学步园

epoll原理简介

通过上面的分析，poll运行效率的两个瓶颈已经找出，现在的问题是怎么改进。首先，每次poll都要把1000个fd 拷入内核，太不科学了，内核干嘛不自己保存已经拷入的fd呢？答对了，epoll就是自己保存拷入的fd，它的API就已经说明了这一点——不是 epoll_wait的时候才传入fd，而是通过epoll_ctl把所有fd传入内核再一起"wait"，这就省掉了不必要的重复拷贝。其次，在
epoll_wait时，也不是把current轮流的加入fd对应的设备等待队列，而是在设备等待队列醒来时调用一个回调函数（当然，这就需要“唤醒回调”机制），把产生事件的fd归入一个链表，然后返回这个链表上的fd。

epoll剖析
epoll是个module，所以先看看module的入口eventpoll_init
[fs/eventpoll.c-->evetpoll_init()]

static int __init eventpoll_init(void)
{
	int error;

	init_MUTEX(&epsem);

	/* Initialize the structure used to perform safe poll wait head wake ups */
	ep_poll_safewake_init(&psw);

	/* Allocates slab cache used to allocate "struct epitem" items */
	epi_cache = kmem_cache_create("eventpoll_epi", sizeof(struct epitem),
	0, SLAB_HWCACHE_ALIGN|EPI_SLAB_DEBUG|SLAB_PANIC,NULL, NULL);

	/* Allocates slab cache used to allocate "struct eppoll_entry" */
	pwq_cache = kmem_cache_create("eventpoll_pwq",
	sizeof(struct eppoll_entry), 0,EPI_SLAB_DEBUG|SLAB_PANIC, NULL, NULL);

	/*
	* Register the virtual file system that will be the source of inodes
	* for the eventpoll files
	*/
	error = register_filesystem(&eventpoll_fs_type);
	if (error)
		goto epanic;

	/* Mount the above commented virtual file system */
	eventpoll_mnt = kern_mount(&eventpoll_fs_type);
	error = PTR_ERR(eventpoll_mnt);
	if (IS_ERR(eventpoll_mnt))
		goto epanic;

	DNPRINTK(3, (KERN_INFO "[%p] eventpoll: successfully initialized.\n",current));
	return 0;

	epanic:
		panic("eventpoll_init() failed\n");
}

很有趣，这个module在初始化时注册了一个新的文件系统，叫"eventpollfs"（在eventpoll_fs_type结构里），然后挂载此文件系统。另外创建两个内核cache（在内核编程中，如果需要频繁分配小块内存，应该创建kmem_cahe来做“内存池”）,分别用于存放struct
epitem和eppoll_entry。如果以后要开发新的文件系统，可以参考这段代码。
现在想想epoll_create为什么会返回一个新的fd？因为它就是在这个叫做"eventpollfs"的文件系统里创建了一个新文件！如下：
[fs/eventpoll.c-->sys_epoll_create()]

asmlinkage long sys_epoll_create(int size)
{
	int error, fd;
	struct inode *inode;
	struct file *file;

	DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d)\n",current, size));

	/* Sanity check on the size parameter */
	error = -EINVAL;
	if (size <= 0)
		goto eexit_1;

	/*
	* Creates all the items needed to setup an eventpoll file. That is,
	* a file structure, and inode and a free file descriptor.
	*/
	error = ep_getfd(&fd, &inode, &file);
	if (error)
		goto eexit_1;

	/* Setup the file internal data structure ( "struct eventpoll" ) */
	error = ep_file_init(file);
	if (error)
		goto eexit_2;

	//...
}

函数很简单，其中ep_getfd看上去是“get”，其实在第一次调用epoll_create时，它是要创建新inode、新的file、新的fd。而ep_file_init则要创建一个struct eventpoll结构，并把它放入file->private_data，注意，这个private_data后面还要用到的。

看到这里，也许有人要问了，为什么epoll的开发者不做一个内核的超级大map把用户要创建的epoll句柄存起来，在epoll_create时返回一个指针？那似乎很直观呀。但是，仔细看看，linux的系统调用有多少是返回指针的？你会发现几乎没有！（特此强调，malloc不是系统调用，malloc调用的brk才是）因为linux做为unix的最杰出的继承人，它遵循了unix的一个巨大优点——一切皆文件，输入输出是文件、socket也是文件，一切皆文件意味着使用这个操作系统的程序可以非常简单，因为一切都是文件操作而已！（unix还不是完全做到，plan
9才算）。而且使用文件系统有个好处：epoll_create返回的是一个fd，而不是该死的指针，指针如果指错了，你简直没办法判断，而fd则可以通过current->files->fd_array[]找到其真伪。

/* 
 * This structure is stored inside the "private_data" member of the file 
 * structure and rapresent the main data sructure for the eventpoll 
 * interface. 
 */  
struct eventpoll {  
    /* Protect the this structure access */  
    rwlock_t lock;  
  
    /* 
     * This semaphore is used to ensure that files are not removed 
     * while epoll is using them. This is read-held during the event 
     * collection loop and it is write-held during the file cleanup 
     * path, the epoll file exit code and the ctl operations. 
     */  
    struct rw_semaphore sem;  
  
    /* 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;  
};  

epoll_create好了，该epoll_ctl了，我们略去判断性的代码：
[fs/eventpoll.c-->sys_epoll_ctl()]

asmlinkage long
sys_epoll_ctl(int epfd, int op, int fd, struct epoll_event __user *event)
{
	int error;
	struct file *file, *tfile;
	struct eventpoll *ep;
	struct epitem *epi;
	struct epoll_event epds;

	//....

