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 8b5ba0a5cd386..93530bd332470 100644 --- a/net/ipv4/tcp_cubic.c +++ b/net/ipv4/tcp_cubic.c @@ -340,8 +340,6 @@ 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; @@ -383,6 +381,9 @@ 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();