6.12-stable review patch. If anyone has any objections, please let me know.
------------------
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
[ Upstream commit 2a77e4be12cb58bbf774e7c717c8bb80e128b7a4 ]
There are 3 sites using set_next_buddy() and only one is conditional
on NEXT_BUDDY, the other two sites are unconditional; to note:
- yield_to_task()
- cgroup dequeue / pick optimization
However, having NEXT_BUDDY control both the wakeup-preemption and the
picking side of things means its near useless.
Fixes: 147f3efaa241 ("sched/fair: Implement an EEVDF-like scheduling policy")
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20241129101541.GA33464@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
kernel/sched/fair.c | 4 ++--
kernel/sched/features.h | 9 +++++++++
2 files changed, 11 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 60be5f8bbe711..a21110b220b7b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5647,9 +5647,9 @@ static struct sched_entity *
pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq)
{
/*
- * Enabling NEXT_BUDDY will affect latency but not fairness.
+ * Picking the ->next buddy will affect latency but not fairness.
*/
- if (sched_feat(NEXT_BUDDY) &&
+ if (sched_feat(PICK_BUDDY) &&
cfs_rq->next && entity_eligible(cfs_rq, cfs_rq->next)) {
/* ->next will never be delayed */
SCHED_WARN_ON(cfs_rq->next->sched_delayed);
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 290874079f60d..050d7503064e3 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -31,6 +31,15 @@ SCHED_FEAT(PREEMPT_SHORT, true)
*/
SCHED_FEAT(NEXT_BUDDY, false)
+/*
+ * Allow completely ignoring cfs_rq->next; which can be set from various
+ * places:
+ * - NEXT_BUDDY (wakeup preemption)
+ * - yield_to_task()
+ * - cgroup dequeue / pick
+ */
+SCHED_FEAT(PICK_BUDDY, true)
+
/*
* Consider buddies to be cache hot, decreases the likeliness of a
* cache buddy being migrated away, increases cache locality.
--
2.39.5
