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tcp_cubic: tweak Hystart detection for short RTT flows

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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>
This commit is contained in:
Eric Dumazet 2019-12-23 12:27:53 -08:00 committed by David S. Miller
parent cff04e2da3
commit 42f3a8aaae
1 changed files with 21 additions and 2 deletions
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23
net/ipv4/tcp_cubic.c
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@ -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) &&