tcp: refactor DCTCP ECN ACK handling

DCTCP has two parts - a new ECN signalling mechanism and the response
function to it. The first part can be used by other congestion
control for DCTCP-ECN deployed networks. This patch moves that part
into a separate tcp_dctcp.h to be used by other congestion control
module (like how Yeah uses Vegas algorithmas). For example, BBR is
experimenting such ECN signal currently
https://tinyurl.com/ietf-102-iccrg-bbr2

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Yousuk Seung <ysseung@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_dctcp.c

@@ -44,6 +44,7 @@
 #include <linux/mm.h>
 #include <net/tcp.h>
 #include <linux/inet_diag.h>
+#include "tcp_dctcp.h"
 
 #define DCTCP_MAX_ALPHA	1024U
 
@@ -118,54 +119,6 @@ static u32 dctcp_ssthresh(struct sock *sk)
 	return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U);
 }
 
-/* Minimal DCTP CE state machine:
- *
- * S:	0 <- last pkt was non-CE
- *	1 <- last pkt was CE
- */
-
-static void dctcp_ce_state_0_to_1(struct sock *sk)
-{
-	struct dctcp *ca = inet_csk_ca(sk);
-	struct tcp_sock *tp = tcp_sk(sk);
-
-	if (!ca->ce_state) {
-		/* State has changed from CE=0 to CE=1, force an immediate
-		 * ACK to reflect the new CE state. If an ACK was delayed,
-		 * send that first to reflect the prior CE state.
-		 */
-		if (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_TIMER)
-			__tcp_send_ack(sk, ca->prior_rcv_nxt);
-		inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
-	}
-
-	ca->prior_rcv_nxt = tp->rcv_nxt;
-	ca->ce_state = 1;
-
-	tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
-}
-
-static void dctcp_ce_state_1_to_0(struct sock *sk)
-{
-	struct dctcp *ca = inet_csk_ca(sk);
-	struct tcp_sock *tp = tcp_sk(sk);
-
-	if (ca->ce_state) {
-		/* State has changed from CE=1 to CE=0, force an immediate
-		 * ACK to reflect the new CE state. If an ACK was delayed,
-		 * send that first to reflect the prior CE state.
-		 */
-		if (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_TIMER)
-			__tcp_send_ack(sk, ca->prior_rcv_nxt);
-		inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
-	}
-
-	ca->prior_rcv_nxt = tp->rcv_nxt;
-	ca->ce_state = 0;
-
-	tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
-}
-
 static void dctcp_update_alpha(struct sock *sk, u32 flags)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
@@ -230,12 +183,12 @@ static void dctcp_state(struct sock *sk, u8 new_state)
 
 static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
 {
+	struct dctcp *ca = inet_csk_ca(sk);
+
 	switch (ev) {
 	case CA_EVENT_ECN_IS_CE:
-		dctcp_ce_state_0_to_1(sk);
-		break;
 	case CA_EVENT_ECN_NO_CE:
-		dctcp_ce_state_1_to_0(sk);
+		dctcp_ece_ack_update(sk, ev, &ca->prior_rcv_nxt, &ca->ce_state);
 		break;
 	default:
 		/* Don't care for the rest. */

net/ipv4/tcp_dctcp.h

@@ -0,0 +1,40 @@
+#ifndef _TCP_DCTCP_H
+#define _TCP_DCTCP_H
+
+static inline void dctcp_ece_ack_cwr(struct sock *sk, u32 ce_state)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (ce_state == 1)
+		tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
+	else
+		tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
+}
+
+/* Minimal DCTP CE state machine:
+ *
+ * S:	0 <- last pkt was non-CE
+ *	1 <- last pkt was CE
+ */
+static inline void dctcp_ece_ack_update(struct sock *sk, enum tcp_ca_event evt,
+					u32 *prior_rcv_nxt, u32 *ce_state)
+{
+	u32 new_ce_state = (evt == CA_EVENT_ECN_IS_CE) ? 1 : 0;
+
+	if (*ce_state != new_ce_state) {
+		/* CE state has changed, force an immediate ACK to
+		 * reflect the new CE state. If an ACK was delayed,
+		 * send that first to reflect the prior CE state.
+		 */
+		if (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_TIMER) {
+			dctcp_ece_ack_cwr(sk, *ce_state);
+			__tcp_send_ack(sk, *prior_rcv_nxt);
+		}
+		inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
+	}
+	*prior_rcv_nxt = tcp_sk(sk)->rcv_nxt;
+	*ce_state = new_ce_state;
+	dctcp_ece_ack_cwr(sk, new_ce_state);
+}
+
+#endif