	epi = ep_find(ep, tfile, fd);

	error = -EINVAL;
	switch (op) {

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

	//...
}

原来就是在一个大的结构（现在先不管是什么大结构）里先ep_find，如果找到了struct epitem而用户操作是ADD，那么返回-EEXIST；如果是DEL，则ep_remove。如果找不到struct epitem而用户操作是ADD，就ep_insert创建并插入一个。很直白。那这个“大结构”是什么呢？看ep_find的调用方式，ep参数应该是指向这个“大结构”的指针，再看ep
= file->private_data，我们才明白，原来这个“大结构”就是那个在epoll_create时创建的struct eventpoll，具体再看看ep_find的实现，发现原来是struct eventpoll的rbr成员（struct rb_root），原来这是一个红黑树的根！而红黑树上挂的都是struct epitem。现在清楚了，一个新创建的epoll文件带有一个struct eventpoll结构，这个结构上再挂一个红黑树，而这个红黑树就是每次epoll_ctl时fd存放的地方！

现在数据结构都已经清楚了，我们来看最核心的:[fs/eventpoll.c-->sys_epoll_wait()]

asmlinkage long 
sys_epoll_wait(int epfd, struct epoll_event __user *events,int maxevents, int timeout)
{
	int error;
	struct file *file;
	struct eventpoll *ep;

	DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_wait(%d, %p, %d, %d)\n",
	current, epfd, events, maxevents, timeout));

	/* The maximum number of event must be greater than zero */
	if (maxevents <= 0)
		return -EINVAL;

	/* Verify that the area passed by the user is writeable */
	if ((error = verify_area(VERIFY_WRITE, events, maxevents * sizeof(struct epoll_event))))
		goto eexit_1;

	/* Get the "struct file *" for the eventpoll file */
	error = -EBADF;
	file = fget(epfd);
	if (!file)
		goto eexit_1;

	/*
	* We have to check that the file structure underneath the fd
	* the user passed to us _is_ an eventpoll file.
	*/
	error = -EINVAL;
	if (!IS_FILE_EPOLL(file))
		goto eexit_2;

	/*
	* At this point it is safe to assume that the "private_data" contains
	* our own data structure.
	*/
	ep = file->private_data;

	/* Time to fish for events ... */
	error = ep_poll(ep, events, maxevents, timeout);

	eexit_2:
		fput(file);

	eexit_1:
	DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_wait(%d, %p, %d, %d) = %d\n",
				current, epfd, events, maxevents, timeout, error));

	return error;
}

故伎重演，从file->private_data中拿到struct eventpoll，再调用ep_poll
[fs/eventpoll.c-->sys_epoll_wait()->ep_poll()]

static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
int maxevents, long timeout)
{
	int res, eavail;
	unsigned long flags;
	long jtimeout;
	wait_queue_t wait;

	/*
	* Calculate the timeout by checking for the "infinite" value ( -1 )
	* and the overflow condition. The passed timeout is in milliseconds,
	* that why (t * HZ) / 1000.
	*/
	jtimeout = timeout == -1 || timeout > (MAX_SCHEDULE_TIMEOUT - 1000) / HZ ?
	MAX_SCHEDULE_TIMEOUT: (timeout * HZ + 999) / 1000;

	retry:
	write_lock_irqsave(&ep->lock, flags);

	res = 0;
	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(&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) || !jtimeout)
			break;
			if (signal_pending(current)) {
			res = -EINTR;
			break;
			}

			write_unlock_irqrestore(&ep->lock, flags);
			jtimeout = schedule_timeout(jtimeout);
			write_lock_irqsave(&ep->lock, flags);
		}
		remove_wait_queue(&ep->wq, &wait);

		set_current_state(TASK_RUNNING);
	}
	
	//....

}

又是一个大循环，不过这个大循环比poll的那个好，因为仔细一看——它居然除了睡觉和判断ep->rdllist是否为空以外，啥也没做！
什么也没做当然效率高了，但到底是谁来让ep->rdllist不为空呢？
答案是ep_insert时设下的回调函数：
[fs/eventpoll.c-->sys_epoll_ctl()-->ep_insert()]

923 static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
924 struct file *tfile, int fd)
925 {
926 int error, revents, pwake = 0;
927 unsigned long flags;
928 struct epitem *epi;
929 struct ep_pqueue epq;
930
931 error = -ENOMEM;
932 if (!(epi = EPI_MEM_ALLOC()))
933 goto eexit_1;
934
935 /* Item initialization follow here ... */
936 EP_RB_INITNODE(&epi->rbn);
937 INIT_LIST_HEAD(&epi->rdllink);
938 INIT_LIST_HEAD(&epi->fllink);
939 INIT_LIST_HEAD(&epi->txlink);
940 INIT_LIST_HEAD(&epi->pwqlist);
941 epi->ep = ep;
942 EP_SET_FFD(&epi->ffd, tfile, fd);
943 epi->event = *event;
944 atomic_set(&epi->usecnt, 1);
945 epi->nwait = 0;
946
947 /* Initialize the poll table using the queue callback */
948 epq.epi = epi;
949 init_poll_funcptr(&epq.pt, ep_ptable_queue_proc);
950
951 /*
952 * Attach the item to the poll hooks and get current event bits.
953 * We can safely use the file* here because its usage count has
954 * been increased by the caller of this function.
955 */
956 revents = tfile->f_op->poll(tfile, &epq.pt);

我们注意949行，其实就是
&(epq.pt)->qproc = ep_ptable_queue_proc;
紧接着 tfile->f_op->poll(tfile, &epq.pt)其实就是调用被监控文件（epoll里叫“target file”)的poll方法，而这个poll其实就是调用poll_wait（还记得poll_wait吗？每个支持poll的设备驱动程序都要调用的），最后就是调用ep_ptable_queue_proc。这是比较难解的一个调用关系，因为不是语言级的直接调用。
ep_insert还把struct epitem放到struct file里的f_ep_links连表里，以方便查找，struct epitem里的fllink就是担负这个使命的。
[fs/eventpoll.c-->ep_ptable_queue_proc()]

static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,poll_table *pt)
{
	struct epitem *epi = EP_ITEM_FROM_EPQUEUE(pt);
	struct eppoll_entry *pwq;

	if (epi->nwait >= 0 && (pwq = PWQ_MEM_ALLOC())) {
		init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
		pwq->whead = whead;
		pwq->base = epi;
		add_wait_queue(whead, &pwq->wait);
		list_add_tail(&pwq->llink, &epi->pwqlist);
		epi->nwait++;
	} 
	else {
		/* We have to signal that an error occurred */
		epi->nwait = -1;
	}
}

