tipc: set default MTU for UDP media

Currently, all bearers are configured with MTU value same as the
underlying L2 device. However, in case of bearers with media type
UDP, higher throughput is possible with a fixed and higher emulated
MTU value than adapting to the underlying L2 MTU.

In this commit, we introduce a parameter mtu in struct tipc_media
and a default value is set for UDP. A default value of 14k
was determined by experimentation and found to have a higher throughput
than 16k. MTU for UDP bearers are assigned the above set value of
media MTU.
Acked-by: NYing Xue <ying.xue@windriver.com>
Acked-by: NJon Maloy <jon.maloy@ericsson.com>
Signed-off-by: NGhantaKrishnamurthy MohanKrishna <mohan.krishna.ghanta.krishnamurthy@ericsson.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/uapi/linux/tipc_config.h

@@ -185,6 +185,11 @@
 #define TIPC_DEF_LINK_WIN 50
 #define TIPC_MAX_LINK_WIN 8191
 
+/*
+ * Default MTU for UDP media
+ */
+
+#define TIPC_DEF_LINK_UDP_MTU 14000
 
 struct tipc_node_info {
 	__be32 addr;			/* network address of node */

net/tipc/udp_media.c

@@ -713,8 +713,7 @@ static int tipc_udp_enable(struct net *net, struct tipc_bearer *b,
 			err = -EINVAL;
 			goto err;
 		}
-		b->mtu = dev->mtu - sizeof(struct iphdr)
-			- sizeof(struct udphdr);
+		b->mtu = b->media->mtu;
 #if IS_ENABLED(CONFIG_IPV6)
 	} else if (local.proto == htons(ETH_P_IPV6)) {
 		udp_conf.family = AF_INET6;
@@ -803,6 +802,7 @@ struct tipc_media udp_media_info = {
 	.priority	= TIPC_DEF_LINK_PRI,
 	.tolerance	= TIPC_DEF_LINK_TOL,
 	.window		= TIPC_DEF_LINK_WIN,
+	.mtu		= TIPC_DEF_LINK_UDP_MTU,
 	.type_id	= TIPC_MEDIA_TYPE_UDP,
 	.hwaddr_len	= 0,
 	.name		= "udp"