832 lines
30 KiB
Markdown
832 lines
30 KiB
Markdown
# 31丨性能篇答疑：epoll源码深度剖析
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你好，我是盛延敏，今天是网络编程实战性能篇的答疑模块，欢迎回来。
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在性能篇中，我主要围绕C10K问题进行了深入剖析，最后引出了事件分发机制和多线程。可以说，基于epoll的事件分发能力，是Linux下高性能网络编程的不二之选。如果你觉得还不过瘾，期望有更深刻的认识和理解，那么在性能篇的答疑中，我就带你一起梳理一下epoll的源代码，从中我们一定可以有更多的发现和领悟。
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今天的代码有些多，建议你配合文稿收听音频。
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## 基本数据结构
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在开始研究源代码之前，我们先看一下epoll中使用的数据结构，分别是eventpoll、epitem和eppoll\_entry。
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我们先看一下eventpoll这个数据结构，这个数据结构是我们在调用epoll\_create之后内核侧创建的一个句柄，表示了一个epoll实例。后续如果我们再调用epoll\_ctl和epoll\_wait等，都是对这个eventpoll数据进行操作，这部分数据会被保存在epoll\_create创建的匿名文件file的private\_data字段中。
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```
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/*
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* This structure is stored inside the "private_data" member of the file
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* structure and represents the main data structure for the eventpoll
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* interface.
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*/
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struct eventpoll {
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/* Protect the access to this structure */
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spinlock_t lock;
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/*
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* This mutex is used to ensure that files are not removed
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* while epoll is using them. This is held during the event
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* collection loop, the file cleanup path, the epoll file exit
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* code and the ctl operations.
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*/
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struct mutex mtx;
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/* Wait queue used by sys_epoll_wait() */
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//这个队列里存放的是执行epoll_wait从而等待的进程队列
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wait_queue_head_t wq;
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/* Wait queue used by file->poll() */
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//这个队列里存放的是该eventloop作为poll对象的一个实例，加入到等待的队列
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//这是因为eventpoll本身也是一个file, 所以也会有poll操作
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wait_queue_head_t poll_wait;
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/* List of ready file descriptors */
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//这里存放的是事件就绪的fd列表，链表的每个元素是下面的epitem
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struct list_head rdllist;
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/* RB tree root used to store monitored fd structs */
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//这是用来快速查找fd的红黑树
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struct rb_root_cached rbr;
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/*
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* This is a single linked list that chains all the "struct epitem" that
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* happened while transferring ready events to userspace w/out
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* holding ->lock.
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*/
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struct epitem *ovflist;
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/* wakeup_source used when ep_scan_ready_list is running */
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struct wakeup_source *ws;
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/* The user that created the eventpoll descriptor */
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struct user_struct *user;
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//这是eventloop对应的匿名文件，充分体现了Linux下一切皆文件的思想
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struct file *file;
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/* used to optimize loop detection check */
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int visited;
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struct list_head visited_list_link;
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#ifdef CONFIG_NET_RX_BUSY_POLL
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/* used to track busy poll napi_id */
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unsigned int napi_id;
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#endif
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};
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```
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你能看到在代码中我提到了epitem，这个epitem结构是干什么用的呢？
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每当我们调用epoll\_ctl增加一个fd时，内核就会为我们创建出一个epitem实例，并且把这个实例作为红黑树的一个子节点，增加到eventpoll结构体中的红黑树中，对应的字段是rbr。这之后，查找每一个fd上是否有事件发生都是通过红黑树上的epitem来操作。
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```
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/*
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* Each file descriptor added to the eventpoll interface will
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* have an entry of this type linked to the "rbr" RB tree.
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* Avoid increasing the size of this struct, there can be many thousands
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* of these on a server and we do not want this to take another cache line.
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*/
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struct epitem {
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union {
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/* RB tree node links this structure to the eventpoll RB tree */
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struct rb_node rbn;
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/* Used to free the struct epitem */
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struct rcu_head rcu;
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};
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/* List header used to link this structure to the eventpoll ready list */
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//将这个epitem连接到eventpoll 里面的rdllist的list指针
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struct list_head rdllink;
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/*
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* Works together "struct eventpoll"->ovflist in keeping the
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* single linked chain of items.
