[TCP]: Fix BIC max_cwnd calculation error.

The BIC TCP cwnd problem as identified by Yee-Ting Li and Doug Leith
is that the computation is recalc_ssthresh is incorrect and
BICTCP_1_OVER_BETA/2 should be BICTCP_1_OVER_BETA*2.

My fix is to implement the code from BIC TCP 1.1 which uses a sysctl
to set the beta.  There are a few variable name changes from the 1.1
code, and made the scaling factor a #define instead of hardcoded.

I validated this using netem and kprobes, for more details see
http://developer.osdl.org/shemminger/bic-beta-patch.pdfSigned-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sysctl.h

@@ -344,6 +344,7 @@ enum
 	NET_TCP_DEFAULT_WIN_SCALE=105,
 	NET_TCP_MODERATE_RCVBUF=106,
 	NET_TCP_TSO_WIN_DIVISOR=107,
+	NET_TCP_BIC_BETA=108,
 };
 
 enum {

include/net/tcp.h

@@ -505,9 +505,8 @@ static __inline__ int tcp_sk_listen_hashfn(struct sock *sk)
 # define TCP_TW_RECYCLE_TICK (12+2-TCP_TW_RECYCLE_SLOTS_LOG)
 #endif
 
-#define BICTCP_1_OVER_BETA	8	/*
-					 * Fast recovery
-					 * multiplicative decrease factor
+#define BICTCP_BETA_SCALE    1024	/* Scale factor beta calculation
+					 * max_cwnd = snd_cwnd * beta
 					 */
 #define BICTCP_MAX_INCREMENT 32		/*
 					 * Limit on the amount of
@@ -606,6 +605,7 @@ extern int sysctl_tcp_nometrics_save;
 extern int sysctl_tcp_bic;
 extern int sysctl_tcp_bic_fast_convergence;
 extern int sysctl_tcp_bic_low_window;
+extern int sysctl_tcp_bic_beta;
 extern int sysctl_tcp_moderate_rcvbuf;
 extern int sysctl_tcp_tso_win_divisor;
 
@@ -1244,15 +1244,16 @@ static inline __u32 tcp_recalc_ssthresh(struct tcp_sock *tp)
 	if (tcp_is_bic(tp)) {
 		if (sysctl_tcp_bic_fast_convergence &&
 		    tp->snd_cwnd < tp->bictcp.last_max_cwnd)
-			tp->bictcp.last_max_cwnd
-				= (tp->snd_cwnd * (2*BICTCP_1_OVER_BETA-1))
-				/ (BICTCP_1_OVER_BETA/2);
+			tp->bictcp.last_max_cwnd = (tp->snd_cwnd * 
+						    (BICTCP_BETA_SCALE
+						     + sysctl_tcp_bic_beta))
+				/ (2 * BICTCP_BETA_SCALE);
 		else
 			tp->bictcp.last_max_cwnd = tp->snd_cwnd;
 
 		if (tp->snd_cwnd > sysctl_tcp_bic_low_window)
-			return max(tp->snd_cwnd - (tp->snd_cwnd/BICTCP_1_OVER_BETA),
-				   2U);
+			return max((tp->snd_cwnd * sysctl_tcp_bic_beta)
+				   / BICTCP_BETA_SCALE, 2U);
 	}
 
 	return max(tp->snd_cwnd >> 1U, 2U);

net/ipv4/sysctl_net_ipv4.c

@@ -682,6 +682,14 @@ ctl_table ipv4_table[] = {
 		.mode		= 0644,
 		.proc_handler	= &proc_dointvec,
 	},
+	{
+		.ctl_name	= NET_TCP_BIC_BETA,
+		.procname	= "tcp_bic_beta",
+		.data		= &sysctl_tcp_bic_beta,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
 	{ .ctl_name = 0 }
 };
 

net/ipv4/tcp_input.c

@@ -102,6 +102,7 @@ int sysctl_tcp_vegas_gamma = 1<<V_PARAM_SHIFT;
 int sysctl_tcp_bic = 1;
 int sysctl_tcp_bic_fast_convergence = 1;
 int sysctl_tcp_bic_low_window = 14;
+int sysctl_tcp_bic_beta = 819;		/* = 819/1024 (BICTCP_BETA_SCALE) */
 
 #define FLAG_DATA		0x01 /* Incoming frame contained data.		*/
 #define FLAG_WIN_UPDATE		0x02 /* Incoming ACK was a window update.	*/