| From 810f6c378b36ed06baba501728ff11403472f3b4 Mon Sep 17 00:00:00 2001 |
| From: Sasha Levin <sashal@kernel.org> |
| Date: Tue, 23 Nov 2021 12:25:35 -0800 |
| Subject: tcp_cubic: fix spurious Hystart ACK train detections for |
| not-cwnd-limited flows |
| |
| From: Eric Dumazet <edumazet@google.com> |
| |
| [ Upstream commit 4e1fddc98d2585ddd4792b5e44433dcee7ece001 ] |
| |
| While testing BIG TCP patch series, I was expecting that TCP_RR workloads |
| with 80KB requests/answers would send one 80KB TSO packet, |
| then being received as a single GRO packet. |
| |
| It turns out this was not happening, and the root cause was that |
| cubic Hystart ACK train was triggering after a few (2 or 3) rounds of RPC. |
| |
| Hystart was wrongly setting CWND/SSTHRESH to 30, while my RPC |
| needed a budget of ~20 segments. |
| |
| Ideally these TCP_RR flows should not exit slow start. |
| |
| Cubic Hystart should reset itself at each round, instead of assuming |
| every TCP flow is a bulk one. |
| |
| Note that even after this patch, Hystart can still trigger, depending |
| on scheduling artifacts, but at a higher CWND/SSTHRESH threshold, |
| keeping optimal TSO packet sizes. |
| |
| Tested: |
| |
| ip link set dev eth0 gro_ipv6_max_size 131072 gso_ipv6_max_size 131072 |
| nstat -n; netperf -H ... -t TCP_RR -l 5 -- -r 80000,80000 -K cubic; nstat|egrep "Ip6InReceives|Hystart|Ip6OutRequests" |
| |
| Before: |
| |
| 8605 |
| Ip6InReceives 87541 0.0 |
| Ip6OutRequests 129496 0.0 |
| TcpExtTCPHystartTrainDetect 1 0.0 |
| TcpExtTCPHystartTrainCwnd 30 0.0 |
| |
| After: |
| |
| 8760 |
| Ip6InReceives 88514 0.0 |
| Ip6OutRequests 87975 0.0 |
| |
| Fixes: ae27e98a5152 ("[TCP] CUBIC v2.3") |
| Co-developed-by: Neal Cardwell <ncardwell@google.com> |
| Signed-off-by: Neal Cardwell <ncardwell@google.com> |
| Signed-off-by: Eric Dumazet <edumazet@google.com> |
| Cc: Stephen Hemminger <stephen@networkplumber.org> |
| Cc: Yuchung Cheng <ycheng@google.com> |
| Cc: Soheil Hassas Yeganeh <soheil@google.com> |
| Link: https://lore.kernel.org/r/20211123202535.1843771-1-eric.dumazet@gmail.com |
| Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
| Signed-off-by: Sasha Levin <sashal@kernel.org> |
| --- |
| net/ipv4/tcp_cubic.c | 5 +++-- |
| 1 file changed, 3 insertions(+), 2 deletions(-) |
| |
| diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c |
| index 9fb3a5e83a7c7..e0b3b194b6049 100644 |
| --- a/net/ipv4/tcp_cubic.c |
| +++ b/net/ipv4/tcp_cubic.c |
| @@ -342,8 +342,6 @@ static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked) |
| return; |
| |
| if (tcp_in_slow_start(tp)) { |
| - if (hystart && after(ack, ca->end_seq)) |
| - bictcp_hystart_reset(sk); |
| acked = tcp_slow_start(tp, acked); |
| if (!acked) |
| return; |
| @@ -394,6 +392,9 @@ static void hystart_update(struct sock *sk, u32 delay) |
| if (ca->found & hystart_detect) |
| return; |
| |
| + if (after(tp->snd_una, ca->end_seq)) |
| + bictcp_hystart_reset(sk); |
| + |
| if (hystart_detect & HYSTART_ACK_TRAIN) { |
| u32 now = bictcp_clock(); |
| |
| -- |
| 2.33.0 |
| |
