sched: pull tasks when CPU is about to run SCHED_IDLE tasks

Before a CPU switches from running SCHED_NORMAL task to
SCHED_IDLE task, trying to pull SCHED_NORMAL tasks from other
CPU by doing load_balance first.

Signed-off-by: Chen Xiaoguang <xiaoggchen@tencent.com>
Signed-off-by: Chen He <heddchen@tencent.com>

kernel/sched/fair.c

@@ -7029,6 +7029,11 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
 	struct task_struct *p;
 	int new_tasks;
 
+	if (prev &&
+	    fair_policy(prev->policy) &&
+	    sched_idle_cpu(rq->cpu))
+		goto idle;
+
 again:
 	if (!sched_fair_runnable(rq))
 		goto idle;