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/sched.h

@@ -1700,12 +1700,15 @@ struct sched_class {
 	void (*check_preempt_curr)(struct rq *rq, struct task_struct *p, int flags);
 
 	/*
-	 * It is the responsibility of the pick_next_task() method that will
-	 * return the next task to call put_prev_task() on the @prev task or
-	 * something equivalent.
+	 * Both @prev and @rf are optional and may be NULL, in which case the
+	 * caller must already have invoked put_prev_task(rq, prev, rf).
 	 *
-	 * May return RETRY_TASK when it finds a higher prio class has runnable
-	 * tasks.
+	 * Otherwise it is the responsibility of the pick_next_task() to call
+	 * put_prev_task() on the @prev task or something equivalent, IFF it
+	 * returns a next task.
+	 *
+	 * In that case (@rf != NULL) it may return RETRY_TASK when it finds a
+	 * higher prio class has runnable tasks.
 	 */
 	struct task_struct * (*pick_next_task)(struct rq *rq,
 					       struct task_struct *prev,