[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>

3 files changed, 9 insertions, 3 deletions

diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
index 9bf73fe5094..de4e83b6da4 100644
--- a/include/net/inet_connection_sock.h
+++ b/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,
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 29e3d606db7..66e9a729f6d 100644
--- a/net/ipv4/tcp.c
+++ b/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;
         }
 
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 511b738f118..b3def0df14f 100644
--- a/net/ipv4/tcp_input.c
+++ b/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;
         }
 }