tcp_cubic: tweak Hystart detection for short RTT flows

After switching ca->delay_min to usec resolution, we exit
slow start prematurely for very low RTT flows, setting
snd_ssthresh to 20.

The reason is that delay_min is fed with RTT of small packet
trains. Then as cwnd is increased, TCP sends bigger TSO packets.

LRO/GRO aggregation and/or interrupt mitigation strategies
on receiver tend to inflate RTT samples.

Fix this by adding to delay_min the expected delay of
two TSO packets, given current pacing rate.

Tested:

Sender uses pfifo_fast qdisc

Before :
$ nstat -n;for f in {1..10}; do ./super_netperf 1 -H lpaa24 -l -4000000; done;nstat|egrep "Hystart"
  11348
  11707
  11562
  11428
  11773
  11534
   9878
  11693
  10597
  10968
TcpExtTCPHystartTrainDetect     10                 0.0
TcpExtTCPHystartTrainCwnd       200                0.0

After :
$ nstat -n;for f in {1..10}; do ./super_netperf 1 -H lpaa24 -l -4000000; done;nstat|egrep "Hystart"
  14877
  14517
  15797
  18466
  17376
  14833
  17558
  17933
  16039
  18059
TcpExtTCPHystartTrainDetect     10                 0.0
TcpExtTCPHystartTrainCwnd       1670               0.0
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_cubic.c

@@ -436,8 +436,27 @@ static void bictcp_acked(struct sock *sk, const struct ack_sample *sample)
 		delay = 1;
 
 	/* first time call or link delay decreases */
+	if (ca->delay_min == 0 || ca->delay_min > delay) {
+		unsigned long rate = READ_ONCE(sk->sk_pacing_rate);
+
+		/* Account for TSO/GRO delays.
+		 * Otherwise short RTT flows could get too small ssthresh,
+		 * since during slow start we begin with small TSO packets
+		 * and could lower ca->delay_min too much.
+		 * Ideally even with a very small RTT we would like to have
+		 * at least one TSO packet being sent and received by GRO,
+		 * and another one in qdisc layer.
+		 * We apply another 100% factor because @rate is doubled at
+		 * this point.
+		 * We cap the cushion to 1ms.
+		 */
+		if (rate)
+			delay += min_t(u64, USEC_PER_MSEC,
+				       div64_ul((u64)GSO_MAX_SIZE *
+						4 * USEC_PER_SEC, rate));
 		if (ca->delay_min == 0 || ca->delay_min > delay)
 			ca->delay_min = delay;
+	}
 
 	/* hystart triggers when cwnd is larger than some threshold */
 	if (!ca->found && hystart && tcp_in_slow_start(tp) &&