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*/
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struct epitem *next;
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/* The file descriptor information this item refers to */
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//epoll监听的fd
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struct epoll_filefd ffd;
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/* Number of active wait queue attached to poll operations */
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//一个文件可以被多个epoll实例所监听，这里就记录了当前文件被监听的次数
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int nwait;
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/* List containing poll wait queues */
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struct list_head pwqlist;
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/* The "container" of this item */
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//当前epollitem所属的eventpoll
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struct eventpoll *ep;
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/* List header used to link this item to the "struct file" items list */
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struct list_head fllink;
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/* wakeup_source used when EPOLLWAKEUP is set */
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struct wakeup_source __rcu *ws;
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/* The structure that describe the interested events and the source fd */
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struct epoll_event event;
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};
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```
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每次当一个fd关联到一个epoll实例，就会有一个eppoll\_entry产生。eppoll\_entry的结构如下：
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```
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/* Wait structure used by the poll hooks */
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struct eppoll_entry {
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/* List header used to link this structure to the "struct epitem" */
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struct list_head llink;
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/* The "base" pointer is set to the container "struct epitem" */
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struct epitem *base;
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/*
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* Wait queue item that will be linked to the target file wait
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* queue head.
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*/
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wait_queue_entry_t wait;
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/* The wait queue head that linked the "wait" wait queue item */
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wait_queue_head_t *whead;
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};
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```
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## epoll\_create
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我们在使用epoll的时候，首先会调用epoll\_create来创建一个epoll实例。这个函数是如何工作的呢?
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首先，epoll\_create会对传入的flags参数做简单的验证。
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```
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/* Check the EPOLL_* constant for consistency. */
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BUILD_BUG_ON(EPOLL_CLOEXEC != O_CLOEXEC);
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if (flags & ~EPOLL_CLOEXEC)
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return -EINVAL;
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/*
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```
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接下来，内核申请分配eventpoll需要的内存空间。
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```
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/* Create the internal data structure ("struct eventpoll").
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*/
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error = ep_alloc(&ep);
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if (error < 0)
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return error;
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```
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在接下来，epoll\_create为epoll实例分配了匿名文件和文件描述字，其中fd是文件描述字，file是一个匿名文件。这里充分体现了UNIX下一切都是文件的思想。注意，eventpoll的实例会保存一份匿名文件的引用，通过调用fd\_install函数将匿名文件和文件描述字完成了绑定。
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这里还有一个特别需要注意的地方，在调用anon\_inode\_get\_file的时候，epoll\_create将eventpoll作为匿名文件file的private\_data保存了起来，这样，在之后通过epoll实例的文件描述字来查找时，就可以快速地定位到eventpoll对象了。
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最后，这个文件描述字作为epoll的文件句柄，被返回给epoll\_create的调用者。
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```
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/*
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* Creates all the items needed to setup an eventpoll file. That is,
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* a file structure and a free file descriptor.
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*/
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fd = get_unused_fd_flags(O_RDWR | (flags & O_CLOEXEC));
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if (fd < 0) {
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error = fd;
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goto out_free_ep;
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}
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file = anon_inode_getfile("[eventpoll]", &eventpoll_fops, ep,
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O_RDWR | (flags & O_CLOEXEC));
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if (IS_ERR(file)) {
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error = PTR_ERR(file);
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goto out_free_fd;
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}
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ep->file = file;
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fd_install(fd, file);
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return fd;
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```
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## epoll\_ctl
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接下来，我们看一下一个套接字是如何被添加到epoll实例中的。这就要解析一下epoll\_ctl函数实现了。
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### 查找epoll实例
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首先，epoll\_ctl函数通过epoll实例句柄来获得对应的匿名文件，这一点很好理解，UNIX下一切都是文件，epoll的实例也是一个匿名文件。
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```
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//获得epoll实例对应的匿名文件
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f = fdget(epfd);
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if (!f.file)
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goto error_return;
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```
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接下来，获得添加的套接字对应的文件，这里tf表示的是target file，即待处理的目标文件。
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```
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/* Get the "struct file *" for the target file */
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//获得真正的文件，如监听套接字、读写套接字
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tf = fdget(fd);
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if (!tf.file)
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goto error_fput;
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```
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再接下来，进行了一系列的数据验证，以保证用户传入的参数是合法的，比如epfd真的是一个epoll实例句柄，而不是一个普通文件描述符。
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```
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/* The target file descriptor must support poll */
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//如果不支持poll，那么该文件描述字是无效的
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error = -EPERM;
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if (!tf.file->f_op->poll)
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goto error_tgt_fput;
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...
