tcp: reduce the bloat caused by tcp_is_cwnd_limited()

tcp_is_cwnd_limited() allows GSO/TSO enabled flows to increase
their cwnd to allow a full size (64KB) TSO packet to be sent.

Non GSO flows only allow an extra room of 3 MSS.

For most flows with a BDP below 10 MSS, this results in a bloat
of cwnd reaching 90, and an inflate of RTT.

Thanks to TSO auto sizing, we can restrict the bloat to the number
of MSS contained in a TSO packet (tp->xmit_size_goal_segs), to keep
original intent without performance impact.

Because we keep cwnd small, it helps to keep TSO packet size to their
optimal value.

Example for a 10Mbit flow, with low TCP Small queue limits (no more than
2 skb in qdisc/device tx ring)

Before patch :

lpk51:~# ./ss -i dst lpk52:44862 | grep cwnd
         cubic wscale:6,6 rto:215 rtt:15.875/2.5 mss:1448 cwnd:96
ssthresh:96
send 70.1Mbps unacked:14 rcv_space:29200

After patch :

lpk51:~# ./ss -i dst lpk52:52916 | grep cwnd
         cubic wscale:6,6 rto:206 rtt:5.206/0.036 mss:1448 cwnd:15
ssthresh:14
send 33.4Mbps unacked:4 rcv_space:29200
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Nandita Dukkipati <nanditad@google.com>
Cc: Van Jacobson <vanj@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_cong.c

@@ -290,8 +290,7 @@ bool tcp_is_cwnd_limited(const struct sock *sk, u32 in_flight)
 	left = tp->snd_cwnd - in_flight;
 	if (sk_can_gso(sk) &&
 	    left * sysctl_tcp_tso_win_divisor < tp->snd_cwnd &&
-	    left * tp->mss_cache < sk->sk_gso_max_size &&
-	    left < sk->sk_gso_max_segs)
+	    left < tp->xmit_size_goal_segs)
 		return true;
 	return left <= tcp_max_tso_deferred_mss(tp);
 }