schedule_timeout函数

比较理想的延迟执行方法是使用schedule_timeout函数，该方法会让需要延迟的任务睡眠到指定的延时时间耗尽后在重新运行。
具体的源代码如下：

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_timeout() (since it
         * should never happens anyway). You just have the printk()
         * that will tell you if something is gone wrong and where.
         */
        if (timeout < 0) {
            printk(KERN_ERR "schedule_timeout: wrong timeout "
                "value %lx\n", timeout);
            dump_stack();
            current->state = TASK_RUNNING;
            goto out;
        }
    }

    expire = timeout + jiffies;

    setup_timer_on_stack(&timer, process_timeout, (unsigned long)current);
    __mod_timer(&timer, expire, false, TIMER_NOT_PINNED);
    schedule();
    del_singleshot_timer_sync(&timer);

    /* Remove the timer from the object tracker */
    destroy_timer_on_stack(&timer);

    timeout = expire - jiffies;

 out:
    return timeout < 0 ? 0 : timeout;
}

该函数使用原始的名字timer创建一个定时器timer；然后设置他的超时时间timeout；设置超时执行函数process_timeout();然后激活定时器而且调用schedule()。因为任务被标识为TASK_INTERRUPTIBLE或者TASK_UNINTERRUPTIBLE,所以调度程序不会在选择该任务投入运行，而会选择其他新任务运行。