This is a note to let you know that I've just added the patch titled
tcp: derive delack_max from rto_min
to the 6.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
tcp-derive-delack_max-from-rto_min.patch
and it can be found in the queue-6.6 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 6bf958a473c640a661d24e4ffcdd1adb26bd6a41
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date: Wed Sep 20 17:29:43 2023 +0000
tcp: derive delack_max from rto_min
[ Upstream commit bbf80d713fe75cfbecda26e7c03a9a8d22af2f4f ]
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 <edumazet@xxxxxxxxxx>
Acked-by: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx>
Acked-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/include/net/tcp.h b/include/net/tcp.h
index a88bf8f6db235..9cbcfb3c95dac 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -723,6 +723,8 @@ static inline void tcp_fast_path_check(struct sock *sk)
tcp_fast_path_on(tp);
}
+u32 tcp_delack_max(const struct sock *sk);
+
/* Compute the actual rto_min value */
static inline u32 tcp_rto_min(struct sock *sk)
{
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index ec46d74c20938..f124f6c639157 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3774,7 +3774,8 @@ 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;
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index cab3c1162c3a6..ab3b7b4b4429b 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -4003,6 +4003,20 @@ int tcp_connect(struct sock *sk)
}
EXPORT_SYMBOL(tcp_connect);
+u32 tcp_delack_max(const struct sock *sk)
+{
+ const struct dst_entry *dst = __sk_dst_get(sk);
+ u32 delack_max = inet_csk(sk)->icsk_delack_max;
+
+ if (dst && dst_metric_locked(dst, RTAX_RTO_MIN)) {
+ u32 rto_min = dst_metric_rtt(dst, RTAX_RTO_MIN);
+ u32 delack_from_rto_min = max_t(int, 1, rto_min - 1);
+
+ delack_max = min_t(u32, delack_max, delack_from_rto_min);
+ }
+ return delack_max;
+}
+
/* Send out a delayed ack, the caller does the policy checking
* to see if we should even be here. See tcp_input.c:tcp_ack_snd_check()
* for details.
@@ -4038,7 +4052,7 @@ void tcp_send_delayed_ack(struct sock *sk)
ato = min(ato, max_ato);
}
- ato = min_t(u32, ato, inet_csk(sk)->icsk_delack_max);
+ ato = min_t(u32, ato, tcp_delack_max(sk));
/* Stay within the limit we were given */
timeout = jiffies + ato;
