Events: support for EPOLLEXCLUSIVE.

This flag appeared in Linux 4.5 and is useful for avoiding thundering herd
problem.

The current Linux kernel implementation walks the list of exclusive waiters,
and queues an event to each epfd, until it finds the first waiter that has
threads blocked on it via epoll_wait().

auto/modules

@@ -43,6 +43,7 @@ fi
 if [ $NGX_TEST_BUILD_EPOLL = YES ]; then
     have=NGX_HAVE_EPOLL . auto/have
     have=NGX_HAVE_EPOLLRDHUP . auto/have
+    have=NGX_HAVE_EPOLLEXCLUSIVE . auto/have
     have=NGX_HAVE_EVENTFD . auto/have
     have=NGX_TEST_BUILD_EPOLL . auto/have
     EVENT_MODULES="$EVENT_MODULES $EPOLL_MODULE"

auto/os/linux

@@ -70,6 +70,22 @@ if [ $ngx_found = yes ]; then
                       ee.data.ptr = NULL;
                       epoll_ctl(efd, EPOLL_CTL_ADD, fd, &ee)"
     . auto/feature
+
+
+    # EPOLLEXCLUSIVE appeared in Linux 4.5, glibc 2.24
+
+    ngx_feature="EPOLLEXCLUSIVE"
+    ngx_feature_name="NGX_HAVE_EPOLLEXCLUSIVE"
+    ngx_feature_run=no
+    ngx_feature_incs="#include <sys/epoll.h>"
+    ngx_feature_path=
+    ngx_feature_libs=
+    ngx_feature_test="int efd = 0, fd = 0;
+                      struct epoll_event ee;
+                      ee.events = EPOLLIN|EPOLLEXCLUSIVE;
+                      ee.data.ptr = NULL;
+                      epoll_ctl(efd, EPOLL_CTL_ADD, fd, &ee)"
+    . auto/feature
 fi
 
 

src/event/modules/ngx_epoll_module.c

@@ -27,6 +27,7 @@
 
 #define EPOLLRDHUP     0x2000
 
+#define EPOLLEXCLUSIVE 0x10000000
 #define EPOLLONESHOT   0x40000000
 #define EPOLLET        0x80000000
 
@@ -610,6 +611,12 @@ ngx_epoll_add_event(ngx_event_t *ev, ngx_int_t event, ngx_uint_t flags)
         op = EPOLL_CTL_ADD;
     }
 
+#if (NGX_HAVE_EPOLLEXCLUSIVE && NGX_HAVE_EPOLLRDHUP)
+    if (flags & NGX_EXCLUSIVE_EVENT) {
+        events &= ~EPOLLRDHUP;
+    }
+#endif
+
     ee.events = events | (uint32_t) flags;
     ee.data.ptr = (void *) ((uintptr_t) c | ev->instance);
 

src/event/ngx_event.c

@@ -822,15 +822,38 @@ ngx_event_process_init(ngx_cycle_t *cycle)
         rev->handler = (c->type == SOCK_STREAM) ? ngx_event_accept
                                                 : ngx_event_recvmsg;
 
-        if (ngx_use_accept_mutex
 #if (NGX_HAVE_REUSEPORT)
-            && !ls[i].reuseport
+
+        if (ls[i].reuseport) {
+            if (ngx_add_event(rev, NGX_READ_EVENT, 0) == NGX_ERROR) {
+                return NGX_ERROR;
+            }
+
+            continue;
+        }
+
 #endif
-           )
+
+        if (ngx_use_accept_mutex) {
+            continue;
+        }
+
+#if (NGX_HAVE_EPOLLEXCLUSIVE)
+
+        if ((ngx_event_flags & NGX_USE_EPOLL_EVENT)
+            && ccf->worker_processes > 1)
         {
+            if (ngx_add_event(rev, NGX_READ_EVENT, NGX_EXCLUSIVE_EVENT)
+                == NGX_ERROR)
+            {
+                return NGX_ERROR;
+            }
+
             continue;
         }
 
+#endif
+
         if (ngx_add_event(rev, NGX_READ_EVENT, 0) == NGX_ERROR) {
             return NGX_ERROR;
         }

src/event/ngx_event.h

@@ -367,6 +367,9 @@ extern ngx_uint_t            ngx_use_epoll_rdhup;
 #define NGX_ONESHOT_EVENT  EPOLLONESHOT
 #endif
 
+#if (NGX_HAVE_EPOLLEXCLUSIVE)
+#define NGX_EXCLUSIVE_EVENT  EPOLLEXCLUSIVE
+#endif
 
 #elif (NGX_HAVE_POLL)
 
@@ -395,6 +398,11 @@ extern ngx_uint_t            ngx_use_epoll_rdhup;
 #endif
 
 
+#if (NGX_TEST_BUILD_EPOLL)
+#define NGX_EXCLUSIVE_EVENT  0
+#endif
+
+
 #ifndef NGX_CLEAR_EVENT
 #define NGX_CLEAR_EVENT    0    /* dummy declaration */
 #endif