its negative lag. When the delayed dequeued task wakes up, it has more neg lag compared to being dequeued immediately or to other tasks that have been dequeued just before theses new enqueues. Ensure that the negative lag of a delayed dequeued task doesn't increase during its delayed dequeued phase while waiting for its neg lag to diseappear. Similarly, we remove any positive lag that the delayed dequeued task could have gain during thsi period. Short slice tasks are particularly impacted in overloaded system. Test on snapdragon rb5: hackbench -T -p -l 16000000 -g 2 1> /dev/null & cyclictest -t 1 -i 2777 -D 333 --policy=fair --mlock -h 20000 -q The scheduling latency of cyclictest is: tip/sched/core tip/sched/core +this patch cyclictest slice (ms) (default)2.8 8 8 hackbench slice (ms) (default)2.8 20 20 Total Samples | 115632 119733 119806 Average (us) | 364 64(-82%) 61(- 5%) Median (P50) (us) | 60 56(- 7%) 56( 0%) 90th Percentile (us) | 1166 62(-95%) 62( 0%) 99th Percentile (us) | 4192 73(-98%) 72(- 1%) 99.9th Percentile (us) | 8528 2707(-68%) 1300(-52%) Maximum (us) | 17735 14273(-20%) 13525(- 5%) Signed-off-by: Vincent Guittot Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20260331162352.551501-1-vincent.guittot@linaro.org --- diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 41293d5d9b75c..597ce5b718d26 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -840,11 +840,33 @@ static s64 entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se, u64 avrunt return clamp(vlag, -limit, limit); } -static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se) +/* + * Delayed dequeue aims to reduce the negative lag of a dequeued task. While + * updating the lag of an entity, check that negative lag didn't increase + * during the delayed dequeue period which would be unfair. + * Similarly, check that the entity didn't gain positive lag when DELAY_ZERO + * is set. + * + * Return true if the lag has been adjusted. + */ +static __always_inline +bool update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se) { + s64 vlag = entity_lag(cfs_rq, se, avg_vruntime(cfs_rq)); + bool ret; + WARN_ON_ONCE(!se->on_rq); - se->vlag = entity_lag(cfs_rq, se, avg_vruntime(cfs_rq)); + if (se->sched_delayed) { + /* previous vlag < 0 otherwise se would not be delayed */ + vlag = max(vlag, se->vlag); + if (sched_feat(DELAY_ZERO)) + vlag = min(vlag, 0); + } + ret = (vlag == se->vlag); + se->vlag = vlag; + + return ret; } /* @@ -5564,13 +5586,6 @@ static void clear_delayed(struct sched_entity *se) } } -static inline void finish_delayed_dequeue_entity(struct sched_entity *se) -{ - clear_delayed(se); - if (sched_feat(DELAY_ZERO) && se->vlag > 0) - se->vlag = 0; -} - static bool dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) { @@ -5596,6 +5611,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) if (sched_feat(DELAY_DEQUEUE) && delay && !entity_eligible(cfs_rq, se)) { update_load_avg(cfs_rq, se, 0); + update_entity_lag(cfs_rq, se); set_delayed(se); return false; } @@ -5635,7 +5651,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) update_cfs_group(se); if (flags & DEQUEUE_DELAYED) - finish_delayed_dequeue_entity(se); + clear_delayed(se); if (cfs_rq->nr_queued == 0) { update_idle_cfs_rq_clock_pelt(cfs_rq); @@ -7084,18 +7100,14 @@ requeue_delayed_entity(struct sched_entity *se) WARN_ON_ONCE(!se->sched_delayed); WARN_ON_ONCE(!se->on_rq); - if (sched_feat(DELAY_ZERO)) { - update_entity_lag(cfs_rq, se); - if (se->vlag > 0) { - cfs_rq->nr_queued--; - if (se != cfs_rq->curr) - __dequeue_entity(cfs_rq, se); - se->vlag = 0; - place_entity(cfs_rq, se, 0); - if (se != cfs_rq->curr) - __enqueue_entity(cfs_rq, se); - cfs_rq->nr_queued++; - } + if (update_entity_lag(cfs_rq, se)) { + cfs_rq->nr_queued--; + if (se != cfs_rq->curr) + __dequeue_entity(cfs_rq, se); + place_entity(cfs_rq, se, 0); + if (se != cfs_rq->curr) + __enqueue_entity(cfs_rq, se); + cfs_rq->nr_queued++; } update_load_avg(cfs_rq, se, 0);