sched/fair: Simplify the code of should_we_balance()

We only consider group_balance_cpu() after there is no idle
cpu. So, just do comparison before return at these two cases.
Signed-off-by: Peng Wang <rocking@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/245c792f0e580b3ca342ad61257f4c066ee0f84f.1586594833.git.rocking@linux.alibaba.com

kernel/sched/fair.c

@@ -9413,7 +9413,7 @@ static int active_load_balance_cpu_stop(void *data);
 static int should_we_balance(struct lb_env *env)
 {
 	struct sched_group *sg = env->sd->groups;
-	int cpu, balance_cpu = -1;
+	int cpu;
 
 	/*
 	 * Ensure the balancing environment is consistent; can happen
@@ -9434,18 +9434,12 @@ static int should_we_balance(struct lb_env *env)
 		if (!idle_cpu(cpu))
 			continue;
 
-		balance_cpu = cpu;
-		break;
+		/* Are we the first idle CPU? */
+		return cpu == env->dst_cpu;
 	}
 
-	if (balance_cpu == -1)
-		balance_cpu = group_balance_cpu(sg);
-
-	/*
-	 * First idle CPU or the first CPU(busiest) in this sched group
-	 * is eligible for doing load balancing at this and above domains.
-	 */
-	return balance_cpu == env->dst_cpu;
+	/* Are we the first CPU of this group ? */
+	return group_balance_cpu(sg) == env->dst_cpu;
 }
 
 /*