tcp: make undo_cwnd mandatory for congestion modules

The undo_cwnd fallback in the stack doubles cwnd based on ssthresh,
which un-does reno halving behaviour.

It seems more appropriate to let congctl algorithms pair .ssthresh
and .undo_cwnd properly. Add a 'tcp_reno_undo_cwnd' function and wire it
up for all congestion algorithms that used to rely on the fallback.

Cc: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -958,6 +958,7 @@ u32 tcp_slow_start(struct tcp_sock *tp, u32 acked);
 void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w, u32 acked);
 
 u32 tcp_reno_ssthresh(struct sock *sk);
+u32 tcp_reno_undo_cwnd(struct sock *sk);
 void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 acked);
 extern struct tcp_congestion_ops tcp_reno;
 

net/ipv4/tcp_cong.c

@@ -68,8 +68,9 @@ int tcp_register_congestion_control(struct tcp_congestion_ops *ca)
 {
 	int ret = 0;
 
-	/* all algorithms must implement ssthresh and cong_avoid ops */
-	if (!ca->ssthresh || !(ca->cong_avoid || ca->cong_control)) {
+	/* all algorithms must implement these */
+	if (!ca->ssthresh || !ca->undo_cwnd ||
+	    !(ca->cong_avoid || ca->cong_control)) {
 		pr_err("%s does not implement required ops\n", ca->name);
 		return -EINVAL;
 	}
@@ -441,10 +442,19 @@ u32 tcp_reno_ssthresh(struct sock *sk)
 }
 EXPORT_SYMBOL_GPL(tcp_reno_ssthresh);
 
+u32 tcp_reno_undo_cwnd(struct sock *sk)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+
+	return max(tp->snd_cwnd, tp->snd_ssthresh << 1);
+}
+EXPORT_SYMBOL_GPL(tcp_reno_undo_cwnd);
+
 struct tcp_congestion_ops tcp_reno = {
 	.flags		= TCP_CONG_NON_RESTRICTED,
 	.name		= "reno",
 	.owner		= THIS_MODULE,
 	.ssthresh	= tcp_reno_ssthresh,
 	.cong_avoid	= tcp_reno_cong_avoid,
+	.undo_cwnd	= tcp_reno_undo_cwnd,
 };

net/ipv4/tcp_dctcp.c

@@ -342,6 +342,7 @@ static struct tcp_congestion_ops dctcp __read_mostly = {
 static struct tcp_congestion_ops dctcp_reno __read_mostly = {
 	.ssthresh	= tcp_reno_ssthresh,
 	.cong_avoid	= tcp_reno_cong_avoid,
+	.undo_cwnd	= tcp_reno_undo_cwnd,
 	.get_info	= dctcp_get_info,
 	.owner		= THIS_MODULE,
 	.name		= "dctcp-reno",

net/ipv4/tcp_hybla.c

@@ -166,6 +166,7 @@ static void hybla_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 static struct tcp_congestion_ops tcp_hybla __read_mostly = {
 	.init		= hybla_init,
 	.ssthresh	= tcp_reno_ssthresh,
+	.undo_cwnd	= tcp_reno_undo_cwnd,
 	.cong_avoid	= hybla_cong_avoid,
 	.set_state	= hybla_state,
 

net/ipv4/tcp_input.c

@@ -2394,10 +2394,7 @@ static void tcp_undo_cwnd_reduction(struct sock *sk, bool unmark_loss)
 	if (tp->prior_ssthresh) {
 		const struct inet_connection_sock *icsk = inet_csk(sk);
 
-		if (icsk->icsk_ca_ops->undo_cwnd)
 		tp->snd_cwnd = icsk->icsk_ca_ops->undo_cwnd(sk);
-		else
-			tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh << 1);
 
 		if (tp->prior_ssthresh > tp->snd_ssthresh) {
 			tp->snd_ssthresh = tp->prior_ssthresh;

net/ipv4/tcp_lp.c

@@ -316,6 +316,7 @@ static void tcp_lp_pkts_acked(struct sock *sk, const struct ack_sample *sample)
 static struct tcp_congestion_ops tcp_lp __read_mostly = {
 	.init = tcp_lp_init,
 	.ssthresh = tcp_reno_ssthresh,
+	.undo_cwnd = tcp_reno_undo_cwnd,
 	.cong_avoid = tcp_lp_cong_avoid,
 	.pkts_acked = tcp_lp_pkts_acked,
 

net/ipv4/tcp_vegas.c

@@ -307,6 +307,7 @@ EXPORT_SYMBOL_GPL(tcp_vegas_get_info);
 static struct tcp_congestion_ops tcp_vegas __read_mostly = {
 	.init		= tcp_vegas_init,
 	.ssthresh	= tcp_reno_ssthresh,
+	.undo_cwnd	= tcp_reno_undo_cwnd,
 	.cong_avoid	= tcp_vegas_cong_avoid,
 	.pkts_acked	= tcp_vegas_pkts_acked,
 	.set_state	= tcp_vegas_state,

net/ipv4/tcp_westwood.c

@@ -278,6 +278,7 @@ static struct tcp_congestion_ops tcp_westwood __read_mostly = {
 	.init		= tcp_westwood_init,
 	.ssthresh	= tcp_reno_ssthresh,
 	.cong_avoid	= tcp_reno_cong_avoid,
+	.undo_cwnd      = tcp_reno_undo_cwnd,
 	.cwnd_event	= tcp_westwood_event,
 	.in_ack_event	= tcp_westwood_ack,
 	.get_info	= tcp_westwood_info,