[PATCH net-next 1/2] RTAX_INITRWND should be able to bring the rcv_ssthresh above 64KiB

[Marek Majkowski <marek@cloudflare.com>]

We already support RTAX_INITRWND / initrwnd path attribute:

 $ ip route change local 127.0.0.0/8 dev lo initrwnd 1024

However normally, the initial advertised receive window is limited to
64KiB by rcv_ssthresh, regardless of initrwnd. This patch changes
that, bumping up rcv_ssthresh to value derived from initrwnd. This
allows for larger initial advertised receive windows, which is useful
for specific types of TCP flows: big BDP ones, where there is a lot of
data to send immediately after the flow is established.

There are three places where we initialize sockets:
 - tcp_output:tcp_connect_init
 - tcp_minisocks:tcp_openreq_init_rwin
 - syncookies

In the first two we already have a call to `tcp_rwnd_init_bpf` and
`dst_metric(RTAX_INITRWND)` which retrieve the bpf/path initrwnd
attribute. We use this value to bring `rcv_ssthresh` up, potentially
above the traditional 64KiB.

With higher initial `rcv_ssthresh` the receiver will open the receive
window more aggresively, which can improve large BDP flows - large
throughput and latency.

This patch does not cover the syncookies case.

Signed-off-by: Marek Majkowski <marek@cloudflare.com>
---
 include/net/inet_sock.h  |  1 +
 net/ipv4/tcp_minisocks.c |  8 ++++++--
 net/ipv4/tcp_output.c    | 10 ++++++++--
 3 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/include/net/inet_sock.h b/include/net/inet_sock.h
index daead5fb389a..bc68c9b70942 100644
--- a/include/net/inet_sock.h
+++ b/include/net/inet_sock.h
@@ -89,6 +89,7 @@ struct inet_request_sock {
 				no_srccheck: 1,
 				smc_ok	   : 1;
 	u32                     ir_mark;
+	u32                     rcv_ssthresh;
 	union {
 		struct ip_options_rcu __rcu	*ireq_opt;
 #if IS_ENABLED(CONFIG_IPV6)
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 6854bb1fb32b..89ba2a30a012 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -360,6 +360,7 @@ void tcp_openreq_init_rwin(struct request_sock *req,
 	u32 window_clamp;
 	__u8 rcv_wscale;
 	u32 rcv_wnd;
+	int adj_mss;
 	int mss;
 
 	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
@@ -378,15 +379,18 @@ void tcp_openreq_init_rwin(struct request_sock *req,
 	else if (full_space < rcv_wnd * mss)
 		full_space = rcv_wnd * mss;
 
+	adj_mss = mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0);
+
 	/* tcp_full_space because it is guaranteed to be the first packet */
 	tcp_select_initial_window(sk_listener, full_space,
-		mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
+		adj_mss,
 		&req->rsk_rcv_wnd,
 		&req->rsk_window_clamp,
 		ireq->wscale_ok,
 		&rcv_wscale,
 		rcv_wnd);
 	ireq->rcv_wscale = rcv_wscale;
+	ireq->rcv_ssthresh = max(req->rsk_rcv_wnd, rcv_wnd * adj_mss);
 }
 EXPORT_SYMBOL(tcp_openreq_init_rwin);
 
@@ -502,7 +506,7 @@ struct sock *tcp_create_openreq_child(const struct sock *sk,
 	newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
 	newtp->rx_opt.sack_ok = ireq->sack_ok;
 	newtp->window_clamp = req->rsk_window_clamp;
-	newtp->rcv_ssthresh = req->rsk_rcv_wnd;
+	newtp->rcv_ssthresh = ireq->rcv_ssthresh;
 	newtp->rcv_wnd = req->rsk_rcv_wnd;
 	newtp->rx_opt.wscale_ok = ireq->wscale_ok;
 	if (newtp->rx_opt.wscale_ok) {
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 18c913a2347a..0f2d4174ea59 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3642,6 +3642,7 @@ static void tcp_connect_init(struct sock *sk)
 	struct tcp_sock *tp = tcp_sk(sk);
 	__u8 rcv_wscale;
 	u32 rcv_wnd;
+	u32 mss;
 
 	/* We'll fix this up when we get a response from the other end.
 	 * See tcp_input.c:tcp_rcv_state_process case TCP_SYN_SENT.
@@ -3679,8 +3680,10 @@ static void tcp_connect_init(struct sock *sk)
 	if (rcv_wnd == 0)
 		rcv_wnd = dst_metric(dst, RTAX_INITRWND);
 
+	mss = tp->advmss - (tp->rx_opt.ts_recent_stamp ?
+			    tp->tcp_header_len - sizeof(struct tcphdr) : 0);
 	tcp_select_initial_window(sk, tcp_full_space(sk),
-				  tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
+				  mss,
 				  &tp->rcv_wnd,
 				  &tp->window_clamp,
 				  sock_net(sk)->ipv4.sysctl_tcp_window_scaling,
@@ -3688,7 +3691,10 @@ static void tcp_connect_init(struct sock *sk)
 				  rcv_wnd);
 
 	tp->rx_opt.rcv_wscale = rcv_wscale;
-	tp->rcv_ssthresh = tp->rcv_wnd;
+	if (rcv_wnd)
+		tp->rcv_ssthresh = max(tp->rcv_wnd, rcv_wnd * mss);
+	else
+		tp->rcv_ssthresh = tp->rcv_wnd;
 
 	sk->sk_err = 0;
 	sock_reset_flag(sk, SOCK_DONE);
-- 
2.25.1