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```
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如果获得了一个真正的epoll实例句柄，就可以通过private\_data获取之前创建的eventpoll实例了。
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```
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/*
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* At this point it is safe to assume that the "private_data" contains
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* our own data structure.
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*/
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ep = f.file->private_data;
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```
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### 红黑树查找
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接下来epoll\_ctl通过目标文件和对应描述字，在红黑树中查找是否存在该套接字，这也是epoll为什么高效的地方。红黑树（RB-tree）是一种常见的数据结构，这里eventpoll通过红黑树跟踪了当前监听的所有文件描述字，而这棵树的根就保存在eventpoll数据结构中。
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```
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/* RB tree root used to store monitored fd structs */
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struct rb_root_cached rbr;
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```
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对于每个被监听的文件描述字，都有一个对应的epitem与之对应，epitem作为红黑树中的节点就保存在红黑树中。
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```
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/*
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* Try to lookup the file inside our RB tree, Since we grabbed "mtx"
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* above, we can be sure to be able to use the item looked up by
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* ep_find() till we release the mutex.
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*/
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epi = ep_find(ep, tf.file, fd);
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```
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红黑树是一棵二叉树，作为二叉树上的节点，epitem必须提供比较能力，以便可以按大小顺序构建出一棵有序的二叉树。其排序能力是依靠epoll\_filefd结构体来完成的，epoll\_filefd可以简单理解为需要监听的文件描述字，它对应到二叉树上的节点。
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可以看到这个还是比较好理解的，按照文件的地址大小排序。如果两个相同，就按照文件文件描述字来排序。
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```
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struct epoll_filefd {
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struct file *file; // pointer to the target file struct corresponding to the fd
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int fd; // target file descriptor number
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} __packed;
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/* Compare RB tree keys */
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static inline int ep_cmp_ffd(struct epoll_filefd *p1,
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struct epoll_filefd *p2)
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{
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return (p1->file > p2->file ? +1:
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(p1->file < p2->file ? -1 : p1->fd - p2->fd));
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}
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```
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在进行完红黑树查找之后，如果发现是一个ADD操作，并且在树中没有找到对应的二叉树节点，就会调用ep\_insert进行二叉树节点的增加。
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```
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case EPOLL_CTL_ADD:
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if (!epi) {
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epds.events |= POLLERR | POLLHUP;
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error = ep_insert(ep, &epds, tf.file, fd, full_check);
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} else
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error = -EEXIST;
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if (full_check)
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clear_tfile_check_list();
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break;
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```
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### ep\_insert
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ep\_insert首先判断当前监控的文件值是否超过了/proc/sys/fs/epoll/max\_user\_watches的预设最大值，如果超过了则直接返回错误。
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```
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user_watches = atomic_long_read(&ep->user->epoll_watches);
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if (unlikely(user_watches >= max_user_watches))
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return -ENOSPC;
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```
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接下来是分配资源和初始化动作。
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```
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if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))
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return -ENOMEM;
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/* Item initialization follow here ... */
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INIT_LIST_HEAD(&epi->rdllink);
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INIT_LIST_HEAD(&epi->fllink);
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INIT_LIST_HEAD(&epi->pwqlist);
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epi->ep = ep;
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ep_set_ffd(&epi->ffd, tfile, fd);
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epi->event = *event;
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epi->nwait = 0;
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epi->next = EP_UNACTIVE_PTR;
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```
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再接下来的事情非常重要，ep\_insert会为加入的每个文件描述字设置回调函数。这个回调函数是通过函数ep\_ptable\_queue\_proc来进行设置的。这个回调函数是干什么的呢？其实，对应的文件描述字上如果有事件发生，就会调用这个函数，比如套接字缓冲区有数据了，就会回调这个函数。这个函数就是ep\_poll\_callback。这里你会发现，原来内核设计也是充满了事件回调的原理。
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```
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/*
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* This is the callback that is used to add our wait queue to the
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* target file wakeup lists.
