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

["tip-bot2 for Peter Zijlstra" <tip-bot2@linutronix.de>]

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

Commit-ID:     2a77e4be12cb58bbf774e7c717c8bb80e128b7a4
Gitweb:        https://git.kernel.org/tip/2a77e4be12cb58bbf774e7c717c8bb80e128b7a4
Author:        Peter Zijlstra <peterz@infradead.org>
AuthorDate:    Fri, 29 Nov 2024 11:15:41 +01:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 09 Dec 2024 11:48:13 +01:00

sched/fair: Untangle NEXT_BUDDY and pick_next_task()

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@infradead.org>
Link: https://lkml.kernel.org/r/20241129101541.GA33464@noisy.programming.kicks-ass.net
---
 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 b505d3d..2c4ebfc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5630,9 +5630,9 @@ pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq)
 	struct sched_entity *se;
 
 	/*
-	 * 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 a3d331d..3c12d9f 100644
--- 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.
  */