[PATCH net-next] epoll: add EPOLLEXCLUSIVE support

[Hagen Paul Pfeifer <hagen@xxxxxxxx>]

High performance server sometimes create one listening socket (e.g. port
80), create a epoll file descriptor and add the socket. Afterwards
create SC_NPROCESSORS_ONLN threads and wait for events. This often
result in a thundering herd problem because all CPUs are scheduled.

This patch add an additional flag to epoll_ctl(2) called EPOLLEXCLUSIVE.
If a descriptor is added with this flag only one CPU is scheduled in.

Signed-off-by: Hagen Paul Pfeifer <hagen@xxxxxxxx>
Reported-by: Li Yu <raise.sail@xxxxxxxxx>
Cc: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>
Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
---
 fs/eventpoll.c            |    7 +++++--
 include/linux/eventpoll.h |    3 +++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index aabdfc3..bb442b1 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -88,7 +88,7 @@
  */
 
 /* Epoll private bits inside the event mask */
-#define EP_PRIVATE_BITS (EPOLLONESHOT | EPOLLET)
+#define EP_PRIVATE_BITS (EPOLLONESHOT | EPOLLET | EPOLLEXCLUSIVE)
 
 /* Maximum number of nesting allowed inside epoll sets */
 #define EP_MAX_NESTS 4
@@ -913,7 +913,10 @@ static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
 		init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
 		pwq->whead = whead;
 		pwq->base = epi;
-		add_wait_queue(whead, &pwq->wait);
+		if (unlikely(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 {
diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h
index 657ab55..d334389 100644
--- a/include/linux/eventpoll.h
+++ b/include/linux/eventpoll.h
@@ -26,6 +26,9 @@
 #define EPOLL_CTL_DEL 2
 #define EPOLL_CTL_MOD 3
 
+/* Set Exclusive wake up behaviour for the target file descriptor */
+#define EPOLLEXCLUSIVE (1 << 29)
+
 /* Set the One Shot behaviour for the target file descriptor */
 #define EPOLLONESHOT (1 << 30)
 
-- 
1.7.9

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