Avoid calling read(2) on eventfd on each event-loop wakeup

0002-Avoid-calling-read-2-on-eventfd-on-each-event-loop-w.patch

@@ -0,0 +1,93 @@
+From b137f071988d9b01c05cb693a56d4359f19cdb2c Mon Sep 17 00:00:00 2001
+From: Andy Pan <i@andypan.me>
+Date: Wed, 17 Apr 2024 10:36:47 +0000
+Subject: [PATCH] Avoid calling read(2) on eventfd on each event-loop wakeup
+
+Register the eventfd with EPOLLET to enable edge-triggered notification
+where we don't need to read the data from the eventfd for every wakeup
+event.
+
+When the eventfd counter reaches the maximum value of the unsigned 64-bit,
+we rewind the counter and retry again. This optimization saves one system
+call on each event-loop wakeup, which eliminates the extra latency for epoll
+as the EVFILT_USER filter does for the kqueue.
+---
+ event.c | 38 ++++++++++++++++++++++++--------------
+ 1 file changed, 24 insertions(+), 14 deletions(-)
+
+diff --git a/event.c b/event.c
+index 7a42b73..e37e588 100644
+--- a/event.c
++++ b/event.c
+@@ -211,7 +211,7 @@ int event_debug_mode_on_ = 0;
+  *        to be shared across threads (if thread support is enabled).
+  *
+  *        When and if evthreads are initialized, this variable will be evaluated,
+- *        and if set to something other than zero, this means the evthread setup 
++ *        and if set to something other than zero, this means the evthread setup
+  *        functions were called out of order.
+  *
+  *        See: "Locks and threading" in the documentation.
+@@ -2523,13 +2523,30 @@ evthread_notify_base_default(struct event_base *base)
+ static int
+ evthread_notify_base_eventfd(struct event_base *base)
+ {
++	int efd = base->th_notify_fd[0];
+ 	ev_uint64_t msg = 1;
+-	int r;
+-	do {
+-		r = write(base->th_notify_fd[0], (void*) &msg, sizeof(msg));
+-	} while (r < 0 && errno == EAGAIN);
++	ev_uint64_t val;
+ 
+-	return (r < 0) ? -1 : 0;
++	int ret;
++	for (;;) {
++		ret = eventfd_write(efd, (eventfd_t) msg);
++		if (ret < 0) {
++			// When EAGAIN occurs, the eventfd counter hits the maximum value of the unsigned 64-bit.
++			// We need to first drain the eventfd and then write again.
++			//
++			// Check out https://man7.org/linux/man-pages/man2/eventfd.2.html for details.
++			if (errno == EAGAIN) {
++				// It's ready to retry.
++				if (eventfd_read(efd, &val) == 0 || errno == EAGAIN) {
++					continue;
++				}
++			}
++			// Unknown error occurs.
++			ret = -1;
++		}
++		break;
++	}
++	return ret;
+ }
+ #endif
+ 
+@@ -3582,14 +3599,7 @@ event_set_mem_functions(void *(*malloc_fn)(size_t sz),
+ static void
+ evthread_notify_drain_eventfd(evutil_socket_t fd, short what, void *arg)
+ {
+-	ev_uint64_t msg;
+-	ev_ssize_t r;
+ 	struct event_base *base = arg;
+-
+-	r = read(fd, (void*) &msg, sizeof(msg));
+-	if (r<0 && errno != EAGAIN) {
+-		event_sock_warn(fd, "Error reading from eventfd");
+-	}
+ 	EVBASE_ACQUIRE_LOCK(base, th_base_lock);
+ 	base->is_notify_pending = 0;
+ 	EVBASE_RELEASE_LOCK(base, th_base_lock);
+@@ -3667,7 +3677,7 @@ evthread_make_base_notifiable_nolock_(struct event_base *base)
+ 
+ 	/* prepare an event that we can use for wakeup */
+ 	event_assign(&base->th_notify, base, base->th_notify_fd[0],
+-				 EV_READ|EV_PERSIST, cb, base);
++				 EV_READ|EV_PERSIST|EV_ET, cb, base);
+ 
+ 	/* we need to mark this as internal event */
+ 	base->th_notify.ev_flags |= EVLIST_INTERNAL;
+-- 
+2.27.0
+

libevent.spec

@@ -1,6 +1,6 @@
 Name:           libevent
 Version:        2.1.12
-Release:        10
+Release:        11
 Summary:        An event notification library
 
 License:        BSD
@@ -23,6 +23,7 @@ Patch6001: backport-http-eliminate-redundant-bev-fd-manipulating-and-cac.patch
 Patch6002: backport-http-fix-fd-leak-on-fd-reset-by-using-bufferevent_re.patch
 Patch6003: backport-bufferevent-introduce-bufferevent_replacefd-like-set.patch
 Patch6004: backport-evutil-don-t-call-memset-before-memcpy.patch
+Patch6005: 0002-Avoid-calling-read-2-on-eventfd-on-each-event-loop-w.patch  
 
 %description
 Libevent additionally provides a sophisticated framework for buffered network IO, with support for sockets,
@@ -83,6 +84,9 @@ rm -f %{buildroot}%{_libdir}/*.la
 
 
 %changelog
+* Wed May 08 2024 baiguo <baiguo@kylinos.cn> - 2.1.12-11
+- Avoid calling read(2) on eventfd on each event-loop wakeup
+
 * Mon Apr 01 2024 shixuantong <shixuantong1@huawei.com> - 2.1.12-10
 - evutil: don't call memset before memcpy