sched: Rework pick_next_task() slow-path

Avoid the RETRY_TASK case in the pick_next_task() slow path.

By doing the put_prev_task() early, we get the rt/deadline pull done,
and by testing rq->nr_running we know if we need newidle_balance().

This then gives a stable state to pick a task from.

Since the fast-path is fair only; it means the other classes will
always have pick_next_task(.prev=NULL, .rf=NULL) and we can simplify.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Aaron Lu <aaron.lwe@gmail.com>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: mingo@kernel.org
Cc: Phil Auld <pauld@redhat.com>
Cc: Julien Desfossez <jdesfossez@digitalocean.com>
Cc: Nishanth Aravamudan <naravamudan@digitalocean.com>
Link: https://lkml.kernel.org/r/aa34d24b36547139248f32a30138791ac6c02bd6.1559129225.git.vpillai@digitalocean.com

kernel/sched/fair.c

@@ -6770,7 +6770,7 @@ again:
 		goto idle;
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
-	if (prev->sched_class != &fair_sched_class)
+	if (!prev || prev->sched_class != &fair_sched_class)
 		goto simple;
 
 	/*
@@ -6847,8 +6847,8 @@ again:
 	goto done;
 simple:
 #endif
-
-	put_prev_task(rq, prev);
+	if (prev)
+		put_prev_task(rq, prev);
 
 	do {
 		se = pick_next_entity(cfs_rq, NULL);
@@ -6876,6 +6876,9 @@ done: __maybe_unused;
 	return p;
 
 idle:
+	if (!rf)
+		return NULL;
+
 	new_tasks = newidle_balance(rq, rf);
 
 	/*