sched/fair: Tweak pick_next_entity()

Currently, pick_next_entity(...) has the following structure
(simplified):

  [...]
  if (last_buddy_ok())
    result = last_buddy;
  if (next_buddy_ok())
    result = next_buddy;
  [...]

The intended behavior is to prefer next buddy over last buddy;
the current code somewhat obfuscates this, and also wastes
cycles checking the last buddy when eventually the next buddy is
picked up.

So this patch refactors two 'ifs' above into

  [...]
  if (next_buddy_ok())
      result = next_buddy;
  else if (last_buddy_ok())
      result = last_buddy;
  [...]

Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guitttot@linaro.org>
Link: https://lkml.kernel.org/r/20200930173532.1069092-1-posk@google.com

kernel/sched/fair.c

@@ -4465,17 +4465,17 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 			se = second;
 	}
 
-	/*
-	 * Prefer last buddy, try to return the CPU to a preempted task.
-	 */
-	if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
-		se = cfs_rq->last;
-
-	/*
-	 * Someone really wants this to run. If it's not unfair, run it.
-	 */
-	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
+	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
+		/*
+		 * Someone really wants this to run. If it's not unfair, run it.
+		 */
 		se = cfs_rq->next;
+	} else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
+		/*
+		 * Prefer last buddy, try to return the CPU to a preempted task.
+		 */
+		se = cfs_rq->last;
+	}
 
 	clear_buddies(cfs_rq, se);