前面已经讲过，高精度timer是通过hrtimer来实现的（见 ），hrtimer通过可编程定时器来现，在等待时，不占用CPU。
在用户态，只要我们调用usleep，则线程在kernel态执行时，则使用hrtimer进行不占CPU的等待。
在Kernel中如何使用的呢？
先看看eventpoll.c中的ep_poll函数：
static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, int maxevents, long timeout) { int res = 0, eavail, timed_out = 0; unsigned long flags; long slack = 0; wait_queue_t wait; ktime_t expires, *to = NULL; if (timeout > 0) { struct timespec end_time = ep_set_mstimeout(timeout); slack = select_estimate_accuracy(&end_time); to = &expires; *to = timespec_to_ktime(end_time); } else if (timeout == 0) { /* * Avoid the unnecessary trip to the wait queue loop, if the * caller specified a non blocking operation. */ timed_out = 1; spin_lock_irqsave(&ep->lock, flags); goto check_events; } fetch_events: spin_lock_irqsave(&ep->lock, flags); if (!ep_events_available(ep)) { /* * We don't have any available event to return to the caller. * We need to sleep here, and we will be wake up by * ep_poll_callback() when events will become available. */ init_waitqueue_entry(&wait, current); __add_wait_queue_exclusive(&ep->wq, &wait); for (;;) { /* * We don't want to sleep if the ep_poll_callback() sends us * a wakeup in between. That's why we set the task state * to TASK_INTERRUPTIBLE before doing the checks. */ set_current_state(TASK_INTERRUPTIBLE); if (ep_events_available(ep) || timed_out) break; if (signal_pending(current)) { res = -EINTR; break; } spin_unlock_irqrestore(&ep->lock, flags); if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS)) timed_out = 1; spin_lock_irqsave(&ep->lock, flags); } __remove_wait_queue(&ep->wq, &wait); set_current_state(TASK_RUNNING); } check_events: /* Is it worth to try to dig for events ? */ eavail = ep_events_available(ep); spin_unlock_irqrestore(&ep->lock, flags); /* * Try to transfer events to user space. In case we get 0 events and * there's still timeout left over, we go trying again in search of * more luck. */ if (!res && eavail && !(res = ep_send_events(ep, events, maxevents)) && !timed_out) goto fetch_events; return res; }