Commit 42f3a8aa authored Dec 23, 2019 by Eric Dumazet Committed by David S. Miller Dec 27, 2019

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>

parent cff04e2d

net/ipv4/tcp_cubic.c

+21 −2

Original line number	Diff line number	Diff line
		@@ -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) &&

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