[TCP]: Add tcp_slow_start_after_idle sysctl.

A lot of people have asked for a way to disable tcp_cwnd_restart(),
and it seems reasonable to add a sysctl to do that.

Signed-off-by: David S. Miller <davem@davemloft.net>

5 files changed, 22 insertions, 1 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index f12007b80a46..d46338af6002 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -362,6 +362,13 @@ tcp_workaround_signed_windows - BOOLEAN
         not receive a window scaling option from them.
         Default: 0
 
+tcp_slow_start_after_idle - BOOLEAN
+        If set, provide RFC2861 behavior and time out the congestion
+        window after an idle period.  An idle period is defined at
+        the current RTO.  If unset, the congestion window will not
+        be timed out after an idle period.
+        Default: 1
+
 IP Variables:
 
 ip_local_port_range - 2 INTEGERS
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 98338ed2c0b6..cee944dbdcd4 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -405,6 +405,7 @@ enum
         NET_TCP_BASE_MSS=114,
         NET_IPV4_TCP_WORKAROUND_SIGNED_WINDOWS=115,
         NET_TCP_DMA_COPYBREAK=116,
+        NET_TCP_SLOW_START_AFTER_IDLE=117,
 };
 
 enum {
diff --git a/include/net/tcp.h b/include/net/tcp.h
index de88c5472bfc..bfc71f954bbe 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -227,6 +227,7 @@ extern int sysctl_tcp_abc;
 extern int sysctl_tcp_mtu_probing;
 extern int sysctl_tcp_base_mss;
 extern int sysctl_tcp_workaround_signed_windows;
+extern int sysctl_tcp_slow_start_after_idle;
 
 extern atomic_t tcp_memory_allocated;
 extern atomic_t tcp_sockets_allocated;
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index e7fb665873c6..ce4cd5f35511 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -690,6 +690,14 @@ ctl_table ipv4_table[] = {
                 .proc_handler   = &proc_dointvec
         },
 #endif
+        {
+                .ctl_name       = NET_TCP_SLOW_START_AFTER_IDLE,
+                .procname       = "tcp_slow_start_after_idle",
+                .data           = &sysctl_tcp_slow_start_after_idle,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = &proc_dointvec
+        },
         { .ctl_name = 0 }
 };
 
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index f33c9dddaa12..07bb5a2b375e 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -59,6 +59,9 @@ int sysctl_tcp_tso_win_divisor = 3;
 int sysctl_tcp_mtu_probing = 0;
 int sysctl_tcp_base_mss = 512;
 
+/* By default, RFC2861 behavior.  */
+int sysctl_tcp_slow_start_after_idle = 1;
+
 static void update_send_head(struct sock *sk, struct tcp_sock *tp,
                              struct sk_buff *skb)
 {
@@ -138,7 +141,8 @@ static void tcp_event_data_sent(struct tcp_sock *tp,
         struct inet_connection_sock *icsk = inet_csk(sk);
         const u32 now = tcp_time_stamp;
 
-        if (!tp->packets_out && (s32)(now - tp->lsndtime) > icsk->icsk_rto)
+        if (sysctl_tcp_slow_start_after_idle &&
+            (!tp->packets_out && (s32)(now - tp->lsndtime) > icsk->icsk_rto))
                 tcp_cwnd_restart(sk, __sk_dst_get(sk));
 
         tp->lsndtime = now;