tcp: add support for PLB in DCTCP

PLB support is added to TCP DCTCP code. As DCTCP uses ECN as the
congestion signal, PLB also uses ECN to make decisions whether to change
the path or not upon sustained congestion.
Signed-off-by: Mubashir Adnan Qureshi <mubashirq@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_dctcp.c

@@ -54,6 +54,7 @@ struct dctcp {
 	u32 next_seq;
 	u32 ce_state;
 	u32 loss_cwnd;
+	struct tcp_plb_state plb;
 };
 
 static unsigned int dctcp_shift_g __read_mostly = 4; /* g = 1/2^4 */
@@ -91,6 +92,8 @@ static void dctcp_init(struct sock *sk)
 		ca->ce_state = 0;
 
 		dctcp_reset(tp, ca);
+		tcp_plb_init(sk, &ca->plb);
+
 		return;
 	}
 
@@ -117,14 +120,28 @@ static void dctcp_update_alpha(struct sock *sk, u32 flags)
 
 	/* Expired RTT */
 	if (!before(tp->snd_una, ca->next_seq)) {
+		u32 delivered = tp->delivered - ca->old_delivered;
 		u32 delivered_ce = tp->delivered_ce - ca->old_delivered_ce;
 		u32 alpha = ca->dctcp_alpha;
+		u32 ce_ratio = 0;
+
+		if (delivered > 0) {
+			/* dctcp_alpha keeps EWMA of fraction of ECN marked
+			 * packets. Because of EWMA smoothing, PLB reaction can
+			 * be slow so we use ce_ratio which is an instantaneous
+			 * measure of congestion. ce_ratio is the fraction of
+			 * ECN marked packets in the previous RTT.
+			 */
+			if (delivered_ce > 0)
+				ce_ratio = (delivered_ce << TCP_PLB_SCALE) / delivered;
+			tcp_plb_update_state(sk, &ca->plb, (int)ce_ratio);
+			tcp_plb_check_rehash(sk, &ca->plb);
+		}
 
 		/* alpha = (1 - g) * alpha + g * F */
 
 		alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
 		if (delivered_ce) {
-			u32 delivered = tp->delivered - ca->old_delivered;
 
 			/* If dctcp_shift_g == 1, a 32bit value would overflow
 			 * after 8 M packets.
@@ -172,8 +189,12 @@ static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
 		dctcp_ece_ack_update(sk, ev, &ca->prior_rcv_nxt, &ca->ce_state);
 		break;
 	case CA_EVENT_LOSS:
+		tcp_plb_update_state_upon_rto(sk, &ca->plb);
 		dctcp_react_to_loss(sk);
 		break;
+	case CA_EVENT_TX_START:
+		tcp_plb_check_rehash(sk, &ca->plb); /* Maybe rehash when inflight is 0 */
+		break;
 	default:
 		/* Don't care for the rest. */
 		break;