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*/
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static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,poll_table *pt)
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{
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struct epitem *epi = ep_item_from_epqueue(pt);
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struct eppoll_entry *pwq;
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if (epi>nwait >= 0 && (pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL))) {
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init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
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pwq->whead = whead;
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pwq->base = epi;
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if (epi->event.events & EPOLLEXCLUSIVE)
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add_wait_queue_exclusive(whead, &pwq->wait);
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else
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add_wait_queue(whead, &pwq->wait);
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list_add_tail(&pwq->llink, &epi->pwqlist);
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epi->nwait++;
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} else {
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/* We have to signal that an error occurred */
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epi->nwait = -1;
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}
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}
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```
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### ep\_poll\_callback
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ep\_poll\_callback函数的作用非常重要，它将内核事件真正地和epoll对象联系了起来。它又是怎么实现的呢？
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首先，通过这个文件的wait\_queue\_entry\_t对象找到对应的epitem对象，因为eppoll\_entry对象里保存了wait\_queue\_entry\_t，根据wait\_queue\_entry\_t这个对象的地址就可以简单计算出eppoll\_entry对象的地址，从而可以获得epitem对象的地址。这部分工作在ep\_item\_from\_wait函数中完成。一旦获得epitem对象，就可以寻迹找到eventpoll实例。
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```
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/*
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* This is the callback that is passed to the wait queue wakeup
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* mechanism. It is called by the stored file descriptors when they
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* have events to report.
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*/
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static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, void *key)
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{
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int pwake = 0;
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unsigned long flags;
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struct epitem *epi = ep_item_from_wait(wait);
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struct eventpoll *ep = epi->ep;
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```
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接下来，进行一个加锁操作。
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```
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spin_lock_irqsave(&ep->lock, flags);
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```
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下面对发生的事件进行过滤，为什么需要过滤呢？为了性能考虑，ep\_insert向对应监控文件注册的是所有的事件，而实际用户侧订阅的事件未必和内核事件对应。比如，用户向内核订阅了一个套接字的可读事件，在某个时刻套接字的可写事件发生时，并不需要向用户空间传递这个事件。
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```
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/*
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* Check the events coming with the callback. At this stage, not
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* every device reports the events in the "key" parameter of the
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* callback. We need to be able to handle both cases here, hence the
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* test for "key" != NULL before the event match test.
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*/
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if (key && !((unsigned long) key & epi->event.events))
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goto out_unlock;
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```
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接下来，判断是否需要把该事件传递给用户空间。
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```
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if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
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if (epi->next == EP_UNACTIVE_PTR) {
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epi->next = ep->ovflist;
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ep->ovflist = epi;
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if (epi->ws) {
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/*
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* Activate ep->ws since epi->ws may get
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* deactivated at any time.
