ANDROID: net: ipv4: tcp: add a sysctl to config the tcp_default_init_rwnd

The default initial rwnd is hardcoded to 10.

Now we allow it to be controlled via
  /proc/sys/net/ipv4/tcp_default_init_rwnd
which limits the values from 3 to 100

This is somewhat needed because ipv6 routes are
autoconfigured by the kernel.

See "An Argument for Increasing TCP's Initial Congestion Window"
in https://developers.google.com/speed/articles/tcp_initcwnd_paper.pdf

Change-Id: I386b2a9d62de0ebe05c1ebe1b4bd91b314af5c54
Signed-off-by: JP Abgrall <jpa@google.com>

Conflicts:
	net/ipv4/sysctl_net_ipv4.c
	net/ipv4/tcp_input.c

[AmitP: Folded following android-4.9 commit changes into this patch
        3823c8b26e6e ("ANDROID: tcp: fix tcp_default_init_rwnd() for 4.1")]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>

include/net/tcp.h

@@ -248,6 +248,8 @@ extern long sysctl_tcp_mem[3];
 #define TCP_RACK_STATIC_REO_WND  0x2 /* Use static RACK reo wnd */
 #define TCP_RACK_NO_DUPTHRESH    0x4 /* Do not use DUPACK threshold in RACK */
 
+extern int sysctl_tcp_default_init_rwnd;
+
 extern atomic_long_t tcp_memory_allocated;
 extern struct percpu_counter tcp_sockets_allocated;
 extern unsigned long tcp_memory_pressure;

net/ipv4/sysctl_net_ipv4.c

@@ -218,6 +218,21 @@ static int ipv4_fwd_update_priority(struct ctl_table *table, int write,
 	return ret;
 }
 
+/* Validate changes from /proc interface. */
+static int proc_tcp_default_init_rwnd(struct ctl_table *ctl, int write,
+				      void __user *buffer,
+				      size_t *lenp, loff_t *ppos)
+{
+	int old_value = *(int *)ctl->data;
+	int ret = proc_dointvec(ctl, write, buffer, lenp, ppos);
+	int new_value = *(int *)ctl->data;
+
+	if (write && ret == 0 && (new_value < 3 || new_value > 100))
+		*(int *)ctl->data = old_value;
+
+	return ret;
+}
+
 static int proc_tcp_congestion_control(struct ctl_table *ctl, int write,
 				       void __user *buffer, size_t *lenp, loff_t *ppos)
 {
@@ -525,6 +540,13 @@ static struct ctl_table ipv4_table[] = {
 		.mode		= 0444,
 		.proc_handler   = proc_tcp_available_ulp,
 	},
+	{
+		.procname       = "tcp_default_init_rwnd",
+		.data           = &sysctl_tcp_default_init_rwnd,
+		.maxlen         = sizeof(int),
+		.mode           = 0644,
+		.proc_handler   = proc_tcp_default_init_rwnd
+	},
 	{
 		.procname	= "icmp_msgs_per_sec",
 		.data		= &sysctl_icmp_msgs_per_sec,

net/ipv4/tcp_input.c

@@ -81,6 +81,7 @@
 #include <net/busy_poll.h>
 
 int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
+int sysctl_tcp_default_init_rwnd __read_mostly = TCP_INIT_CWND * 2;
 
 #define FLAG_DATA		0x01 /* Incoming frame contained data.		*/
 #define FLAG_WIN_UPDATE		0x02 /* Incoming ACK was a window update.	*/

net/ipv4/tcp_output.c

@@ -187,7 +187,7 @@ u32 tcp_default_init_rwnd(u32 mss)
 	 * (RFC 3517, Section 4, NextSeg() rule (2)). Further place a
 	 * limit when mss is larger than 1460.
 	 */
-	u32 init_rwnd = TCP_INIT_CWND * 2;
+	u32 init_rwnd = sysctl_tcp_default_init_rwnd;
 
 	if (mss > 1460)
 		init_rwnd = max((1460 * init_rwnd) / mss, 2U);