Linux进程调度时机

我们之前有分析过，update_process_times会判断当前进程的时间片是否用完，如果用完了会设置TIF_NEED_RESCHED，在合适的时机，调用schedule函数做实际的进程调度。
进程调度的时机，主要分成以下四种：

1. 直接调用schedule
2. 间接调用schedule，如果msleep函数
3. 中断返回时检测TIF_NEED_RESCHED，如果设置则调用schedule
4. 系统调用返回用户空间的时候检测TIF_NEED_RESCHED，如果设置该标记，则调用schedule。
   直接调用schedule没什么可说的，下面主要分析后面四种调度时机的代码。

间接调用schedule

---

void msleep(unsigned int msecs)
{
   unsigned long timeout = msecs_to_jiffies(msecs) + 1;

   while (timeout)
   	timeout = schedule_timeout_uninterruptible(timeout);
}

首先计算超时时间，并调用 schedule_timeout_uninterruptible函数，该函数返回剩余的超时时间。

signed long __sched schedule_timeout_uninterruptible(signed long timeout)
{
	__set_current_state(TASK_UNINTERRUPTIBLE);
	return schedule_timeout(timeout);
}

首先设置当前进程的状态为TASK_UNINTERRUPTIBLE，然后调用 schedule_timeout。

signed long __sched schedule_timeout(signed long timeout)
{
	struct timer_list timer;
	unsigned long expire;

	switch (timeout)
	{
	case MAX_SCHEDULE_TIMEOUT:
		/*
		 * These two special cases are useful to be comfortable
		 * in the caller. Nothing more. We could take
		 * MAX_SCHEDULE_TIMEOUT from one of the negative value
		 * but I' d like to return a valid offset (>=0) to allow
		 * the caller to do everything it want with the retval.
		 */
		schedule();
		goto out;
	default:
		/*
		 * Another bit of PARANOID. Note that the retval will be
		 * 0 since no piece of kernel is supposed to do a check
		 * for a negative retval of schedule