[RFC][PATCH 6/8] sched/fair: Add newidle balance to pick_task_fair()

[Peter Zijlstra <peterz@infradead.org>]

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@infradead.org>
---
 kernel/sched/fair.c |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8892,16 +8892,18 @@ static void wakeup_preempt_fair(struct r
 }
 
 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;
 
@@ -8922,6 +8924,14 @@ static struct task_struct *pick_task_fai
 	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);
@@ -9011,6 +9021,7 @@ pick_next_task_fair(struct rq *rq, struc
 
 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);
 }