net-tcp: add new ca opts flag TCP_CONG_WANTS_CE_EVENTS

Add a a new ca opts flag TCP_CONG_WANTS_CE_EVENTS that allows a
congestion control module to receive CE events.

Currently congestion control modules have to set the TCP_CONG_NEEDS_ECN
bit in opts flag to receive CE events but this may incur changes in ECN
behavior elsewhere. This patch adds a new bit TCP_CONG_WANTS_CE_EVENTS
that allows congestion control modules to receive CE events
independently of TCP_CONG_NEEDS_ECN.

Effort: net-tcp
Origin-9xx-SHA1: 9f7e14716cde760bc6c67ef8ef7e1ee48501d95b
Change-Id: I2255506985242f376d910c6fd37daabaf4744f24

include/net/tcp.h

@@ -1011,7 +1011,11 @@ enum tcp_ca_ack_event_flags {
 #define TCP_CONG_NON_RESTRICTED 0x1
 /* Requires ECN/ECT set on all packets */
 #define TCP_CONG_NEEDS_ECN	0x2
-#define TCP_CONG_MASK	(TCP_CONG_NON_RESTRICTED | TCP_CONG_NEEDS_ECN)
+/* Wants notification of CE events (CA_EVENT_ECN_IS_CE, CA_EVENT_ECN_NO_CE). */
+#define TCP_CONG_WANTS_CE_EVENTS	0x4
+#define TCP_CONG_MASK	(TCP_CONG_NON_RESTRICTED | \
+			 TCP_CONG_NEEDS_ECN | \
+			 TCP_CONG_WANTS_CE_EVENTS)
 
 union tcp_cc_info;
 
@@ -1138,6 +1142,14 @@ static inline char *tcp_ca_get_name_by_key(u32 key, char *buffer)
 }
 #endif
 
+static inline bool tcp_ca_wants_ce_events(const struct sock *sk)
+{
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+
+	return icsk->icsk_ca_ops->flags & (TCP_CONG_NEEDS_ECN |
+					   TCP_CONG_WANTS_CE_EVENTS);
+}
+
 static inline bool tcp_ca_needs_ecn(const struct sock *sk)
 {
 	const struct inet_connection_sock *icsk = inet_csk(sk);

net/ipv4/tcp_input.c

@@ -348,7 +348,7 @@ static void __tcp_ecn_check_ce(struct sock *sk, const struct sk_buff *skb)
 			tcp_enter_quickack_mode(sk, 2);
 		break;
 	case INET_ECN_CE:
-		if (tcp_ca_needs_ecn(sk))
+		if (tcp_ca_wants_ce_events(sk))
 			tcp_ca_event(sk, CA_EVENT_ECN_IS_CE);
 
 		if (!(tp->ecn_flags & TCP_ECN_DEMAND_CWR)) {
@@ -359,7 +359,7 @@ static void __tcp_ecn_check_ce(struct sock *sk, const struct sk_buff *skb)
 		tp->ecn_flags |= TCP_ECN_SEEN;
 		break;
 	default:
-		if (tcp_ca_needs_ecn(sk))
+		if (tcp_ca_wants_ce_events(sk))
 			tcp_ca_event(sk, CA_EVENT_ECN_NO_CE);
 		tp->ecn_flags |= TCP_ECN_SEEN;
 		break;