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*/
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__pm_stay_awake(ep->ws);
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}
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}
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goto out_unlock;
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}
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```
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|
||
如果需要，而且该事件对应的event\_item不在eventpoll对应的已完成队列中，就把它放入该队列，以便将该事件传递给用户空间。
|
||
|
||
```
|
||
/* 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);
|
||
ep_pm_stay_awake_rcu(epi);
|
||
}
|
||
|
||
```
|
||
|
||
我们知道，当我们调用epoll\_wait的时候，调用进程被挂起，在内核看来调用进程陷入休眠。如果该epoll实例上对应描述字有事件发生，这个休眠进程应该被唤醒，以便及时处理事件。下面的代码就是起这个作用的，wake\_up\_locked函数唤醒当前eventpoll上的等待进程。
|
||
|
||
```
|
||
/*
|
||
* Wake up ( if active ) both the eventpoll wait list and the ->poll()
|
||
* wait list.
|
||
*/
|
||
if (waitqueue_active(&ep->wq)) {
|
||
if ((epi->event.events & EPOLLEXCLUSIVE) &&
|
||
!((unsigned long)key & POLLFREE)) {
|
||
switch ((unsigned long)key & EPOLLINOUT_BITS) {
|
||
case POLLIN:
|
||
if (epi->event.events & POLLIN)
|
||
ewake = 1;
|
||
break;
|
||
case POLLOUT:
|
||
if (epi->event.events & POLLOUT)
|
||
ewake = 1;
|
||
break;
|
||
case 0:
|
||
ewake = 1;
|
||
break;
|
||
}
|
||
}
|
||
wake_up_locked(&ep->wq);
|
||
}
|
||
|
||
```
|
||
|
||
### 查找epoll实例
|
||
|
||
epoll\_wait函数首先进行一系列的检查，例如传入的maxevents应该大于0。
|
||
|
||
```
|
||
/* The maximum number of event must be greater than zero */
|
||
if (maxevents <= 0 || maxevents > EP_MAX_EVENTS)
|
||
return -EINVAL;
|
||
|
||
/* Verify that the area passed by the user is writeable */
|
||
if (!access_ok(VERIFY_WRITE, events, maxevents * sizeof(struct epoll_event)))
|
||
return -EFAULT;
|
||
|
||
```
|
||
|
||
和前面介绍的epoll\_ctl一样，通过epoll实例找到对应的匿名文件和描述字，并且进行检查和验证。
|
||
|
||
```
|
||
/* Get the "struct file *" for the eventpoll file */
|
||
f = fdget(epfd);
|
||
if (!f.file)
|
||
return -EBADF;
|
||
|
||
/*
|
||
* 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(f.file))
|
||
goto error_fput;
|
||
|
||
```
|
||
|
||
还是通过读取epoll实例对应匿名文件的private\_data得到eventpoll实例。
|
||
|
||
```
|
||
/*
|
||
* At this point it is safe to assume that the "private_data" contains
|
||
* our own data structure.
|
||
*/
|
||
ep = f.file->private_data;
|
||
|
||
```
|
||
|
||
接下来调用ep\_poll来完成对应的事件收集并传递到用户空间。
|
||
|
||
```
|
||
/* Time to fish for events ... */
|
||
error = ep_poll(ep, events, maxevents, timeout);
|
||
|
||
```
|
||
|
||
### ep\_poll
|
||
|
||
还记得[第23讲](https://time.geekbang.org/column/article/143245)里介绍epoll函数的时候，对应的timeout值可以是大于0，等于0和小于0么？这里ep\_poll就分别对timeout不同值的场景进行了处理。如果大于0则产生了一个超时时间，如果等于0则立即检查是否有事件发生。
|
||
|
||
```
|
||
*/
|
||
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;
|
||
u64 slack = 0;
|
||
wait_queue_entry_t wait;
|
||
ktime_t expires, *to = NULL;
|
||
|
||
if (timeout > 0) {
|
||
struct timespec64 end_time = ep_set_mstimeout(timeout);
|
||
slack = select_estimate_accuracy(&end_time);
|
||
to = &expires;
|
||
*to = timespec64_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;
|
||
}
|
||
|
||
```
|
||
|
||
接下来尝试获得eventpoll上的锁：
|
||
|
||
```
|
||
spin_lock_irqsave(&ep->lock, flags);
|
||
|
||
```
|
||
|
||
获得这把锁之后，检查当前是否有事件发生，如果没有，就把当前进程加入到eventpoll的等待队列wq中，这样做的目的是当事件发生时，ep\_poll\_callback函数可以把该等待进程唤醒。
|
||
|
||
```
|
||
if (!ep_events_available(ep)) {
|
||
/*
|
||
* Busy poll timed out. Drop NAPI ID for now, we can add
|
||
* it back in when we have moved a socket with a valid NAPI
|
||
* ID onto the ready list.
