tcp: Refactor pingpong code

Instead of using pingpong as a single bit information, we refactor the
code to treat it as a counter. When interactive session is detected,
we set pingpong count to TCP_PINGPONG_THRESH. And when pingpong count
is >= TCP_PINGPONG_THRESH, we consider the session in pingpong mode.

This patch is a pure refactor and sets foundation for the next patch.
This patch itself does not change any pingpong logic.

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>
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 12ba214..6f8d292 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1551,7 +1551,7 @@
 		    (copied > 0 &&
 		     ((icsk->icsk_ack.pending & ICSK_ACK_PUSHED2) ||
 		      ((icsk->icsk_ack.pending & ICSK_ACK_PUSHED) &&
-		       !icsk->icsk_ack.pingpong)) &&
+		       !inet_csk_in_pingpong_mode(sk))) &&
 		      !atomic_read(&sk->sk_rmem_alloc)))
 			time_to_ack = true;
 	}
@@ -2984,16 +2984,16 @@
 
 	case TCP_QUICKACK:
 		if (!val) {
-			icsk->icsk_ack.pingpong = 1;
+			inet_csk_enter_pingpong_mode(sk);
 		} else {
-			icsk->icsk_ack.pingpong = 0;
+			inet_csk_exit_pingpong_mode(sk);
 			if ((1 << sk->sk_state) &
 			    (TCPF_ESTABLISHED | TCPF_CLOSE_WAIT) &&
 			    inet_csk_ack_scheduled(sk)) {
 				icsk->icsk_ack.pending |= ICSK_ACK_PUSHED;
 				tcp_cleanup_rbuf(sk, 1);
 				if (!(val & 1))
-					icsk->icsk_ack.pingpong = 1;
+					inet_csk_enter_pingpong_mode(sk);
 			}
 		}
 		break;
@@ -3407,7 +3407,7 @@
 		return 0;
 	}
 	case TCP_QUICKACK:
-		val = !icsk->icsk_ack.pingpong;
+		val = !inet_csk_in_pingpong_mode(sk);
 		break;
 
 	case TCP_CONGESTION: