sched/core: Make pick_next_task_idle() more consistent

Only pick_next_task_fair() needs the @prev and @rf argument; these are
required to implement the cpu-cgroup optimization. None of the other
pick_next_task() methods need this. Make pick_next_task_idle() more
consistent.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bsegall@google.com
Cc: dietmar.eggemann@arm.com
Cc: juri.lelli@redhat.com
Cc: ktkhai@virtuozzo.com
Cc: mgorman@suse.de
Cc: qais.yousef@arm.com
Cc: qperret@google.com
Cc: rostedt@goodmis.org
Cc: valentin.schneider@arm.com
Cc: vincent.guittot@linaro.org
Link: https://lkml.kernel.org/r/20191108131909.545730862@infradead.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -3922,8 +3922,10 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 			goto restart;
 
 		/* Assumes fair_sched_class->next == idle_sched_class */
-		if (unlikely(!p))
-			p = idle_sched_class.pick_next_task(rq, prev, rf);
+		if (unlikely(!p)) {
+			put_prev_task(rq, prev);
+			p = idle_sched_class.pick_next_task(rq, NULL, NULL);
+		}
 
 		return p;
 	}

kernel/sched/idle.c

@@ -396,8 +396,7 @@ pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
 {
 	struct task_struct *next = rq->idle;
 
-	if (prev)
-		put_prev_task(rq, prev);
+	WARN_ON_ONCE(prev || rf);
 
 	set_next_task_idle(rq, next);