上面的代码就是ep_insert中要做的最重要的事：创建struct eppoll_entry，设置其唤醒回调函数为ep_poll_callback，然后加入设备等待队列（注意这里的whead就是上一章所说的每个设备驱动都要带的等待队列）。只有这样，当设备就绪，唤醒等待队列上的等待着时，ep_poll_callback就会被调用。每次调用poll系统调用，操作系统都要把current（当前进程）挂到fd对应的所有设备的等待队列上，可以想象，fd多到上千的时候，这样“挂”法很费事；而每次调用epoll_wait则没有这么罗嗦，epoll只在epoll_ctl时把current挂一遍（这第一遍是免不了的）并给每个fd一个命令“好了就调回调函数”，如果设备有事件了，通过回调函数，会把fd放入rdllist，而每次调用epoll_wait就只是收集rdllist里的fd就可以了
——epoll巧妙的利用回调函数，实现了更高效的事件驱动模型。
现在我们猜也能猜出来ep_poll_callback会干什么了——肯定是把红黑树上的收到event的epitem（代表
每个fd）插入ep->rdllist中，这样，当epoll_wait返回时，rdllist里就都是就绪的fd了！
[fs/eventpoll.c-->ep_poll_callback()]

static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key)
{
	int pwake = 0;
	unsigned long flags;
	struct epitem *epi = EP_ITEM_FROM_WAIT(wait);
	struct eventpoll *ep = epi->ep;

	DNPRINTK(3, (KERN_INFO "[%p] eventpoll: poll_callback(%p) epi=%p ep=%p\n",1214 current, epi->file, epi, ep));

	write_lock_irqsave(&ep->lock, flags);

	/*
	* If the event mask does not contain any poll(2) event, we consider the
	* descriptor to be disabled. This condition is likely the effect of the
	* EPOLLONESHOT bit that disables the descriptor when an event is received,
	* until the next EPOLL_CTL_MOD will be issued.
	*/
	if (!(epi->event.events & ~EP_PRIVATE_BITS))
		goto is_disabled;

	/* If this file is already in the ready list we exit soon */
	if (EP_IS_LINKED(&epi->rdllink))
		goto is_linked;

	list_add_tail(&epi->rdllink, &ep->rdllist);

	is_linked:
	/*
	* Wake up ( if active ) both the eventpoll wait list and the ->poll()
	* wait list.
	*/
	if (waitqueue_active(&ep->wq))
		wake_up(&ep->wq);
	if (waitqueue_active(&ep->poll_wait))
		pwake++;

	is_disabled:
		write_unlock_irqrestore(&ep->lock, flags);

	/* We have to call this outside the lock */
	if (pwake)
		ep_poll_safewake(&psw, &ep->poll_wait);

	return 1;
}

真正重要的只有1231行的只一句，就是把struct epitem放到struct eventpoll的rdllist中去。现在我们可以画出epoll的核心数据结构图了：

epoll独有的EPOLLET

EPOLLET是epoll系统调用独有的flag，ET就是Edge Trigger（边缘触发）的意思，具体含义和应用大家可google之。有了EPOLLET，重复的事件就不会总是出来打扰程序的判断，故而常被使用。那EPOLLET的原理是什么呢？
上篇我们讲到epoll把fd都挂上一个回调函数，当fd对应的设备有消息时，就把fd放入rdllist链表，这样epoll_wait只要检查这个rdllist链表就可以知道哪些fd有事件了。我们看看ep_poll的最后几行代码：
[fs/eventpoll.c->ep_poll()]

1524
1525 /*
1526 * Try to transfer events to user space. In case we get 0 events and
1527 * there's still timeout left over, we go trying again in search of
1528 * more luck.
1529 */
1530 if (!res && eavail &&
1531 !(res = ep_events_transfer(ep, events, maxevents)) && jtimeout)
1532 goto retry;
1533
1534 return res;
1535 }
把rdllist里的fd拷到用户空间，这个任务是ep_events_transfer做的：
[fs/eventpoll.c->ep_events_transfer()]
1439 static int ep_events_transfer(struct eventpoll *ep,
1440 struct epoll_event __user *events, int maxevents)
1441 {
1442 int eventcnt = 0;
1443 struct list_head txlist;
1444
1445 INIT_LIST_HEAD(&txlist);
1446
1447 /*
1448 * We need to lock this because we could be hit by
1449 * eventpoll_release_file() and epoll_ctl(EPOLL_CTL_DEL).
1450 */
1451 down_read(&ep->sem);
1452
1453 /* Collect/extract ready items */
1454 if (ep_collect_ready_items(ep, &txlist, maxevents) > 0) {
1455 /* Build result set in userspace */
1456 eventcnt = ep_send_events(ep, &txlist, events);
1457
1458 /* Reinject ready items into the ready list */
1459 ep_reinject_items(ep, &txlist);
1460 }
1461
1462 up_read(&ep->sem);
1463
1464 return eventcnt;
1465 }

代码很少，其中ep_collect_ready_items把rdllist里的fd挪到txlist里（挪完后rdllist就空了），接着ep_send_events把txlist里的fd拷给用户空间，然后ep_reinject_items把一部分fd从txlist里“返还”给rdllist以便下次还能从rdllist里发现它。其中ep_send_events的实现：
[fs/eventpoll.c->ep_send_events()]