|
||
*/
|
||
ep_reset_busy_poll_napi_id(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);
|
||
|
||
```
|
||
|
||
紧接着是一个无限循环, 这个循环中通过调用schedule\_hrtimeout\_range，将当前进程陷入休眠，CPU时间被调度器调度给其他进程使用，当然，当前进程可能会被唤醒，唤醒的条件包括有以下四种：
|
||
|
||
1. 当前进程超时；
|
||
2. 当前进程收到一个signal信号；
|
||
3. 某个描述字上有事件发生；
|
||
4. 当前进程被CPU重新调度，进入for循环重新判断，如果没有满足前三个条件，就又重新进入休眠。
|
||
|
||
对应的1、2、3都会通过break跳出循环，直接返回。
|
||
|
||
```
|
||
//这个循环里，当前进程可能会被唤醒，唤醒的途径包括
|
||
//1.当前进程超时
|
||
//2.当前进行收到一个signal信号
|
||
//3.某个描述字上有事件发生
|
||
//对应的1.2.3都会通过break跳出循环
|
||
//第4个可能是当前进程被CPU重新调度，进入for循环的判断，如果没有满足1.2.3的条件，就又重新进入休眠
|
||
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);
|
||
/*
|
||
* Always short-circuit for fatal signals to allow
|
||
* threads to make a timely exit without the chance of
|
||
* finding more events available and fetching
|
||
* repeatedly.
|
||
*/
|
||
if (fatal_signal_pending(current)) {
|
||
res = -EINTR;
|
||
break;
|
||
}
|
||
if (ep_events_available(ep) || timed_out)
|
||
break;
|
||
if (signal_pending(current)) {
|
||
res = -EINTR;
|
||
break;
|
||
}
|
||
|
||
spin_unlock_irqrestore(&ep->lock, flags);
|
||
|
||
//通过调用schedule_hrtimeout_range，当前进程进入休眠，CPU时间被调度器调度给其他进程使用
|
||
if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
|
||
timed_out = 1;
|
||
|
||
spin_lock_irqsave(&ep->lock, flags);
|
||
}
|
||
|
||
```
|
||
|
||
如果进程从休眠中返回，则将当前进程从eventpoll的等待队列中删除，并且设置当前进程为TASK\_RUNNING状态。
|
||
|
||
```
|
||
//从休眠中结束，将当前进程从wait队列中删除，设置状态为TASK_RUNNING，接下来进入check_events，来判断是否是有事件发生
|
||
__remove_wait_queue(&ep->wq, &wait);
|
||
__set_current_state(TASK_RUNNING);
|
||
|
||
```
|
||
|
||
最后，调用ep\_send\_events将事件拷贝到用户空间。
|
||
|
||
```
|
||
//ep_send_events将事件拷贝到用户空间
|
||
/*
|
||
* 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;
|
||
|
||
```
|
||
|
||
### ep\_send\_events
|
||
|
||
ep\_send\_events这个函数会将ep\_send\_events\_proc作为回调函数并调用ep\_scan\_ready\_list函数，ep\_scan\_ready\_list函数调用ep\_send\_events\_proc对每个已经就绪的事件循环处理。
|
||
|
||
ep\_send\_events\_proc循环处理就绪事件时，会再次调用每个文件描述符的poll方法，以便确定确实有事件发生。为什么这样做呢？这是为了确定注册的事件在这个时刻还是有效的。
|
||
|
||
可以看到，尽管ep\_send\_events\_proc已经尽可能的考虑周全，使得用户空间获得的事件通知都是真实有效的，但还是有一定的概率，当ep\_send\_events\_proc再次调用文件上的poll函数之后，用户空间获得的事件通知已经不再有效，这可能是用户空间已经处理掉了，或者其他什么情形。还记得[第22讲](https://time.geekbang.org/column/article/141573)吗，在这种情况下，如果套接字不是非阻塞的，整个进程将会被阻塞，这也是为什么将非阻塞套接字配合epoll使用作为最佳实践的原因。
|
||
|
||
在进行简单的事件掩码校验之后，ep\_send\_events\_proc将事件结构体拷贝到用户空间需要的数据结构中。这是通过\_\_put\_user方法完成的。
|
||
|
||
```
|
||
//这里对一个fd再次进行poll操作，以确认事件
|
||
revents = ep_item_poll(epi, &pt);
|
||
|
||
/*
|
||
* If the event mask intersect the caller-requested one,
|
||
* deliver the event to userspace. Again, ep_scan_ready_list()
|
||
* is holding "mtx", so no operations coming from userspace
|
||
* can change the item.
