[PATCH] sched/fair: Untangle NEXT_BUDDY and pick_next_task()

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 29, 2024 at 10:55:00AM +0100, Peter Zijlstra wrote:

> Anyway..  I'm sure I started a patch series cleaning up the whole next
> buddy thing months ago (there's more problems here), but I can't seem to
> find it in a hurry :/

There was this..

---
Subject: sched/fair: Untangle NEXT_BUDDY and pick_next_task()
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri Nov 29 10:36:59 CET 2024

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.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/fair.c     |    4 ++--
 kernel/sched/features.h |    9 +++++++++
 2 files changed, 11 insertions(+), 2 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5613,9 +5613,9 @@ 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);
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -32,6 +32,15 @@ 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.
  */
> 
>