From 8de5c4fa15d612bd70456881f8fef13b8fe37e2a Mon Sep 17 00:00:00 2001
From: Willy Tarreau <w@1wt.eu>
Date: Wed, 4 Mar 2020 17:45:21 +0100
Subject: [PATCH] MEDIUM: connection: only call ->wake() for connect() without
 I/O

We used to call ->wake() for any I/O event for which there was no
subscriber. But this is a problem because this causes massive wake()
storms since we disabled fd_stop_recv() to save syscalls.

The only reason for the io_available condition is to detect that an
asynchronous connect() just finished and will not be handled by any
registered event handler. Since we now properly handle synchronous
connects, we can detect this situation by the fact that we had a
success on conn_fd_check() and no requested I/O took over.
---
 src/connection.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/src/connection.c b/src/connection.c
index ae53d56df..fea092b76 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -73,7 +73,7 @@ void conn_fd_handler(int fd)
 {
 	struct connection *conn = fdtab[fd].owner;
 	unsigned int flags;
-	int io_available = 0;
+	int need_wake = 0;
 
 	if (unlikely(!conn)) {
 		activity[tid].conn_dead++;
@@ -91,6 +91,7 @@ void conn_fd_handler(int fd)
 		 */
 		if (!conn_fd_check(conn))
 			goto leave;
+		need_wake = 1;
 	}
 
 	if (fd_send_ready(fd) && fd_send_active(fd)) {
@@ -100,12 +101,12 @@ void conn_fd_handler(int fd)
 		 */
 		flags = 0;
 		if (conn->subs && conn->subs->events & SUB_RETRY_SEND) {
+			need_wake = 0; // wake will be called after this I/O
 			tasklet_wakeup(conn->subs->tasklet);
 			conn->subs->events &= ~SUB_RETRY_SEND;
 			if (!conn->subs->events)
 				conn->subs = NULL;
-		} else
-			io_available = 1;
+		}
 		fd_stop_send(fd);
 	}
 
@@ -120,12 +121,12 @@ void conn_fd_handler(int fd)
 		 */
 		flags = 0;
 		if (conn->subs && conn->subs->events & SUB_RETRY_RECV) {
+			need_wake = 0; // wake will be called after this I/O
 			tasklet_wakeup(conn->subs->tasklet);
 			conn->subs->events &= ~SUB_RETRY_RECV;
 			if (!conn->subs->events)
 				conn->subs = NULL;
-		} else
-			io_available = 1;
+		}
 	}
 
  leave:
@@ -152,7 +153,7 @@ void conn_fd_handler(int fd)
 	 * Note that the wake callback is allowed to release the connection and
 	 * the fd (and return < 0 in this case).
 	 */
-	if ((io_available || ((conn->flags ^ flags) & CO_FL_NOTIFY_DONE) ||
+	if ((need_wake || ((conn->flags ^ flags) & CO_FL_NOTIFY_DONE) ||
 	     ((flags & CO_FL_WAIT_XPRT) && !(conn->flags & CO_FL_WAIT_XPRT))) &&
 	    conn->mux && conn->mux->wake && conn->mux->wake(conn) < 0)
 		return;