|
||
*/
|
||
if (revents) {
|
||
if (__put_user(revents, &uevent->events) ||
|
||
__put_user(epi->event.data, &uevent->data)) {
|
||
list_add(&epi->rdllink, head);
|
||
ep_pm_stay_awake(epi);
|
||
return eventcnt ? eventcnt : -EFAULT;
|
||
}
|
||
eventcnt++;
|
||
uevent++;
|
||
|
||
```
|
||
|
||
## Level-triggered VS Edge-triggered
|
||
|
||
在[前面的](https://time.geekbang.org/column/article/143245)[文章](https://time.geekbang.org/column/article/143245)里，我们一直都在强调level-triggered和edge-triggered之间的区别。
|
||
|
||
从实现角度来看其实非常简单，在ep\_send\_events\_proc函数的最后，针对level-triggered情况，当前的epoll\_item对象被重新加到eventpoll的就绪列表中，这样在下一次epoll\_wait调用时，这些epoll\_item对象就会被重新处理。
|
||
|
||
在前面我们提到，在最终拷贝到用户空间有效事件列表中之前，会调用对应文件的poll方法，以确定这个事件是不是依然有效。所以，如果用户空间程序已经处理掉该事件，就不会被再次通知；如果没有处理，意味着该事件依然有效，就会被再次通知。
|
||
|
||
```
|
||
//这里是Level-triggered的处理，可以看到，在Level-triggered的情况下，这个事件被重新加回到ready list里面
|
||
//这样，下一轮epoll_wait的时候，这个事件会被重新check
|
||
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, no one 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);
|
||
ep_pm_stay_awake(epi);
|
||
}
|
||
|
||
```
|
||
|
||
## epoll VS poll/select
|
||
|
||
最后，我们从实现角度来说明一下为什么epoll的效率要远远高于poll/select。
|
||
|
||
首先，poll/select先将要监听的fd从用户空间拷贝到内核空间, 然后在内核空间里面进行处理之后，再拷贝给用户空间。这里就涉及到内核空间申请内存，释放内存等等过程，这在大量fd情况下，是非常耗时的。而epoll维护了一个红黑树，通过对这棵黑红树进行操作，可以避免大量的内存申请和释放的操作，而且查找速度非常快。
|
||
|
||
下面的代码就是poll/select在内核空间申请内存的展示。可以看到select 是先尝试申请栈上资源, 如果需要监听的fd比较多, 就会去申请堆空间的资源。
|
||
|
||
```
|
||
int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
|
||
fd_set __user *exp, struct timespec64 *end_time)
|
||
{
|
||
fd_set_bits fds;
|
||
void *bits;
|
||
int ret, max_fds;
|
||
size_t size, alloc_size;
|
||
struct fdtable *fdt;
|
||
/* Allocate small arguments on the stack to save memory and be faster */
|
||
long stack_fds[SELECT_STACK_ALLOC/sizeof(long)];
|
||
|
||
ret = -EINVAL;
|
||
if (n < 0)
|
||
goto out_nofds;
|
||
|
||
/* max_fds can increase, so grab it once to avoid race */
|
||
rcu_read_lock();
|
||
fdt = files_fdtable(current->files);
|
||
max_fds = fdt->max_fds;
|
||
rcu_read_unlock();
|
||
if (n > max_fds)
|
||
n = max_fds;
|
||
|
||
/*
|
||
* We need 6 bitmaps (in/out/ex for both incoming and outgoing),
|
||
* since we used fdset we need to allocate memory in units of
|
||
* long-words.
