net: tcp: split ack slow/fast events from cwnd_event

The congestion control ops "cwnd_event" currently supports
CA_EVENT_FAST_ACK and CA_EVENT_SLOW_ACK events (among others).
Both FAST and SLOW_ACK are only used by Westwood congestion
control algorithm.

This removes both flags from cwnd_event and adds a new
in_ack_event callback for this. The goal is to be able to
provide more detailed information about ACKs, such as whether
ECE flag was set, or whether the ACK resulted in a window
update.

It is required for DataCenter TCP (DCTCP) congestion control
algorithm as it makes a different choice depending on ECE being
set or not.

Joint work with Daniel Borkmann and Glenn Judd.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Glenn Judd <glenn.judd@morganstanley.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_westwood.c

@@ -220,32 +220,35 @@ static u32 tcp_westwood_bw_rttmin(const struct sock *sk)
 	return max_t(u32, (w->bw_est * w->rtt_min) / tp->mss_cache, 2);
 }
 
+static void tcp_westwood_ack(struct sock *sk, u32 ack_flags)
+{
+	if (ack_flags & CA_ACK_SLOWPATH) {
+		struct westwood *w = inet_csk_ca(sk);
+
+		westwood_update_window(sk);
+		w->bk += westwood_acked_count(sk);
+
+		update_rtt_min(w);
+		return;
+	}
+
+	westwood_fast_bw(sk);
+}
+
 static void tcp_westwood_event(struct sock *sk, enum tcp_ca_event event)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct westwood *w = inet_csk_ca(sk);
 
 	switch (event) {
-	case CA_EVENT_FAST_ACK:
-		westwood_fast_bw(sk);
-		break;
-
 	case CA_EVENT_COMPLETE_CWR:
 		tp->snd_cwnd = tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
 		break;
-
 	case CA_EVENT_LOSS:
 		tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
 		/* Update RTT_min when next ack arrives */
 		w->reset_rtt_min = 1;
 		break;
-
-	case CA_EVENT_SLOW_ACK:
-		westwood_update_window(sk);
-		w->bk += westwood_acked_count(sk);
-		update_rtt_min(w);
-		break;
-
 	default:
 		/* don't care */
 		break;
@@ -274,6 +277,7 @@ static struct tcp_congestion_ops tcp_westwood __read_mostly = {
 	.ssthresh	= tcp_reno_ssthresh,
 	.cong_avoid	= tcp_reno_cong_avoid,
 	.cwnd_event	= tcp_westwood_event,
+	.in_ack_event	= tcp_westwood_ack,
 	.get_info	= tcp_westwood_info,
 	.pkts_acked	= tcp_westwood_pkts_acked,