昨晚分析了poll，通过代码的阅读可以发现，poll操作有很多可以优化的地方。epoll是eventpoll的简称，他的效率是非常高的，我们今天来看看他的实现。他的实现在FS/Eventpoll.c，代码有1500多行，呵呵，怕了吧。
大家都知道，epoll有三个系统调用，C库封装成以下三个：
- 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的源码这么多，我们就干脆跟着他们三个走着瞧。今天先搞定第一个---epoll_create
第一个是
- /*
- * 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) = %d\n",
- current, size, fd));
- return fd;
- eexit_2:
- sys_close(fd);
- eexit_1:
- DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d) = %d\n",
- 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：
- static int ep_file_init(struct file *file)
- {
- struct eventpoll *ep;
- if (!(ep = kmalloc(sizeof(struct eventpoll), GFP_KERNEL)))
- return -ENOMEM;
- memset(ep, 0, sizeof(*ep));
- rwlock_init(&ep->lock);
- init_rwsem(&ep->sem);
- init_waitqueue_head(&ep->wq);
- init_waitqueue_head(&ep->poll_wait);
- INIT_LIST_HEAD(&ep->rdllist);
- ep->rbr = RB_ROOT;
- file->private_data = ep;
- DNPRINTK(3, (KERN_INFO "[%p] eventpoll: ep_file_init() ep=%p\n",
- current, ep));
- return 0;
- }
其实也就是eventpoll结构体的初始化。
sys_epoll_create函数大概就这样了，明天接着看sys_epoll_ctl。