1337 static int ep_send_events(struct eventpoll *ep, struct list_head *txlist,
1338 struct epoll_event __user *events)
1339 {
1340 int eventcnt = 0;
1341 unsigned int revents;
1342 struct list_head *lnk;
1343 struct epitem *epi;
1344
1345 /*
1346 * We can loop without lock because this is a task private list.
1347 * The test done during the collection loop will guarantee us that
1348 * another task will not try to collect this file. Also, items
1349 * cannot vanish during the loop because we are holding "sem".
1350 */
1351 list_for_each(lnk, txlist) {
1352 epi = list_entry(lnk, struct epitem, txlink);
1353
1354 /*
1355 * Get the ready file event set. We can safely use the file
1356 * because we are holding the "sem" in read and this will
1357 * guarantee that both the file and the item will not vanish.
1358 */
1359 revents = epi->ffd.file->f_op->poll(epi->ffd.file, NULL);
1360
1361 /*
1362 * Set the return event set for the current file descriptor.
1363 * Note that only the task task was successfully able to link
1364 * the item to its "txlist" will write this field.
1365 */
1366 epi->revents = revents & epi->event.events;
1367
1368 if (epi->revents) {
1369 if (__put_user(epi->revents,
1370 &events[eventcnt].events) ||
1371 __put_user(epi->event.data,
1372 &events[eventcnt].data))
1373 return -EFAULT;
1374 if (epi->event.events & EPOLLONESHOT)
1375 epi->event.events &= EP_PRIVATE_BITS;
1376 eventcnt++;
1377 }
1378 }
1379 return eventcnt;
1380 }

这个拷贝实现其实没什么可看的，但是请注意1359行，这个poll很狡猾，它把第二个参数置为NULL来调
用。我们先看一下设备驱动通常是怎么实现poll的：

static unsigned int scull_p_poll(struct file *filp, poll_table *wait)
{
struct scull_pipe *dev = filp->private_data;
unsigned int mask = 0;
/*
* The buffer is circular; it is considered full
* if "wp" is right behind "rp" and empty if the
* two are equal.
*/
down(&dev->sem);
poll_wait(filp, &dev->inq, wait);
poll_wait(filp, &dev->outq, wait);
if (dev->rp != dev->wp)
mask |= POLLIN | POLLRDNORM; /* readable */
if (spacefree(dev))
mask |= POLLOUT | POLLWRNORM; /* writable */
up(&dev->sem);
return mask;
}

上面这段代码摘自《linux设备驱动程序（第三版）》，绝对经典，设备先要把current（当前进程）挂在
inq和outq两个队列上（这个“挂”操作是wait回调函数指针做的），然后等设备来唤醒，唤醒后就能通过
mask拿到事件掩码了（注意那个mask参数，它就是负责拿事件掩码的）。那如果wait为NULL，
poll_wait会做些什么呢？
[include/linux/poll.h->poll_wait]

25 static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address,
poll_table *p)
26 {
27 if (p && wait_address)
28 p->qproc(filp, wait_address, p);
29 }

喏，看见了，如果poll_table为空，什么也不做。我们倒回ep_send_events，那句标红的poll，实际上
就是“我不想休眠，我只想拿到事件掩码”的意思。然后再把拿到的事件掩码拷给用户空间。
ep_send_events完成后，就轮到ep_reinject_items了：
[fs/eventpoll.c->ep_reinject_items]

1389 static void ep_reinject_items(struct eventpoll *ep, struct list_head *txlist)
1390 {
1391 int ricnt = 0, pwake = 0;
1392 unsigned long flags;
1393 struct epitem *epi;
1394
1395 write_lock_irqsave(&ep->lock, flags);
1396
1397 while (!list_empty(txlist)) {
1398 epi = list_entry(txlist->next, struct epitem, txlink);
1399
1400 /* Unlink the current item from the transfer list */
1401 EP_LIST_DEL(&epi->txlink);
1402
1403 /*
1404 * If the item is no more linked to the interest set, we don't
1405 * have to push it inside the ready list because the following
1406 * ep_release_epitem() is going to drop it. Also, if the current
1407 * item is set to have an Edge Triggered behaviour, we don't have
1408 * to push it back either.
1409 */
1410 if (EP_RB_LINKED(&epi->rbn) && !(epi->event.events & EPOLLET) &&
1411 (epi->revents & epi->event.events) && !EP_IS_LINKED(&epi->rdllink)) {
1412 list_add_tail(&epi->rdllink, &ep->rdllist);
1413 ricnt++;
1414 }
1415 }
1416
1417 if (ricnt) {
1418 /*
1419 * Wake up ( if active ) both the eventpoll wait list and the ->poll()
1420 * wait list.
1421 */
1422 if (waitqueue_active(&ep->wq))
1423 wake_up(&ep->wq);
1424 if (waitqueue_active(&ep->poll_wait))
1425 pwake++;
1426 }
1427
1428 write_unlock_irqrestore(&ep->lock, flags);
1429
1430 /* We have to call this outside the lock */
1431 if (pwake)
1432 ep_poll_safewake(&psw, &ep->poll_wait);
1433 }

ep_reinject_items把txlist里的一部分fd又放回rdllist，那么，是把哪一部分fd放回去呢？看上面1410行的那个判断——是哪些“没有标上EPOLLET”（标红代码）且“事件被关注”（标蓝代码）的fd被重新放回了rdllist。那么下次epoll_wait当然会又把rdllist里的fd拿来拷给用户了。

举个例子。假设一个socket，只是connect，还没有收发数据，那么它的poll事件掩码总是有POLLOUT的（参见上面的驱动示例），每次调用epoll_wait总是返回POLLOUT事件（比较烦），因为它的fd就总是被放回rdllist；假如此时有人往这个socket里写了一大堆数据，造成socket塞住（不可写了），那么1411行里标蓝色的判断就不成立了（没有POLLOUT了），fd不会放回rdllist，epoll_wait将不会再返回用户POLLOUT事件。现在我们给这个socket加上EPOLLET，然后connect，没有收发数据，此时，1410行标红的判断又不成立了，所以epoll_wait只会返回一次POLLOUT通知给用户（因为此fd不会再回到rdllist了），接下来的epoll_wait都不会有任何事件通知了。

函数详解：

1. int epoll_create(int size);
2. int epoll_ctl(int epfd, int op, int fd, struct epoll_event
   *event);
3. int epoll_wait(int epfd, struct epoll_event
   *events,int maxevents, int timeout);

