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>

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 068775b..0e5428e 100644 (file)
--- a/net/ipv4/tcp_cubic.c
+++ b/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)
-               ca->delay_min = delay;
+       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) &&