From 42f3a8aaae66d31d87850fb4b02979a0fc5dc541 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Mon, 23 Dec 2019 12:27:53 -0800 Subject: 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 Signed-off-by: David S. Miller --- net/ipv4/tcp_cubic.c | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-) (limited to 'net/ipv4/tcp_cubic.c') diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c index 068775b91fb5..0e5428ed04fe 100644 --- 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) && -- cgit v1.2.3-59-g8ed1b