dctcp: update cwnd on congestion event

draft-ietf-tcpm-dctcp-02 says:

... when the sender receives an indication of congestion
(ECE), the sender SHOULD update cwnd as follows:

         cwnd = cwnd * (1 - DCTCP.Alpha / 2)

So, lets do this and reduce cwnd more smoothly (and faster), as per
current congestion estimate.

Cc: Lawrence Brakmo <brakmo@fb.com>
Cc: Andrew Shewmaker <agshew@gmail.com>
Cc: Glenn Judd <glenn.judd@morganstanley.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_dctcp.c

@@ -188,8 +188,8 @@ static void dctcp_ce_state_1_to_0(struct sock *sk)
 
 static void dctcp_update_alpha(struct sock *sk, u32 flags)
 {
-	const struct tcp_sock *tp = tcp_sk(sk);
 	struct dctcp *ca = inet_csk_ca(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
 	u32 acked_bytes = tp->snd_una - ca->prior_snd_una;
 
 	/* If ack did not advance snd_una, count dupack as MSS size.
@@ -229,6 +229,13 @@ static void dctcp_update_alpha(struct sock *sk, u32 flags)
 		WRITE_ONCE(ca->dctcp_alpha, alpha);
 		dctcp_reset(tp, ca);
 	}
+
+	if (flags & CA_ACK_ECE) {
+		unsigned int cwnd = dctcp_ssthresh(sk);
+
+		if (cwnd != tp->snd_cwnd)
+			tp->snd_cwnd = cwnd;
+	}
 }
 
 static void dctcp_state(struct sock *sk, u8 new_state)