/*
 * It opens an eventpoll file descriptor by suggesting a storage of "size"
 * file descriptors. The size parameter is just an hint about how to size
 * data structures. It won't prevent the user to store more than "size"
 * file descriptors inside the epoll interface. It is the kernel part of
 * the userspace epoll_create(2).
 */
asmlinkage long sys_epoll_create(int size)
{
    int error, fd;
    struct inode *inode;
    struct file *file;

    DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d)n",
         current, size));

    /* Sanity check on the size parameter */
    error = -EINVAL;
    if (size <= 0)
        goto eexit_1;

    /*
     * Creates all the items needed to setup an eventpoll file. That is,
     * a file structure, and inode and a free file descriptor.
     */
    error = ep_getfd(&fd, &inode, &file);                  //(1)
    if (error)
        goto eexit_1;

    /* Setup the file internal data structure ( "struct eventpoll" ) */
    error = ep_file_init(file);                            //(2)
    if (error)
        goto eexit_2;


    DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d) = %dn",
         current, size, fd));

    return fd;

eexit_2:
    sys_close(fd);
eexit_1:
    DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d) = %dn",
         current, size, error));
    return error;
}

(1)这里用到了一个ep_getfd函数，从注释我们知道，这个函数建立eventpoll相关的file，当然，一个file要包括文件描述符、inode、还有文件对象，这也是我们传的三个参数。废话不说，看源码：

/*
 * Creates the file descriptor to be used by the epoll interface.
 */
static int ep_getfd(int *efd, struct inode **einode, struct file **efile)
{
    struct qstr this;
    char name[32];
    struct dentry *dentry;
    struct inode *inode;
    struct file *file;
    int error, fd;

    /* Get an ready to use file */
    error = -ENFILE;
    file = get_empty_filp();
    if (!file)
        goto eexit_1;

    /* Allocates an inode from the eventpoll file system */
    inode = ep_eventpoll_inode();
    error = PTR_ERR(inode);
    if (IS_ERR(inode))
        goto eexit_2;

    /* Allocates a free descriptor to plug the file onto */
    error = get_unused_fd();
    if (error < 0)
        goto eexit_3;
    fd = error;

    /*
     * Link the inode to a directory entry by creating a unique name
     * using the inode number.
     */
    error = -ENOMEM;
    sprintf(name, "[%lu]", inode->i_ino);
    this.name = name;
    this.len = strlen(name);
    this.hash = inode->i_ino;
    dentry = d_alloc(eventpoll_mnt->mnt_sb->s_root, &this);
    if (!dentry)
        goto eexit_4;
    dentry->d_op = &eventpollfs_dentry_operations;
    d_add(dentry, inode);
    file->f_vfsmnt = mntget(eventpoll_mnt);
    file->f_dentry = dentry;
    file->f_mapping = inode->i_mapping;

    file->f_pos = 0;
    file->f_flags = O_RDONLY;
    file->f_op = &eventpoll_fops;
    file->f_mode = FMODE_READ;
    file->f_version = 0;
    file->private_data = NULL;

    /* Install the new setup file into the allocated fd. */
    fd_install(fd, file);

    *efd = fd;
    *einode = inode;
    *efile = file;
    return 0;

eexit_4:
    put_unused_fd(fd);
eexit_3:
    iput(inode);
eexit_2:
    put_filp(file);
eexit_1:
    return error;
}

这个函数的注释都比较全，这里简单提一下，况且因为涉及到的函数太多，要深究起来涉及的知识太多，也不可能逐一去列代码。不过这个函数个人觉得比较经典，这函数就是创建一个文件的流程。

首先，我们得拿到一个file结构体，通过内核分配给我们；然后我们要拿到inode，调用这个ep_eventpoll_inode()就可以了；接着是get_unused_fd()拿到文件描述符；接着d_alloc()函数为我们拿到一个dentry；d_add(dentry, inode)函数把dentry建立hash里面并且绑定inode；后面是继续填充文件对象file；fd_install(fd, file)向进程注册文件，并通过这样的方式把文件描述符和文件对象关联起来。

(2)在跟踪ep_file_init函数之前，我们先来看一下eventpoll结构体：

/*
 * This structure is stored inside the "private_data" member of the file
 * structure and rapresent the main data sructure for the eventpoll
 * interface.
 */
struct eventpoll {
    /* Protect the this structure access */
    rwlock_t lock;

    /*
     * This semaphore is used to ensure that files are not removed
     * while epoll is using them. This is read-held during the event
     * collection loop and it is write-held during the file cleanup
     * path, the epoll file exit code and the ctl operations.
     */
    struct rw_semaphore sem;

    /* 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;
};

注释也是相当清楚。这个eventpoll可以看得出来，是epoll的核心，它将会存储你想要监听的文件描述符，这也是为什么epoll高效之所在。

好，我们回到sys_epoll_create函数，开始跟踪ep_file_init函数：

其实也就是eventpoll结构体的初始化。

补充技术知识：

作者：董昊 （要转载的同学帮忙把名字和博客链接http://donghao.org/uii/带上，多谢了！)
epoll
通过上面的分析，poll运行效率的两个瓶颈已经找出，现在的问题是怎么改进。首先，每次poll都要把
1000个fd 拷入内核，太不科学了，内核干嘛不自己保存已经拷入的fd呢？答对了，epoll就是自己保存拷
入的fd，它的API就已经说明了这一点——不是 epoll_wait的时候才传入fd，而是通过epoll_ctl把所有fd
传入内核再一起"wait"，这就省掉了不必要的重复拷贝。其次，在 epoll_wait时，也不是把current轮流
的加入fd对应的设备等待队列，而是在设备等待队列醒来时调用一个回调函数（当然，这就需要“唤醒回
调”机制），把产生事件的fd归入一个链表，然后返回这个链表上的fd。
epoll
epoll是个module，所以先看看module的入口eventpoll_init
[fs/eventpoll.c-->evetpoll_init()]

