[TCP]: Add tcp_tso_win_divisor sysctl.

This allows control over what percentage of
the congestion window can be consumed by a
single TSO frame.

The setting of this parameter is a choice
between burstiness and building larger TSO
frames.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sysctl.h

@@ -341,6 +341,7 @@ enum
 	NET_TCP_BIC_LOW_WINDOW=104,
 	NET_TCP_DEFAULT_WIN_SCALE=105,
 	NET_TCP_MODERATE_RCVBUF=106,
+	NET_TCP_TSO_WIN_DIVISOR=107,
 };
 
 enum {

include/net/tcp.h

@@ -609,6 +609,7 @@ extern int sysctl_tcp_bic;
 extern int sysctl_tcp_bic_fast_convergence;
 extern int sysctl_tcp_bic_low_window;
 extern int sysctl_tcp_moderate_rcvbuf;
+extern int sysctl_tcp_tso_win_divisor;
 
 extern atomic_t tcp_memory_allocated;
 extern atomic_t tcp_sockets_allocated;

net/ipv4/sysctl_net_ipv4.c

@@ -674,6 +674,14 @@ ctl_table ipv4_table[] = {
 		.mode		= 0644,
 		.proc_handler	= &proc_dointvec,
 	},
+	{
+		.ctl_name	= NET_TCP_TSO_WIN_DIVISOR,
+		.procname	= "tcp_tso_win_divisor",
+		.data		= &sysctl_tcp_tso_win_divisor,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
 	{ .ctl_name = 0 }
 };
 

net/ipv4/tcp_output.c

@@ -45,6 +45,12 @@
 /* People can turn this off for buggy TCP's found in printers etc. */
 int sysctl_tcp_retrans_collapse = 1;
 
+/* This limits the percentage of the congestion window which we
+ * will allow a single TSO frame to consume.  Building TSO frames
+ * which are too large can cause TCP streams to be bursty.
+ */
+int sysctl_tcp_tso_win_divisor = 8;
+
 static __inline__
 void update_send_head(struct sock *sk, struct tcp_opt *tp, struct sk_buff *skb)
 {
@@ -658,7 +664,7 @@ unsigned int tcp_current_mss(struct sock *sk, int large)
 {
 	struct tcp_opt *tp = tcp_sk(sk);
 	struct dst_entry *dst = __sk_dst_get(sk);
-	int do_large, mss_now;
+	unsigned int do_large, mss_now;
 
 	mss_now = tp->mss_cache_std;
 	if (dst) {
@@ -673,7 +679,7 @@ unsigned int tcp_current_mss(struct sock *sk, int large)
 		    !tp->urg_mode);
 
 	if (do_large) {
-		int large_mss, factor;
+		unsigned int large_mss, factor, limit;
 
 		large_mss = 65535 - tp->af_specific->net_header_len -
 			tp->ext_header_len - tp->ext2_header_len -
@@ -683,13 +689,19 @@ unsigned int tcp_current_mss(struct sock *sk, int large)
 			large_mss = max((tp->max_window>>1),
 					68U - tp->tcp_header_len);
 
-		/* Always keep large mss multiple of real mss, but
-		 * do not exceed 1/4 of the congestion window so we
-		 * can keep the ACK clock ticking.
-		 */
 		factor = large_mss / mss_now;
-		if (factor > (tp->snd_cwnd >> 2))
-			factor = max(1, tp->snd_cwnd >> 2);
+
+		/* Always keep large mss multiple of real mss, but
+		 * do not exceed 1/tso_win_divisor of the congestion window
+		 * so we can keep the ACK clock ticking and minimize
+		 * bursting.
+		 */
+		limit = tp->snd_cwnd;
+		if (sysctl_tcp_tso_win_divisor)
+			limit /= sysctl_tcp_tso_win_divisor;
+		limit = max(1U, limit);
+		if (factor > limit)
+			factor = limit;
 
 		tp->mss_cache = mss_now * factor;