[TCP]: Add two new spurious RTO responses to FRTO

New sysctl tcp_frto_response is added to select amongst these
responses:
	- Rate halving based; reuses CA_CWR state (default)
	- Very conservative; used to be the only one available (=1)
	- Undo cwr; undoes ssthresh and cwnd reductions (=2)

The response with rate halving requires a new parameter to
tcp_enter_cwr because FRTO has already reduced ssthresh and
doing a second reduction there has to be prevented. In addition,
to keep things nice on 80 cols screen, a local variable was
added.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/sysctl.h

@@ -439,6 +439,7 @@ enum
 	NET_TCP_AVAIL_CONG_CONTROL=122,
 	NET_TCP_ALLOWED_CONG_CONTROL=123,
 	NET_TCP_MAX_SSTHRESH=124,
+	NET_TCP_FRTO_RESPONSE=125,
 };
 
 enum {

include/net/tcp.h

@@ -220,6 +220,7 @@ extern int sysctl_tcp_app_win;
 extern int sysctl_tcp_adv_win_scale;
 extern int sysctl_tcp_tw_reuse;
 extern int sysctl_tcp_frto;
+extern int sysctl_tcp_frto_response;
 extern int sysctl_tcp_low_latency;
 extern int sysctl_tcp_dma_copybreak;
 extern int sysctl_tcp_nometrics_save;
@@ -738,7 +739,7 @@ static inline void tcp_sync_left_out(struct tcp_sock *tp)
 	tp->left_out = tp->sacked_out + tp->lost_out;
 }
 
-extern void tcp_enter_cwr(struct sock *sk);
+extern void tcp_enter_cwr(struct sock *sk, const int set_ssthresh);
 extern __u32 tcp_init_cwnd(struct tcp_sock *tp, struct dst_entry *dst);
 
 /* Slow start with delack produces 3 packets of burst, so that

net/ipv4/sysctl_net_ipv4.c

@@ -646,6 +646,14 @@ ctl_table ipv4_table[] = {
 		.mode		= 0644,
 		.proc_handler	= &proc_dointvec
 	},
+	{
+		.ctl_name	= NET_TCP_FRTO_RESPONSE,
+		.procname	= "tcp_frto_response",
+		.data		= &sysctl_tcp_frto_response,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec
+	},
 	{
 		.ctl_name	= NET_TCP_LOW_LATENCY,
 		.procname	= "tcp_low_latency",

net/ipv4/tcp_input.c

@@ -86,6 +86,7 @@ int sysctl_tcp_stdurg __read_mostly;
 int sysctl_tcp_rfc1337 __read_mostly;
 int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
 int sysctl_tcp_frto __read_mostly;
+int sysctl_tcp_frto_response __read_mostly;
 int sysctl_tcp_nometrics_save __read_mostly;
 
 int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
@@ -762,15 +763,17 @@ __u32 tcp_init_cwnd(struct tcp_sock *tp, struct dst_entry *dst)
 }
 
 /* Set slow start threshold and cwnd not falling to slow start */
-void tcp_enter_cwr(struct sock *sk)
+void tcp_enter_cwr(struct sock *sk, const int set_ssthresh)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
+	const struct inet_connection_sock *icsk = inet_csk(sk);
 
 	tp->prior_ssthresh = 0;
 	tp->bytes_acked = 0;
 	if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
 		tp->undo_marker = 0;
-		tp->snd_ssthresh = inet_csk(sk)->icsk_ca_ops->ssthresh(sk);
+		if (set_ssthresh)
+			tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
 		tp->snd_cwnd = min(tp->snd_cwnd,
 				   tcp_packets_in_flight(tp) + 1U);
 		tp->snd_cwnd_cnt = 0;
@@ -2003,7 +2006,7 @@ static void tcp_try_to_open(struct sock *sk, struct tcp_sock *tp, int flag)
 		tp->retrans_stamp = 0;
 
 	if (flag&FLAG_ECE)
-		tcp_enter_cwr(sk);
+		tcp_enter_cwr(sk, 1);
 
 	if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
 		int state = TCP_CA_Open;
@@ -2579,6 +2582,21 @@ static void tcp_conservative_spur_to_response(struct tcp_sock *tp)
 	tcp_moderate_cwnd(tp);
 }
 
+/* A conservative spurious RTO response algorithm: reduce cwnd using
+ * rate halving and continue in congestion avoidance.
+ */
+static void tcp_ratehalving_spur_to_response(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	tcp_enter_cwr(sk, 0);
+	tp->high_seq = tp->frto_highmark; 	/* Smoother w/o this? - ij */
+}
+
+static void tcp_undo_spur_to_response(struct sock *sk)
+{
+	tcp_undo_cwr(sk, 1);
+}
+
 /* F-RTO spurious RTO detection algorithm (RFC4138)
  *
  * F-RTO affects during two new ACKs following RTO (well, almost, see inline
@@ -2661,7 +2679,17 @@ static int tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag)
 		tp->frto_counter = 2;
 		return 1;
 	} else /* frto_counter == 2 */ {
-		tcp_conservative_spur_to_response(tp);
+		switch (sysctl_tcp_frto_response) {
+		case 2:
+			tcp_undo_spur_to_response(sk);
+			break;
+		case 1:
+			tcp_conservative_spur_to_response(tp);
+			break;
+		default:
+			tcp_ratehalving_spur_to_response(sk);
+			break;
+		};
 		tp->frto_counter = 0;
 	}
 	return 0;

net/ipv4/tcp_output.c

@@ -545,7 +545,7 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 	if (likely(err <= 0))
 		return err;
 
-	tcp_enter_cwr(sk);
+	tcp_enter_cwr(sk, 1);
 
 	return net_xmit_eval(err);