linux kernel中epoll的设计和实现

这里就不贴源码了，源码分析的话，网上一大堆，我这里只是简要的描述下epoll的实现和一些关键的代码片段。

相关的文件在 fs/eventpoll.c中,我看的是2.6.38的内核代码.

1 epoll在创建的时候会调用anon_inode_getfd新建一个file instance，也就是epoll可以看成一个文件。因此我们可以看到epoll_create会返回一个fd.

	error = anon_inode_getfd("[eventpoll]", &eventpoll_fops, ep,
				 O_RDWR | (flags & O_CLOEXEC));

2 epoll所管理的所有的句柄都是放在一个大的结构eventpoll(红黑树)中,而这个结构是保存在file 的private_data域中的(因为epoll本身就是一个文件).这样每次通过epoll fd就可以直接得到eventpoll.

	file = fget(epfd);
	/* Get the "struct file *" for the target file */
	tfile = fget(fd);
...................................
	ep = file->private_data;

3 每一个加入到epoll监听的句柄(也就是红黑树的一个节点)都是一个epitem.它包含了一个 struct eventpoll *ep，也就是它所属于的eventpoll(epoll实例).

	if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))
		return -ENOMEM;

	/* Item initialization follow here ... */
	INIT_LIST_HEAD(&epi->rdllink);
	INIT_LIST_HEAD(&epi->fllink);
	INIT_LIST_HEAD(&epi->pwqlist);
	epi->ep = ep;
	ep_set_ffd(&epi->ffd, tfile, fd);
	epi->event = *event;
	epi->nwait = 0;
	epi->next = EP_UNACTIVE_PTR;

4 在eventpoll中包含两个wait queue，一个是被epoll_wait使用的(wq)，一个是被file->poll使用的(poll_wait).这两个都是属于eventpoll.而在epitem也有一个wait queue(pwqlist)，这个queue是fd私有的wait queue，所以它是保存在epitem中的。

struct epitem {
	/* RB tree node used to link this structure to the eventpoll RB tree */
	struct rb_node rbn;
................................................
	/* List containing poll wait queues */
	struct list_head pwqlist;

	/* The "container" of this item */
	struct eventpoll *ep;
.............................................
};

struct eventpoll {
	/* Protect the this structure access */
	spinlock_t lock;
	/*
	 * This mutex is used to ensure that files are not removed
	 * while epoll is using them. This is held during the event
	 * collection loop, the file cleanup path, the epoll file exit
	 * code and the ctl operations.
	 */
	struct mutex mtx;

	/* Wait queue used by sys_epoll_wait() */
	wait_queue_head_t wq;

	/* Wait queue used by file->poll() */
	wait_queue_head_t poll_wait;

	/* List of ready file descriptors */
	struct list_head rdllist;

	/* RB tree root used to store monitored fd structs */
	struct rb_root rbr;

	/*
	 * This is a single linked list that chains all the "struct epitem" that
	 * happened while transfering ready events to userspace w/out
	 * holding ->lock.
	 */
	struct epitem *ovflist;
.................................................
};

5 当我们添加一个句柄到一个epoll fd的时候，默认是会包含POLLERR和POLLHUP事件.并将epitem插入到红黑树中(eventpoll).然后会初始化一个poll_table,然后设置它的回调函数为ep_poll_callback，紧接着调用file->poll,如果是socket，则会调用tcp_poll,这个函数将会调用ep_poll_callback.

	switch (op) {
	case EPOLL_CTL_ADD:
		if (!epi) {
			epds.events |= POLLERR | POLLHUP;
			error = ep_insert(ep, &epds, tfile, fd);
		} else
			error = -EEXIST;
		break;
.............................................