1582 static int __init eventpoll_init(void)
1583 {
1584 int error;
1585
1586 init_MUTEX(&epsem);
1587
1588 /* Initialize the structure used to perform safe poll wait head wake ups */
1589 ep_poll_safewake_init(&psw);
1590
1591 /* Allocates slab cache used to allocate "struct epitem" items */
1592 epi_cache = kmem_cache_create("eventpoll_epi", sizeof(struct epitem),
1593 0, SLAB_HWCACHE_ALIGN|EPI_SLAB_DEBUG|SLAB_PANIC,
1594 NULL, NULL);
1595
1596 /* Allocates slab cache used to allocate "struct eppoll_entry" */
1597 pwq_cache = kmem_cache_create("eventpoll_pwq",
1598 sizeof(struct eppoll_entry), 0,
1599 EPI_SLAB_DEBUG|SLAB_PANIC, NULL, NULL);
1600
1601 /*
1602 * Register the virtual file system that will be the source of inodes
1603 * for the eventpoll files
1604 */
1605 error = register_filesystem(&eventpoll_fs_type);
1606 if (error)
1607 goto epanic;
1608
1609 /* Mount the above commented virtual file system */
1610 eventpoll_mnt = kern_mount(&eventpoll_fs_type);
1611 error = PTR_ERR(eventpoll_mnt);
1612 if (IS_ERR(eventpoll_mnt))
1613 goto epanic;
1614
1615 DNPRINTK(3, (KERN_INFO "[%p] eventpoll: successfully initialized.\n",
1616 current));
1617 return 0;
16181619 epanic:
1620 panic("eventpoll_init() failed\n");
1621 }

很有趣，这个module在初始化时注册了一个新的文件系统，叫"eventpollfs"（在eventpoll_fs_type结
构里），然后挂载此文件系统。另外创建两个内核cache（在内核编程中，如果需要频繁分配小块内存，
应该创建kmem_cahe来做“内存池”）,分别用于存放struct epitem和eppoll_entry。如果以后要开发新
的文件系统，可以参考这段代码。
现在想想epoll_create为什么会返回一个新的fd？因为它就是在这个叫做"eventpollfs"的文件系统里创建
了一个新文件！如下：
[fs/eventpoll.c-->sys_epoll_create()]

476 asmlinkage long sys_epoll_create(int size)
477 {
478 int error, fd;
479 struct inode *inode;
480 struct file *file;
481
482 DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d)\n",
483 current, size));
484
485 /* Sanity check on the size parameter */
486 error = -EINVAL;
487 if (size <= 0)
488 goto eexit_1;
489
490 /*
491 * Creates all the items needed to setup an eventpoll file. That is,
492 * a file structure, and inode and a free file descriptor.
493 */
494 error = ep_getfd(&fd, &inode, &file);
495 if (error)
496 goto eexit_1;
497
498 /* Setup the file internal data structure ( "struct eventpoll" ) */
499 error = ep_file_init(file);
500 if (error)
501 goto eexit_2;

函数很简单，其中ep_getfd看上去是“get”，其实在第一次调用epoll_create时，它是要创建新inode、
新的file、新的fd。而ep_file_init则要创建一个struct eventpoll结构，并把它放入file-
>private_data，注意，这个private_data后面还要用到的。
看到这里，也许有人要问了，为什么epoll的开发者不做一个内核的超级大map把用户要创建的epoll句柄
存起来，在epoll_create时返回一个指针？那似乎很直观呀。但是，仔细看看，linux的系统调用有多少是
返回指针的？你会发现几乎没有！（特此强调，malloc不是系统调用，malloc调用的brk才是）因为linux
做为unix的最杰出的继承人，它遵循了unix的一个巨大优点——一切皆文件，输入输出是文件、socket也
是文件，一切皆文件意味着使用这个操作系统的程序可以非常简单，因为一切都是文件操作而已！（unix
还不是完全做到，plan 9才算）。而且使用文件系统有个好处：epoll_create返回的是一个fd，而不是该
死的指针，指针如果指错了，你简直没办法判断，而fd则可以通过current->files->fd_array[]找到其真
伪。
epoll_create好了，该epoll_ctl了，我们略去判断性的代码：
[fs/eventpoll.c-->sys_epoll_ctl()]

524 asmlinkage long
525 sys_epoll_ctl(int epfd, int op, int fd, struct epoll_event __user *event)
526 {527 int error;
528 struct file *file, *tfile;
529 struct eventpoll *ep;
530 struct epitem *epi;
531 struct epoll_event epds;
....
575 epi = ep_find(ep, tfile, fd);
576
577 error = -EINVAL;
578 switch (op) {
579 case EPOLL_CTL_ADD:
580 if (!epi) {
581 epds.events |= POLLERR | POLLHUP;
582
583 error = ep_insert(ep, &epds, tfile, fd);
584 } else
585 error = -EEXIST;
586 break;
587 case EPOLL_CTL_DEL:
588 if (epi)
589 error = ep_remove(ep, epi);
590 else
591 error = -ENOENT;
592 break;
593 case EPOLL_CTL_MOD:
594 if (epi) {
595 epds.events |= POLLERR | POLLHUP;
596 error = ep_modify(ep, epi, &epds);
597 } else
598 error = -ENOENT;
599 break;
600 }

原来就是在一个大的结构（现在先不管是什么大结构）里先ep_find，如果找到了struct epitem而用户操
作是ADD，那么返回-EEXIST；如果是DEL，则ep_remove。如果找不到struct epitem而用户操作是
ADD，就ep_insert创建并插入一个。很直白。那这个“大结构”是什么呢？看ep_find的调用方式，ep参数
应该是指向这个“大结构”的指针，再看ep = file->private_data，我们才明白，原来这个“大结构”就是那
个在epoll_create时创建的struct eventpoll，具体再看看ep_find的实现，发现原来是struct eventpoll
的rbr成员（struct rb_root），原来这是一个红黑树的根！而红黑树上挂的都是struct epitem。
现在清楚了，一个新创建的epoll文件带有一个struct eventpoll结构，这个结构上再挂一个红黑树，而这
个红黑树就是每次epoll_ctl时fd存放的地方！
现在数据结构都已经清楚了，我们来看最核心的:
[fs/eventpoll.c-->sys_epoll_wait()]

