[tip: sched/core] sched/fair: Implement DELAY_ZERO

tip-bot2 for Peter Zijlstra
2024-08-18T06:23:11Z

The following commit has been merged into the sched/core branch of tip:

Commit-ID: 54a58a78779169f9c92a51facf6de7ce94962328
Gitweb: https://git.kernel.org/tip/54a58a78779169f9c92a51facf6de7ce94962328
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Thu, 23 May 2024 12:26:06 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Sat, 17 Aug 2024 11:06:44 +02:00

sched/fair: Implement DELAY_ZERO

'Extend' DELAY_DEQUEUE by noting that since we wanted to dequeued them
at the 0-lag point, truncate lag (eg. don't let them earn positive
lag).

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Tested-by: Valentin Schneider <vschneid@redhat.com>
Link: https://lkml.kernel.org/r/20240727105030.403750550@infradead.org
---
kernel/sched/fair.c | 20 ++++++++++++++++++--
kernel/sched/features.h | 3 +++
2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index da5065a..1a59339 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5447,8 +5447,11 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) != DEQUEUE_SAVE)
update_min_vruntime(cfs_rq);

- if (flags & DEQUEUE_DELAYED)
+ if (flags & DEQUEUE_DELAYED) {
se->sched_delayed = 0;
+ if (sched_feat(DELAY_ZERO) && se->vlag > 0)
+ se->vlag = 0;
+ }

if (cfs_rq->nr_running == 0)
update_idle_cfs_rq_clock_pelt(cfs_rq);
@@ -5527,7 +5530,6 @@ pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq)
dequeue_entities(rq, se, DEQUEUE_SLEEP | DEQUEUE_DELAYED);
SCHED_WARN_ON(se->sched_delayed);
SCHED_WARN_ON(se->on_rq);
-
return NULL;
}
return se;
@@ -6825,6 +6827,20 @@ requeue_delayed_entity(struct sched_entity *se)
SCHED_WARN_ON(!se->sched_delayed);
SCHED_WARN_ON(!se->on_rq);

+ if (sched_feat(DELAY_ZERO)) {
+ update_entity_lag(cfs_rq, se);
+ if (se->vlag > 0) {
+ cfs_rq->nr_running--;
+ 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_running++;
+ }
+ }
+
se->sched_delayed = 0;
}

diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 1feaa7b..7fdeb55 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -34,8 +34,11 @@ SCHED_FEAT(CACHE_HOT_BUDDY, true)
* By delaying the dequeue for non-eligible tasks, they remain in the
* competition and can burn off their negative lag. When they get selected
* they'll have positive lag by definition.
+ *
+ * DELAY_ZERO clips the lag on dequeue (or wakeup) to 0.
*/
SCHED_FEAT(DELAY_DEQUEUE, true)
+SCHED_FEAT(DELAY_ZERO, true)

/*
* Allow wakeup-time preemption of the current task: