tcp: avoid cwnd moderation in undo

In the current undo logic, cwnd is moderated after it was restored
to the value prior entering fast-recovery. It was moderated first
in tcp_try_undo_recovery then again in tcp_complete_cwr.

Since the undo indicates recovery was false, these moderations
are not necessary. If the undo is triggered when most of the
outstanding data have been acknowledged, the (restored) cwnd is
falsely pulled down to a small value.

This patch removes these cwnd moderations if cwnd is undone
  a) during fast-recovery
	b) by receiving DSACKs past fast-recovery
Signed-off-by: NYuchung Cheng <ycheng@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -2659,7 +2659,7 @@ static void DBGUNDO(struct sock *sk, const char *msg)
 #define DBGUNDO(x...) do { } while (0)
 #endif
 
-static void tcp_undo_cwr(struct sock *sk, const int undo)
+static void tcp_undo_cwr(struct sock *sk, const int undo_ssthresh)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
@@ -2671,14 +2671,13 @@ static void tcp_undo_cwr(struct sock *sk, const int undo)
 		else
 			tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh << 1);
 
-		if (undo && tp->prior_ssthresh > tp->snd_ssthresh) {
+		if (undo_ssthresh && tp->prior_ssthresh > tp->snd_ssthresh) {
 			tp->snd_ssthresh = tp->prior_ssthresh;
 			TCP_ECN_withdraw_cwr(tp);
 		}
 	} else {
 		tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh);
 	}
-	tcp_moderate_cwnd(tp);
 	tp->snd_cwnd_stamp = tcp_time_stamp;
 }
 
@@ -2822,8 +2821,11 @@ static int tcp_try_undo_loss(struct sock *sk)
 static inline void tcp_complete_cwr(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
-	tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
-	tp->snd_cwnd_stamp = tcp_time_stamp;
+	/* Do not moderate cwnd if it's already undone in cwr or recovery */
+	if (tp->undo_marker && tp->snd_cwnd > tp->snd_ssthresh) {
+		tp->snd_cwnd = tp->snd_ssthresh;
+		tp->snd_cwnd_stamp = tcp_time_stamp;
+	}
 	tcp_ca_event(sk, CA_EVENT_COMPLETE_CWR);
 }