/* Epoll private bits inside the event mask */
#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET | EPOLLEXCLUSIVE)
主要是看下：惊群源：
1、socket wake_up
2、epoll_wait 中wake_up
目前data ready的时候调用sk_data_ready 唤醒进程，此时唤醒进程选择了 只唤醒一个
/ nr_exclusive是1 static void __wake_up_common(wait_queue_head_t *q, unsigned int mode, int nr_exclusive, int wake_flags, void *key) { wait_queue_t *curr, *next; list_for_each_entry_safe(curr, next, &q->task_list, task_list) { unsigned flags = curr->flags; if (curr->func(curr, mode, wake_flags, key) && (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive) break; } }
(flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
传进来的nr_exclusive是1, 所以flags & WQ_FLAG_EXCLUSIVE为真的时候，执行一次，就会跳出循环。
Epoll_create()在fork子进程之前
所有进程都共享一个 epfd， 所以data ready 唤醒进程的时候即使加上 nr_exclusive = 1 只唤醒一个进程， 那么唤醒那个一个呢？
也就是当连接到来时，我们需要选择一个进程来accept，这个时候，任何一个accept都是可以的。当连接建立以后，后续的读写事件，却与进程有了关联。一个请求与a进程建立连接后，后续的读写也应该由a进程来做。
当读写事件发生时，应该通知哪个进程呢？Epoll并不知道，因此，事件有可能错误通知另一个进程处理
Epoll_create()在fork子进程之后
每个进程的读写事件，只注册在自己进程的epoll中。所以不会出现竞争
但是accept呢？？？
目前有的内核版本说是会出现有的不会！！！
这就需要看内核版本实现了 无非就是唤醒的时候加上一些标志。。。当然是用reuseport 一劳永逸！！
如果不是是用reuseport实现只唤醒一个进程，那么wake_up的时候就是唤醒等待队列的头一个。。那怎么做到负载均衡呢？？？
所以还是reuseport好！！！！
*/ static void __wake_up_common(wait_queue_head_t *q, unsigned int mode, int nr_exclusive, int wake_flags, void *key) { wait_queue_t *curr, *next; list_for_each_entry_safe(curr, next, &q->task_list, task_list) { unsigned flags = curr->flags; if (curr->func(curr, mode, wake_flags, key) && (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive) break; } } void __wake_up(wait_queue_head_t *q, unsigned int mode, int nr_exclusive, void *key) { unsigned long flags; spin_lock_irqsave(&q->lock, flags); __wake_up_common(q, mode, nr_exclusive, 0, key); spin_unlock_irqrestore(&q->lock, flags); } /* * This is the callback that is passed to the wait queue wakeup * machanism. It is called by the stored file descriptors when they * have events to report. */ 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; spin_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 out_unlock; /* * Check the events coming with the callback. At this stage, not * every device reports the events in the "key" parameter of the * callback. We need to be able to handle both cases here, hence the * test for "key" != NULL before the event match test. */ if (key && !((unsigned long) key & epi->event.events)) goto out_unlock; /* * 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; } /* 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); /* * Wake up ( if active ) both the eventpoll wait list and the ->poll() * wait list. */ if (waitqueue_active(&ep->wq)) wake_up_locked(&ep->wq); if (waitqueue_active(&ep->poll_wait)) pwake++; out_unlock: spin_unlock_irqrestore(&ep->lock, flags); /* We have to call this outside the lock */ if (pwake) ep_poll_safewake(&ep->poll_wait); return 1; } 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; int ewake = 0; if ((unsigned long)key & POLLFREE) { ep_pwq_from_wait(wait)->whead = NULL; /* * whead = NULL above can race with ep_remove_wait_queue() * which can do another remove_wait_queue() after us, so we * can't use __remove_wait_queue(). whead->lock is held by * the caller. */ list_del_init(&wait->task_list); } spin_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 out_unlock; /* * Check the events coming with the callback. At this stage, not * every device reports the events in the "key" parameter of the * callback. We need to be able to handle both cases here, hence the * test for "key" != NULL before the event match test. */ if (key && !((unsigned long) key & epi->event.events)) goto out_unlock; /* * If we are transferring 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 happen 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; if (epi->ws) { /* * Activate ep->ws since epi->ws may get * deactivated at any time. */ __pm_stay_awake(ep->ws); } } goto out_unlock; } /* 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); } /* * 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); } if (waitqueue_active(&ep->poll_wait)) pwake++; out_unlock: spin_unlock_irqrestore(&ep->lock, flags); /* We have to call this outside the lock */ if (pwake) ep_poll_safewake(&ep->poll_wait); if (epi->event.events & EPOLLEXCLUSIVE) return ewake; return 1; }
/* * This is the callback that is used to add our wait queue to the * target file wakeup lists. */ 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 = kmem_cache_alloc(pwq_cache, GFP_KERNEL))) { init_waitqueue_func_entry(&pwq->wait, ep_poll_callback); pwq->whead = whead; pwq->base = epi; if (epi->event.events & EPOLLEXCLUSIVE) add_wait_queue_exclusive(whead, &pwq->wait); else 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; } }
根据EPOLLEXCLUSIVE 加入到不同的唤醒 队列add_wait_queue_exclusive add_wait_queue
在wake_up的时候 通过nr-exclu 控制 但是 要想break还需要返回 ep_poll_callback返回 true；
对于epoll_oneshot
在epoll_wait后 send fd 到user时，
if (epi->event.events & EPOLLONESHOT) 不会重复添加进去 导致后续链式唤醒
epi->event.events &= EP_PRIVATE_BITS;
else if (!(epi->event.events & EPOLLET)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
ep_pm_stay_awake(epi);
}
同时在ep_call_back的时候也会 继续检查 EPOLLONESHOT ，房子 epoll_wait返回时，在处理data中，fd又有数据需要相应，此时多线程 中别的线程可以相应。。。。乱序了！！！
/*
* 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 out_unlock;