From bbf80d713fe75cfbecda26e7c03a9a8d22af2f4f Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 20 Sep 2023 17:29:43 +0000 Subject: tcp: derive delack_max from rto_min While BPF allows to set icsk->->icsk_delack_max and/or icsk->icsk_rto_min, we have an ip route attribute (RTAX_RTO_MIN) to be able to tune rto_min, but nothing to consequently adjust max delayed ack, which vary from 40ms to 200 ms (TCP_DELACK_{MIN|MAX}). This makes RTAX_RTO_MIN of almost no practical use, unless customers are in big trouble. Modern days datacenter communications want to set rto_min to ~5 ms, and the max delayed ack one jiffie smaller to avoid spurious retransmits. After this patch, an "rto_min 5" route attribute will effectively lower max delayed ack timers to 4 ms. Note in the following ss output, "rto:6 ... ato:4" $ ss -temoi dst XXXXXX State Recv-Q Send-Q Local Address:Port Peer Address:Port Process ESTAB 0 0 [2002:a05:6608:295::]:52950 [2002:a05:6608:297::]:41597 ino:255134 sk:1001 <-> skmem:(r0,rb1707063,t872,tb262144,f0,w0,o0,bl0,d0) ts sack cubic wscale:8,8 rto:6 rtt:0.02/0.002 ato:4 mss:4096 pmtu:4500 rcvmss:536 advmss:4096 cwnd:10 bytes_sent:54823160 bytes_acked:54823121 bytes_received:54823120 segs_out:1370582 segs_in:1370580 data_segs_out:1370579 data_segs_in:1370578 send 16.4Gbps pacing_rate 32.6Gbps delivery_rate 1.72Gbps delivered:1370579 busy:26920ms unacked:1 rcv_rtt:34.615 rcv_space:65920 rcv_ssthresh:65535 minrtt:0.015 snd_wnd:65536 While we could argue this patch fixes a bug with RTAX_RTO_MIN, I do not add a Fixes: tag, so that we can soak it a bit before asking backports to stable branches. Signed-off-by: Eric Dumazet Acked-by: Soheil Hassas Yeganeh Acked-by: Neal Cardwell Signed-off-by: David S. Miller --- net/ipv4/tcp.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'net/ipv4/tcp.c') diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index 69b8d7073708..e54f91eb943b 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -3762,7 +3762,8 @@ void tcp_get_info(struct sock *sk, struct tcp_info *info) info->tcpi_options |= TCPI_OPT_SYN_DATA; info->tcpi_rto = jiffies_to_usecs(icsk->icsk_rto); - info->tcpi_ato = jiffies_to_usecs(icsk->icsk_ack.ato); + info->tcpi_ato = jiffies_to_usecs(min(icsk->icsk_ack.ato, + tcp_delack_max(sk))); info->tcpi_snd_mss = tp->mss_cache; info->tcpi_rcv_mss = icsk->icsk_ack.rcv_mss; -- cgit v1.2.3