[TCP]: Add tcp_low_latency sysctl.

Currently it turns of prequeue processing, but more decisions
may be guided by it in the future.
Based upon a patch from Andi Kleen.

Documentation/networking/ip-sysctl.txt

@@ -281,6 +281,14 @@ tcp_rfc1337 - BOOLEAN
 	assassination.   
 	Default: 0
 
+tcp_low_latency - BOOLEAN
+	If set, the TCP stack makes decisions that prefer lower
+	latency as opposed to higher throughput.  By default, this
+	option is not set meaning that higher throughput is preferred.
+	An example of an application where this default should be
+	changed would be a Beowulf compute cluster.
+	Default: 0
+
 ip_local_port_range - 2 INTEGERS
 	Defines the local port range that is used by TCP and UDP to
 	choose the local port. The first number is the first, the 

include/linux/sysctl.h

@@ -305,7 +305,8 @@ enum
 	NET_IPV4_ICMP_RATELIMIT=89,
 	NET_IPV4_ICMP_RATEMASK=90,
 	NET_TCP_TW_REUSE=91,
-	NET_TCP_FRTO=92
+	NET_TCP_FRTO=92,
+	NET_TCP_LOW_LATENCY=93
 };
 
 enum {

include/net/tcp.h

@@ -474,6 +474,7 @@ extern int sysctl_tcp_app_win;
 extern int sysctl_tcp_adv_win_scale;
 extern int sysctl_tcp_tw_reuse;
 extern int sysctl_tcp_frto;
+extern int sysctl_tcp_low_latency;
 
 extern atomic_t tcp_memory_allocated;
 extern atomic_t tcp_sockets_allocated;
@@ -1349,7 +1350,7 @@ static __inline__ int tcp_prequeue(struct sock *sk, struct sk_buff *skb)
 {
 	struct tcp_opt *tp = tcp_sk(sk);
 
-	if (tp->ucopy.task) {
+	if (!sysctl_tcp_low_latency && tp->ucopy.task) {
 		__skb_queue_tail(&tp->ucopy.prequeue, skb);
 		tp->ucopy.memory += skb->truesize;
 		if (tp->ucopy.memory > sk->rcvbuf) {

net/ipv4/sysctl_net_ipv4.c

@@ -223,6 +223,8 @@ ctl_table ipv4_table[] = {
 	 &sysctl_tcp_tw_reuse, sizeof(int), 0644, NULL, &proc_dointvec},
 	{NET_TCP_FRTO, "tcp_frto",
 	 &sysctl_tcp_frto, sizeof(int), 0644, NULL, &proc_dointvec},
+	{NET_TCP_LOW_LATENCY, "tcp_low_latency",
+	 &sysctl_tcp_low_latency, sizeof(int), 0644, NULL, &proc_dointvec},
 	{0}
 };
 

net/ipv4/tcp_ipv4.c

@@ -75,6 +75,7 @@
 extern int sysctl_ip_dynaddr;
 extern int sysctl_ip_default_ttl;
 int sysctl_tcp_tw_reuse;
+int sysctl_tcp_low_latency;
 
 /* Check TCP sequence numbers in ICMP packets. */
 #define ICMP_MIN_LENGTH 8

net/netsyms.c

@@ -429,6 +429,7 @@ EXPORT_SYMBOL(sysctl_tcp_wmem);
 EXPORT_SYMBOL(sysctl_tcp_ecn);
 EXPORT_SYMBOL(tcp_cwnd_application_limited);
 EXPORT_SYMBOL(tcp_sendpage);
+EXPORT_SYMBOL(sysctl_tcp_low_latency);
 
 EXPORT_SYMBOL(tcp_write_xmit);