tcp: change pingpong threshold to 3

In order to be more confident about an on-going interactive session, we
increment pingpong count by 1 for every interactive transaction and we
adjust TCP_PINGPONG_THRESH to 3.
This means, we only consider a session in pingpong mode after we see 3
interactive transactions, and start to activate delayed acks in quick
ack mode.
And in order to not over-count the credits, we only increase pingpong
count for the first packet sent in response for the previous received
packet.
This is mainly to prevent delaying the ack immediately after some
handshake protocol but no real interactive traffic pattern afterwards.
Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/inet_connection_sock.h

@@ -315,7 +315,7 @@ int inet_csk_compat_setsockopt(struct sock *sk, int level, int optname,
 
 struct dst_entry *inet_csk_update_pmtu(struct sock *sk, u32 mtu);
 
-#define TCP_PINGPONG_THRESH	1
+#define TCP_PINGPONG_THRESH	3
 
 static inline void inet_csk_enter_pingpong_mode(struct sock *sk)
 {
@@ -331,4 +331,12 @@ static inline bool inet_csk_in_pingpong_mode(struct sock *sk)
 {
 	return inet_csk(sk)->icsk_ack.pingpong >= TCP_PINGPONG_THRESH;
 }
+
+static inline void inet_csk_inc_pingpong_cnt(struct sock *sk)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+
+	if (icsk->icsk_ack.pingpong < U8_MAX)
+		icsk->icsk_ack.pingpong++;
+}
 #endif /* _INET_CONNECTION_SOCK_H */

net/ipv4/tcp_output.c

@@ -165,13 +165,16 @@ static void tcp_event_data_sent(struct tcp_sock *tp,
 	if (tcp_packets_in_flight(tp) == 0)
 		tcp_ca_event(sk, CA_EVENT_TX_START);
 
-	tp->lsndtime = now;
-
-	/* If it is a reply for ato after last received
-	 * packet, enter pingpong mode.
+	/* If this is the first data packet sent in response to the
+	 * previous received data,
+	 * and it is a reply for ato after last received packet,
+	 * increase pingpong count.
 	 */
-	if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
-		inet_csk_enter_pingpong_mode(sk);
+	if (before(tp->lsndtime, icsk->icsk_ack.lrcvtime) &&
+	    (u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
+		inet_csk_inc_pingpong_cnt(sk);
+
+	tp->lsndtime = now;
 }
 
 /* Account for an ACK we sent. */