627 asmlinkage long sys_epoll_wait(int epfd, struct epoll_event __user *events,
628 int maxevents, int timeout)
629 {
630 int error;
631 struct file *file;
632 struct eventpoll *ep;
633
634 DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_wait(%d, %p, %d, %d)\n",
635 current, epfd, events, maxevents, timeout));
636
637 /* The maximum number of event must be greater than zero */638 if (maxevents <= 0)
639 return -EINVAL;
640
641 /* Verify that the area passed by the user is writeable */
642 if ((error = verify_area(VERIFY_WRITE, events, maxevents * sizeof(struct
epoll_event))))
643 goto eexit_1;
644
645 /* Get the "struct file *" for the eventpoll file */
646 error = -EBADF;
647 file = fget(epfd);
648 if (!file)
649 goto eexit_1;
650
651 /*
652 * We have to check that the file structure underneath the fd
653 * the user passed to us _is_ an eventpoll file.
654 */
655 error = -EINVAL;
656 if (!IS_FILE_EPOLL(file))
657 goto eexit_2;
658
659 /*
660 * At this point it is safe to assume that the "private_data" contains
661 * our own data structure.
662 */
663 ep = file->private_data;
664
665 /* Time to fish for events ... */
666 error = ep_poll(ep, events, maxevents, timeout);
667
668 eexit_2:
669 fput(file);
670 eexit_1:
671 DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_wait(%d, %p, %d, %d) =
%d\n",
672 current, epfd, events, maxevents, timeout, error));
673
674 return error;
675 }

故伎重演，从file->private_data中拿到struct eventpoll，再调用ep_poll
[fs/eventpoll.c-->sys_epoll_wait()->ep_poll()]

1468 static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
1469 int maxevents, long timeout)
1470 {
1471 int res, eavail;
1472 unsigned long flags;
1473 long jtimeout;
1474 wait_queue_t wait;
1475
1476 /*
1477 * Calculate the timeout by checking for the "infinite" value ( -1 )
1478 * and the overflow condition. The passed timeout is in milliseconds,1479 * that why (t * HZ) / 1000.
1480 */
1481 jtimeout = timeout == -1 || timeout > (MAX_SCHEDULE_TIMEOUT - 1000) / HZ ?
1482 MAX_SCHEDULE_TIMEOUT: (timeout * HZ + 999) / 1000;
1483
1484 retry:
1485 write_lock_irqsave(&ep->lock, flags);
1486
1487 res = 0;
1488 if (list_empty(&ep->rdllist)) {
1489 /*
1490 * We don't have any available event to return to the caller.
1491 * We need to sleep here, and we will be wake up by
1492 * ep_poll_callback() when events will become available.
1493 */
1494 init_waitqueue_entry(&wait, current);
1495 add_wait_queue(&ep->wq, &wait);
1496
1497 for (;;) {
1498 /*
1499 * We don't want to sleep if the ep_poll_callback() sends us
1500 * a wakeup in between. That's why we set the task state
1501 * to TASK_INTERRUPTIBLE before doing the checks.
1502 */
1503 set_current_state(TASK_INTERRUPTIBLE);
1504 if (!list_empty(&ep->rdllist) || !jtimeout)
1505 break;
1506 if (signal_pending(current)) {
1507 res = -EINTR;
1508 break;
1509 }
1510
1511 write_unlock_irqrestore(&ep->lock, flags);
1512 jtimeout = schedule_timeout(jtimeout);
1513 write_lock_irqsave(&ep->lock, flags);
1514 }
1515 remove_wait_queue(&ep->wq, &wait);
1516
1517 set_current_state(TASK_RUNNING);
1518 }
....

又是一个大循环，不过这个大循环比poll的那个好，因为仔细一看——它居然除了睡觉和判断ep->rdllist
是否为空以外，啥也没做！
什么也没做当然效率高了，但到底是谁来让ep->rdllist不为空呢？
答案是ep_insert时设下的回调函数：
[fs/eventpoll.c-->sys_epoll_ctl()-->ep_insert()]

923 static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
924 struct file *tfile, int fd)
925 {
926 int error, revents, pwake = 0;
927 unsigned long flags;
928 struct epitem *epi;
929 struct ep_pqueue epq;930
931 error = -ENOMEM;
932 if (!(epi = EPI_MEM_ALLOC()))
933 goto eexit_1;
934
935 /* Item initialization follow here ... */
936 EP_RB_INITNODE(&epi->rbn);
937 INIT_LIST_HEAD(&epi->rdllink);
938 INIT_LIST_HEAD(&epi->fllink);
939 INIT_LIST_HEAD(&epi->txlink);
940 INIT_LIST_HEAD(&epi->pwqlist);
941 epi->ep = ep;
942 EP_SET_FFD(&epi->ffd, tfile, fd);
943 epi->event = *event;
944 atomic_set(&epi->usecnt, 1);
945 epi->nwait = 0;
946
947 /* Initialize the poll table using the queue callback */
948 epq.epi = epi;
949 init_poll_funcptr(&epq.pt, ep_ptable_queue_proc);
950
951 /*
952 * Attach the item to the poll hooks and get current event bits.
953 * We can safely use the file* here because its usage count has
954 * been increased by the caller of this function.
955 */
956 revents = tfile->f_op->poll(tfile, &epq.pt);

我们注意949行，其实就是
&(epq.pt)->qproc = ep_ptable_queue_proc;
紧接着 tfile->f_op->poll(tfile, &epq.pt)其实就是调用被监控文件（epoll里叫“target file”)的poll方
法，而这个poll其实就是调用poll_wait（还记得poll_wait吗？每个支持poll的设备驱动程序都要调用
的），最后就是调用ep_ptable_queue_proc。这是比较难解的一个调用关系，因为不是语言级的直接调
用。
ep_insert还把struct epitem放到struct file里的f_ep_links连表里，以方便查找，struct epitem里的
fllink就是担负这个使命的。
[fs/eventpoll.c-->ep_ptable_queue_proc()]

