[TCP]: Send ACKs each 2nd received segment.

It does not affect either mss-sized connections (obviously) or
connections controlled by Nagle (because there is only one small
segment in flight).

The idea is to record the fact that a small segment arrives on a
connection, where one small segment has already been received and
still not-ACKed. In this case ACK is forced after tcp_recvmsg() drains
receive buffer.

In other words, it is a "soft" each-2nd-segment ACK, which is enough
to preserve ACK clock even when ABC is enabled.
Signed-off-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/inet_connection_sock.h

@@ -147,7 +147,8 @@ extern struct sock *inet_csk_clone(struct sock *sk,
 enum inet_csk_ack_state_t {
 	ICSK_ACK_SCHED	= 1,
 	ICSK_ACK_TIMER  = 2,
-	ICSK_ACK_PUSHED = 4
+	ICSK_ACK_PUSHED = 4,
+	ICSK_ACK_PUSHED2 = 8
 };
 
 extern void inet_csk_init_xmit_timers(struct sock *sk,

net/ipv4/tcp.c

@@ -955,8 +955,11 @@ void tcp_cleanup_rbuf(struct sock *sk, int copied)
 		     * receive buffer and there was a small segment
 		     * in queue.
 		     */
-		    (copied > 0 && (icsk->icsk_ack.pending & ICSK_ACK_PUSHED) &&
-		     !icsk->icsk_ack.pingpong && !atomic_read(&sk->sk_rmem_alloc)))
+		    (copied > 0 &&
+		     ((icsk->icsk_ack.pending & ICSK_ACK_PUSHED2) ||
+		      ((icsk->icsk_ack.pending & ICSK_ACK_PUSHED) &&
+		       !icsk->icsk_ack.pingpong)) &&
+		      !atomic_read(&sk->sk_rmem_alloc)))
 			time_to_ack = 1;
 	}
 

net/ipv4/tcp_input.c

@@ -156,6 +156,8 @@ static void tcp_measure_rcv_mss(struct sock *sk,
 				return;
 			}
 		}
+		if (icsk->icsk_ack.pending & ICSK_ACK_PUSHED)
+			icsk->icsk_ack.pending |= ICSK_ACK_PUSHED2;
 		icsk->icsk_ack.pending |= ICSK_ACK_PUSHED;
 	}
 }