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这个系统调用会sleep 一段时间其源码分析如下：SYSCALL_DEFINE2(nanosleep, struct timespec __user *, rqtp,		struct timespec __user *, rmtp){	struct timespec64 tu;	#得到user space需要设置sleep的时间	if (get_timespec64(&tu, rqtp))		return -EFAULT;	#检查要睡眠的时间是否合法	if (!timespec64_valid(&tu))		return -EINVAL;	#修改当前current的restart_block	current->restart_block.nanosleep.type = rmtp ? TT_NATIVE : TT_NONE;	current->restart_block.nanosleep.rmtp = rmtp;	#调用高精度时钟来睡眠	return hrtimer_nanosleep(&tu, HRTIMER_MODE_REL, CLOCK_MONOTONIC);}long hrtimer_nanosleep(const struct timespec64 *rqtp,		       const enum hrtimer_mode mode, const clockid_t clockid){	struct restart_block *restart;	struct hrtimer_sleeper t;	int ret = 0;	u64 slack;	slack = current->timer_slack_ns;	if (dl_task(current) || rt_task(current))		slack = 0;	#在栈上新建一个高精度的timer	hrtimer_init_on_stack(&t.timer, clockid, mode);	#设置到期时间	hrtimer_set_expires_range_ns(&t.timer, timespec64_to_ktime(*rqtp), slack);	#开始睡眠，在do_nanosleep中会调用shedule 让出当前cpu	ret = do_nanosleep(&t, mode);	if (ret != -ERESTART_RESTARTBLOCK)		goto out;	/* Absolute timers do not update the rmtp value and restart: */	if (mode == HRTIMER_MODE_ABS) {		ret = -ERESTARTNOHAND;		goto out;	}	#sleep时间到期后，修改回当前进程的restart_block	restart = ¤t->restart_block;	restart->fn = hrtimer_nanosleep_restart;	restart->nanosleep.clockid = t.timer.base->clockid;	restart->nanosleep.expires = hrtimer_get_expires_tv64(&t.timer);out:	#销毁栈上的高精度timer	destroy_hrtimer_on_stack(&t.timer);	return ret;}static int __sched do_nanosleep(struct hrtimer_sleeper *t, enum hrtimer_mode mode){	struct restart_block *restart;	hrtimer_init_sleeper(t, current);	do {		set_current_state(TASK_INTERRUPTIBLE);		hrtimer_start_expires(&t->timer, mode);		if (likely(t->task))		#核心code 调用shedule 让出cpu 其中#define freezable_schedule()  schedule()			freezable_schedule();		hrtimer_cancel(&t->timer);		mode = HRTIMER_MODE_ABS;	} while (t->task && !signal_pending(current));	__set_current_state(TASK_RUNNING);	return -ERESTART_RESTARTBLOCK;}

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