[tip: sched/core] sched/fair: Add newidle balance to pick_task_fair()

[tip-bot2 for Peter Zijlstra]

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

Commit-ID:     b3a2dfa8b42e5b97dd144aa59374f4e045725cac
Gitweb:        https://git.kernel.org/tip/b3a2dfa8b42e5b97dd144aa59374f4e045725cac
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Mon, 11 May 2026 13:31:12 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 26 May 2026 13:53:13 +02:00

sched/fair: Add newidle balance to pick_task_fair()

With commit 50653216e4ff ("sched: Add support to pick functions to
take rf") removing the balance callback, the pick_task() callback is
in charge of newidle balancing.

This means pick_task_fair() should do so too. This hasn't been a
problem in practise because pick_next_task_fair() is used. However,
since we'll be removing that one shortly, make sure pick_next_task()
is up to scratch.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Link: https://patch.msgid.link/20260511120627.944705718@xxxxxxxxxxxxx
---
 kernel/sched/fair.c | 38 +++++++++++++++-----------------------
 1 file changed, 15 insertions(+), 23 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8e858ca..5f48af7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9863,16 +9863,18 @@ preempt:
 }
 
 static struct task_struct *pick_task_fair(struct rq *rq, struct rq_flags *rf)
+	__must_hold(__rq_lockp(rq))
 {
 	struct sched_entity *se;
 	struct cfs_rq *cfs_rq;
 	struct task_struct *p;
 	bool throttled;
+	int new_tasks;
 
 again:
 	cfs_rq = &rq->cfs;
 	if (!cfs_rq->nr_queued)
-		return NULL;
+		goto idle;
 
 	throttled = false;
 
@@ -9893,6 +9895,14 @@ again:
 	if (unlikely(throttled))
 		task_throttle_setup_work(p);
 	return p;
+
+idle:
+	new_tasks = sched_balance_newidle(rq, rf);
+	if (new_tasks < 0)
+		return RETRY_TASK;
+	if (new_tasks > 0)
+		goto again;
+	return NULL;
 }
 
 static void __set_next_task_fair(struct rq *rq, struct task_struct *p, bool first);
@@ -9904,12 +9914,12 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
 {
 	struct sched_entity *se;
 	struct task_struct *p;
-	int new_tasks;
 
-again:
 	p = pick_task_fair(rq, rf);
+	if (unlikely(p == RETRY_TASK))
+		return p;
 	if (!p)
-		goto idle;
+		return p;
 	se = &p->se;
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
@@ -9959,29 +9969,11 @@ simple:
 #endif /* CONFIG_FAIR_GROUP_SCHED */
 	put_prev_set_next_task(rq, prev, p);
 	return p;
-
-idle:
-	if (rf) {
-		new_tasks = sched_balance_newidle(rq, rf);
-
-		/*
-		 * Because sched_balance_newidle() releases (and re-acquires)
-		 * rq->lock, it is possible for any higher priority task to
-		 * appear. In that case we must re-start the pick_next_entity()
-		 * loop.
-		 */
-		if (new_tasks < 0)
-			return RETRY_TASK;
-
-		if (new_tasks > 0)
-			goto again;
-	}
-
-	return NULL;
 }
 
 static struct task_struct *
 fair_server_pick_task(struct sched_dl_entity *dl_se, struct rq_flags *rf)
+	__must_hold(__rq_lockp(dl_se->rq))
 {
 	return pick_task_fair(dl_se->rq, rf);
 }