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 <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -718,6 +718,8 @@ 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(const struct sock *sk)
 {

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;
 

net/ipv4/tcp_output.c

@@ -3977,6 +3977,20 @@ 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.
@@ -4012,7 +4026,7 @@ 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;