[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>

2 files changed, 13 insertions, 1 deletions

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index e7fb665873c..ce4cd5f3551 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 f33c9dddaa1..07bb5a2b375 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;