|
||
*/
|
||
size = FDS_BYTES(n);
|
||
bits = stack_fds;
|
||
if (size > sizeof(stack_fds) / 6) {
|
||
/* Not enough space in on-stack array; must use kmalloc */
|
||
ret = -ENOMEM;
|
||
if (size > (SIZE_MAX / 6))
|
||
goto out_nofds;
|
||
|
||
|
||
alloc_size = 6 * size;
|
||
bits = kvmalloc(alloc_size, GFP_KERNEL);
|
||
if (!bits)
|
||
goto out_nofds;
|
||
}
|
||
fds.in = bits;
|
||
fds.out = bits + size;
|
||
fds.ex = bits + 2*size;
|
||
fds.res_in = bits + 3*size;
|
||
fds.res_out = bits + 4*size;
|
||
fds.res_ex = bits + 5*size;
|
||
...
|
||
|
||
```
|
||
|
||
第二，select/poll从休眠中被唤醒时，如果监听多个fd，只要其中有一个fd有事件发生，内核就会遍历内部的list去检查到底是哪一个事件到达，并没有像epoll一样, 通过fd直接关联eventpoll对象，快速地把fd直接加入到eventpoll的就绪列表中。
|
||
|
||
```
|
||
static int do_select(int n, fd_set_bits *fds, struct timespec64 *end_time)
|
||
{
|
||
...
|
||
retval = 0;
|
||
for (;;) {
|
||
unsigned long *rinp, *routp, *rexp, *inp, *outp, *exp;
|
||
bool can_busy_loop = false;
|
||
|
||
inp = fds->in; outp = fds->out; exp = fds->ex;
|
||
rinp = fds->res_in; routp = fds->res_out; rexp = fds->res_ex;
|
||
|
||
for (i = 0; i < n; ++rinp, ++routp, ++rexp) {
|
||
unsigned long in, out, ex, all_bits, bit = 1, mask, j;
|
||
unsigned long res_in = 0, res_out = 0, res_ex = 0;
|
||
|
||
in = *inp++; out = *outp++; ex = *exp++;
|
||
all_bits = in | out | ex;
|
||
if (all_bits == 0) {
|
||
i += BITS_PER_LONG;
|
||
continue;
|
||
}
|
||
|
||
if (!poll_schedule_timeout(&table, TASK_INTERRUPTIBLE,
|
||
to, slack))
|
||
timed_out = 1;
|
||
...
|
||
|
||
```
|
||
|
||
## 总结
|
||
|
||
在这次答疑中，我希望通过深度分析epoll的源码实现，帮你理解epoll的实现原理。
|
||
|
||
epoll维护了一棵红黑树来跟踪所有待检测的文件描述字，黑红树的使用减少了内核和用户空间大量的数据拷贝和内存分配，大大提高了性能。
|
||
|
||
同时，epoll维护了一个链表来记录就绪事件，内核在每个文件有事件发生时将自己登记到这个就绪事件列表中，通过内核自身的文件file-eventpoll之间的回调和唤醒机制，减少了对内核描述字的遍历，大大加速了事件通知和检测的效率，这也为level-triggered和edge-triggered的实现带来了便利。
|
||
|
||
通过对比poll/select的实现，我们发现epoll确实克服了poll/select的种种弊端，不愧是Linux下高性能网络编程的皇冠。我们应该感谢Linux社区的大神们设计了这么强大的事件分发机制，让我们在Linux下可以享受高性能网络服务器带来的种种技术红利。
|
||
|