参考文档：
https://blog.csdn.net/weixin_35691921/article/details/124961404

Android: alarm定时很短时，比如500ms，然后执行mPowerManager.forceSuspend()后，系统不会suspend，原因分析：

static int alarmtimer_suspend(struct device *dev)
{
    ...
	/* Find the soonest timer to expire*/
	for (i = 0; i < ALARM_NUMTYPE; i++) {


	if (ktime_to_ns(min) < 2 * NSEC_PER_SEC) {
		pm_wakeup_event(dev, 2 * MSEC_PER_SEC);
		return -EBUSY;  // 原因是在这里返回了
	}
	....
}