Linux Kernel中如何使用高精度timer(hrtimer)

前面已經講過，高精度timer是通過hrtimer來實現的（見 http://www.linuxidc.com/Linux/2012-03/55892.htm ），hrtimer通過可編程定時器來現，在等待時，不占用CPU。

在用戶態，只要我們調用usleep，則線程在kernel態執行時，則使用hrtimer進行不占CPU的等待。

在Kernel中如何使用的呢？

先看看eventpoll.c中的ep_poll函數：

1. static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
2. int maxevents, long timeout)
3. {
4. int res = 0, eavail, timed_out = 0;
5. unsigned long flags;
6. long slack = 0;
7. wait_queue_t wait;
8. ktime_t expires, *to = NULL;
10. if (timeout > 0) {
11. struct timespec end_time = ep_set_mstimeout(timeout);
13. slack = select_estimate_accuracy(&end_time);
14. to = &expires;
15. *to = timespec_to_ktime(end_time);
16. } else if (timeout == 0) {
17. /*
18. * Avoid the unnecessary trip to the wait queue loop, if the
19. * caller specified a non blocking operation.
20. */
21. timed_out = 1;
22. spin_lock_irqsave(&ep->lock, flags);
23. goto check_events;
24. }
26. fetch_events:
27. spin_lock_irqsave(&ep->lock, flags);
29. if (!ep_events_available(ep)) {
30. /*
31. * We don't have any available event to return to the caller.
32. * We need to sleep here, and we will be wake up by
33. * ep_poll_callback() when events will become available.
34. */
35. init_waitqueue_entry(&wait, current);
36. __add_wait_queue_exclusive(&ep->wq, &wait);
38. for (;;) {
39. /*
40. * We don't want to sleep if the ep_poll_callback() sends us
41. * a wakeup in between. That's why we set the task state
42. * to TASK_INTERRUPTIBLE before doing the checks.
43. */
44. set_current_state(TASK_INTERRUPTIBLE);
45. if (ep_events_available(ep) || timed_out)
46. break;
47. if (signal_pending(current)) {
48. res = -EINTR;
49. break;
50. }
52. spin_unlock_irqrestore(&ep->lock, flags);
53. if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
54. timed_out = 1;
56. spin_lock_irqsave(&ep->lock, flags);
57. }
58. __remove_wait_queue(&ep->wq, &wait);
60. set_current_state(TASK_RUNNING);
61. }
62. check_events:
63. /* Is it worth to try to dig for events ? */
64. eavail = ep_events_available(ep);
66. spin_unlock_irqrestore(&ep->lock, flags);
68. /*
69. * Try to transfer events to user space. In case we get 0 events and
70. * there's still timeout left over, we go trying again in search of
71. * more luck.
72. */
73. if (!res && eavail &&
74. !(res = ep_send_events(ep, events, maxevents)) && !timed_out)
75. goto fetch_events;
77. return res;
78. }