[PATCH v2] tcp: Disable Nagle's algorithm (set TCP_NODELAY) on all sockets

[Stefano Brivio <sbrivio@redhat.com>]

Following up on 725acd111ba3 ("tcp_splice: Set (again) TCP_NODELAY on
both sides"), David argues that, in general, we don't know what kind
of TCP traffic we're dealing with, on any side or path.

TCP segments might have been delivered to our socket with a PSH flag,
but we don't have a way to know about it.

Similarly, the guest might send us segments with PSH or URG set, but
we don't know if we should generally TCP_CORK sockets and uncork on
those flags, because that would assume they're running a Linux kernel
(and a particular version of it) matching the kernel that delivers
outbound packets for us.

Given that we can't make any assumption and everything might very well
be interactive traffic, disable Nagle's algorithm on all non-spliced
sockets as well.

After all, John Nagle himself is nowadays recommending that delayed
ACKs should never be enabled together with his algorithm, but we
don't have a practical way to ensure that our environment is free from
delayed ACKs (TCP_QUICKACK is not really usable for this purpose):

  https://news.ycombinator.com/item?id=34180239

Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
v2: Set TCP_NODELAY on inbound socket after accept4(), not on the
    listening sockets, and change failure message to debug()
    instead of trace()

 tcp.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/tcp.c b/tcp.c
index a012b81..4d6a6b3 100644
--- a/tcp.c
+++ b/tcp.c
@@ -756,6 +756,19 @@ static void tcp_sock_set_bufsize(const struct ctx *c, int s)
 		trace("TCP: failed to set SO_SNDBUF to %i", v);
 }
 
+/**
+ * tcp_sock_set_nodelay() - Set TCP_NODELAY option (disable Nagle's algorithm)
+ * @s:		Socket, can be -1 to avoid check in the caller
+ */
+static void tcp_sock_set_nodelay(int s)
+{
+	if (s == -1)
+		return;
+
+	if (setsockopt(s, SOL_TCP, TCP_NODELAY, &((int){ 1 }), sizeof(int)))
+		debug("TCP: failed to set TCP_NODELAY on socket %i", s);
+}
+
 /**
  * tcp_update_csum() - Calculate TCP checksum
  * @psum:	Unfolded partial checksum of the IPv4 or IPv6 pseudo-header
@@ -1285,6 +1298,7 @@ static int tcp_conn_new_sock(const struct ctx *c, sa_family_t af)
 		return -errno;
 
 	tcp_sock_set_bufsize(c, s);
+	tcp_sock_set_nodelay(s);
 
 	return s;
 }
@@ -2058,6 +2072,7 @@ void tcp_listen_handler(const struct ctx *c, union epoll_ref ref,
 		goto cancel;
 
 	tcp_sock_set_bufsize(c, s);
+	tcp_sock_set_nodelay(s);
 
 	/* FIXME: When listening port has a specific bound address, record that
 	 * as our address
-- 
@@ -756,6 +756,19 @@ static void tcp_sock_set_bufsize(const struct ctx *c, int s)
 		trace("TCP: failed to set SO_SNDBUF to %i", v);
 }
 
+/**
+ * tcp_sock_set_nodelay() - Set TCP_NODELAY option (disable Nagle's algorithm)
+ * @s:		Socket, can be -1 to avoid check in the caller
+ */
+static void tcp_sock_set_nodelay(int s)
+{
+	if (s == -1)
+		return;
+
+	if (setsockopt(s, SOL_TCP, TCP_NODELAY, &((int){ 1 }), sizeof(int)))
+		debug("TCP: failed to set TCP_NODELAY on socket %i", s);
+}
+
 /**
  * tcp_update_csum() - Calculate TCP checksum
  * @psum:	Unfolded partial checksum of the IPv4 or IPv6 pseudo-header
@@ -1285,6 +1298,7 @@ static int tcp_conn_new_sock(const struct ctx *c, sa_family_t af)
 		return -errno;
 
 	tcp_sock_set_bufsize(c, s);
+	tcp_sock_set_nodelay(s);
 
 	return s;
 }
@@ -2058,6 +2072,7 @@ void tcp_listen_handler(const struct ctx *c, union epoll_ref ref,
 		goto cancel;
 
 	tcp_sock_set_bufsize(c, s);
+	tcp_sock_set_nodelay(s);
 
 	/* FIXME: When listening port has a specific bound address, record that
 	 * as our address
-- 
2.43.0