883 static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
884 poll_table *pt)
885 {
886 struct epitem *epi = EP_ITEM_FROM_EPQUEUE(pt);
887 struct eppoll_entry *pwq;
888
889 if (epi->nwait >= 0 && (pwq = PWQ_MEM_ALLOC())) {
890 init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
891 pwq->whead = whead;
892 pwq->base = epi;
893 add_wait_queue(whead, &pwq->wait);
894 list_add_tail(&pwq->llink, &epi->pwqlist);
895 epi->nwait++;
896 } else {
897 /* We have to signal that an error occurred */
898 epi->nwait = -1;
899 }900 }

上面的代码就是ep_insert中要做的最重要的事：创建struct eppoll_entry，设置其唤醒回调函数为
ep_poll_callback，然后加入设备等待队列（注意这里的whead就是上一章所说的每个设备驱动都要带的
等待队列）。只有这样，当设备就绪，唤醒等待队列上的等待着时，ep_poll_callback就会被调用。每次
调用poll系统调用，操作系统都要把current（当前进程）挂到fd对应的所有设备的等待队列上，可以想
象，fd多到上千的时候，这样“挂”法很费事；而每次调用epoll_wait则没有这么罗嗦，epoll只在epoll_ctl
时把current挂一遍（这第一遍是免不了的）并给每个fd一个命令“好了就调回调函数”，如果设备有事件
了，通过回调函数，会把fd放入rdllist，而每次调用epoll_wait就只是收集rdllist里的fd就可以了
——epoll巧妙的利用回调函数，实现了更高效的事件驱动模型。
现在我们猜也能猜出来ep_poll_callback会干什么了——肯定是把红黑树上的收到event的epitem（代表
每个fd）插入ep->rdllist中，这样，当epoll_wait返回时，rdllist里就都是就绪的fd了！
[fs/eventpoll.c-->ep_poll_callback()]

1206 static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key)
1207 {
1208 int pwake = 0;
1209 unsigned long flags;
1210 struct epitem *epi = EP_ITEM_FROM_WAIT(wait);
1211 struct eventpoll *ep = epi->ep;
1212
1213 DNPRINTK(3, (KERN_INFO "[%p] eventpoll: poll_callback(%p) epi=%p
ep=%p\n",
1214 current, epi->file, epi, ep));
1215
1216 write_lock_irqsave(&ep->lock, flags);
1217
1218 /*
1219 * If the event mask does not contain any poll(2) event, we consider the
1220 * descriptor to be disabled. This condition is likely the effect of the
1221 * EPOLLONESHOT bit that disables the descriptor when an event is received,
1222 * until the next EPOLL_CTL_MOD will be issued.
1223 */
1224 if (!(epi->event.events & ~EP_PRIVATE_BITS))
1225 goto is_disabled;
1226
1227 /* If this file is already in the ready list we exit soon */
1228 if (EP_IS_LINKED(&epi->rdllink))
1229 goto is_linked;
1230
1231 list_add_tail(&epi->rdllink, &ep->rdllist);
1232
1233 is_linked:
1234 /*
1235 * Wake up ( if active ) both the eventpoll wait list and the ->poll()
1236 * wait list.
1237 */
1238 if (waitqueue_active(&ep->wq))
1239 wake_up(&ep->wq);
1240 if (waitqueue_active(&ep->poll_wait))
1241 pwake++;
1242
1243 is_disabled:
1244 write_unlock_irqrestore(&ep->lock, flags);
12451246 /* We have to call this outside the lock */
1247 if (pwake)
1248 ep_poll_safewake(&psw, &ep->poll_wait);
1249
1250 return 1;
1251 }

真正重要的只有1231行的只一句，就是把struct epitem放到struct eventpoll的rdllist中去。现在我们
可以画出epoll的核心数据结构图了：

http://donghao.org/docs/linux_kernel_poll_epoll_2.pdf

本文转自：作者：董昊 博客链接http://donghao.org/uii/

############################################################

epoll的实现原理

1 功能介绍
     epoll与select/poll不同的一点是，它是由一组系统调用组成。
     int epoll_create(int size);
     int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);
     int epoll_wait(int epfd, struct epoll_event *events,int maxevents, int timeout);
     epoll相关系统调用是在Linux 2.5.44开始引入的。该系统调用针对传统的select/poll系统调用的不足，设计上作了很大的改动。select/poll的缺点在于：
     1.每次调用时要重复地从用户态读入参数。
     2.每次调用时要重复地扫描文件描述符。
     3.每次在调用开始时，要把当前进程放入各个文件描述符的等待队列。在调用结束后，又把进程从各个等待队列中删除。
     在实际应用中，select/poll监视的文件描述符可能会非常多，如果每次只是返回一小部分，那么，这种情况下select/poll显得不够高效。epoll的设计思路，是把select/poll单个的操作拆分为1个epoll_create+多个epoll_ctrl+一个wait。此外，
内核针对epoll操作添加了一个文件系统”eventpollfs”，每一个或者多个要监视的文件描述符都有一个对应的eventpollfs文件系统的inode节点，主要信息保存在eventpoll结构体中。而被监视的文件的重要信息则保存在epitem结构体中。所以他们
是一对多的关系。
     由于在执行epoll_create和epoll_ctrl时，已经把用户态的信息保存到内核态了，所以之后即使反复地调用epoll_wait，也不会重复地拷贝参数，扫描文件描述符，反复地把当前进程放入/放出等待队列。这样就避免了以上的三个缺点。
     接下去看看它们的实现：
2 关键结构体：
/* Wrapper struct used by poll queueing */
struct ep_pqueue {
         poll_table pt;
         struct epitem *epi;
};
     这个结构体类似于select/poll中的struct poll_wqueues。由于epoll需要在内核态保存大量信息，所以光光一个回调函数指针已经不能满足要求，所以在这里引入了一个新的结构体struct epitem。
/*
 * Each file descriptor added to the eventpoll interface will
 * have an entry of this type linked to the hash.
 */