6 ep_poll_callback这个函数主要是绑定epitem的wait queue的回调为ep_poll_callback.也就是对应fd如果有事件，则就会调用ep_poll_callback。

7 epoll中保存了一个read list(rdllist)，所有的已经有通知事件的句柄，都会放到这个list中，而对应的操作就是在ep_poll_callback中，在ep_poll_callback中主要就是由wait queue指针来取得对应的epitem，然后再取得eventpoll，并将这个epitem加入到ready list，唤醒epoll_wait(wq).这里可以看到由于一个句柄只会对应一个epitem，所以在rdllist中，也不会有重复的epitem，在ep_poll_callback会判断是否rdllist中是否已经包含了将要插入的epitem，如果包含，则直接返回.

static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key)
{
	struct epitem *epi = ep_item_from_wait(wait);
	struct eventpoll *ep = epi->ep;
...............................................
	/* If this file is already in the ready list we exit soon */
	if (!ep_is_linked(&epi->rdllink))
		list_add_tail(&epi->rdllink, &ep->rdllist);

8 当系统调用epoll_wait,如果ready list(rdllist)为空，则休眠等待被唤醒。当被唤醒之后(第7条),将rdllist复制(指针)到一个新的list(主要是针对LT),然后调用ep_send_events_proc对这个新的list进行遍历(对应的会从rdllist中删除这个epitem).遍历完毕后，最终会返回给用户对应的数据.

	if (list_empty(&ep->rdllist)) {
		/*
		 * 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 (!list_empty(&ep->rdllist) || 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);
	}

9 LT和ET的区别是在ep_send_events_proc中处理的，如果是LT，不但会将对应的数据返回给用户，并且会将当前的epitem再次加入到rdllist中。这样子，如果下次再次被唤醒就会给用户空间再次返回事件.

static int ep_send_events_proc(struct eventpoll *ep, struct list_head *head,
			       void *priv)
{
....................................................
			if (__put_user(revents, &uevent->events) ||
			    __put_user(epi->event.data, &uevent->data)) {
				list_add(&epi->rdllink, head);
				return eventcnt ? eventcnt : -EFAULT;
			}
			eventcnt++;
			uevent++;
			if (epi->event.events & EPOLLONESHOT)
				epi->event.events &= EP_PRIVATE_BITS;
			else if (!(epi->event.events & EPOLLET)) {
				/*
				 * If this file has been added with Level
				 * Trigger mode, we need to insert back inside
				 * the ready list, so that the next call to
				 * epoll_wait() will check again the events
				 * availability. At this point, noone can insert
				 * into ep->rdllist besides us. The epoll_ctl()
				 * callers are locked out by
				 * ep_scan_ready_list() holding "mtx" and the
				 * poll callback will queue them in ep->ovflist.
				 */
				list_add_tail(&epi->rdllink, &ep->rdllist);
			}
..............................
}

10 eventpol还有一个list，叫做ovflist，主要是解决当内核在传输数据给用户空间(ep_send_events_proc)时的锁(eventpoll->mtx)，此时epoll就是将这个时候传递上来的事件保存到ovflist中。

static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key)
{
......................................
	/*
	 * If we are trasfering events to userspace, we can hold no locks
	 * (because we're accessing user memory, and because of linux f_op->poll()
	 * semantics). All the events that happens during that period of time are
	 * chained in ep->ovflist and requeued later on.
	 */
	if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
		if (epi->next == EP_UNACTIVE_PTR) {
			epi->next = ep->ovflist;
			ep->ovflist = epi;
		}
		goto out_unlock;
	}
.........................
}

11 当从ep_send_events_proc返回(ep_scan_ready_list)后，会遍历ovflist，然后将ready的epitem保存到rdllist,以便与下次再次被唤醒时进行操作.

static int ep_scan_ready_list(struct eventpoll *ep,
			      int (*sproc)(struct eventpoll *,
					   struct list_head *, void *),
			      void *priv)
{
...........................................
	/*
	 * During the time we spent inside the "sproc" callback, some
	 * other events might have been queued by the poll callback.
	 * We re-insert them inside the main ready-list here.
	 */
	for (nepi = ep->ovflist; (epi = nepi) != NULL;
	     nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {
		/*
		 * We need to check if the item is already in the list.
		 * During the "sproc" callback execution time, items are
		 * queued into ->ovflist but the "txlist" might already
		 * contain them, and the list_splice() below takes care of them.
		 */
		if (!ep_is_linked(&epi->rdllink))
			list_add_tail(&epi->rdllink, &ep->rdllist);
	}
........................
}

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