【内核调度、hotplug进程迁移】【migrate_tasks】

在cpu_down()时，需要调用migration_call() -> migrate_tasks()把cpu上所有runnable进程迁移到其他cpu；在cpu_up()时，并不需要在函数中迁移进程，直接等待负载均衡算法的迁移。

migrate_tasks

需要注意的是被迁移的task的dst cpu很有可能都是不一样的。

/*
 * Migrate all tasks from the rq, sleeping tasks will be migrated by
 * try_to_wake_up()->select_task_rq().
 *
 * Called with rq->lock held even though we'er in stop_machine() and
 * there's no concurrency possible, we hold the required locks anyway
 * because of lock validation efforts.
 */
static void migrate_tasks(struct rq *dead_rq, struct rq_flags *rf)
{
	struct rq *rq = dead_rq;
	struct task_struct *next, *stop = rq->stop;
	struct rq_flags orf = *rf;
	int dest_cpu;

	/*
	 * Fudge the rq selection such that the below task selection loop
	 * doesn't get stuck on the currently eligible stop task.
	 *
	 * We're currently inside stop_machine() and the rq is either stuck
	 * in the stop_machine_cpu_stop() loop, or we&