[PATCH net] tcp: fix receive buffer autotuning to trigger for any valid advertised MSS

[Hazem Mohamed Abuelfotoh <abuehaze@amazon.com>]

    Previously receiver buffer auto-tuning starts after receiving
    one advertised window amount of data.After the initial
    receiver buffer was raised by
    commit a337531b942b ("tcp: up initial rmem to 128KB
    and SYN rwin to around 64KB"),the receiver buffer may
    take too long for TCP autotuning to start raising
    the receiver buffer size.
    commit 041a14d26715 ("tcp: start receiver buffer autotuning sooner")
    tried to decrease the threshold at which TCP auto-tuning starts
    but it's doesn't work well in some environments
    where the receiver has large MTU (9001) especially with high RTT
    connections as in these environments rcvq_space.space will be the same
    as rcv_wnd so TCP autotuning will never start because
    sender can't send more than rcv_wnd size in one round trip.
    To address this issue this patch is decreasing the initial
    rcvq_space.space so TCP autotuning kicks in whenever the sender is
    able to send more than 5360 bytes in one round trip regardless the
    receiver's configured MTU.

    Fixes: a337531b942b ("tcp: up initial rmem to 128KB and SYN rwin to around 64KB")
    Fixes: 041a14d26715 ("tcp: start receiver buffer autotuning sooner")

Signed-off-by: Hazem Mohamed Abuelfotoh <abuehaze@amazon.com>
---
 net/ipv4/tcp_input.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 389d1b340248..f0ffac9e937b 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -504,13 +504,14 @@ static void tcp_grow_window(struct sock *sk, const struct sk_buff *skb)
 static void tcp_init_buffer_space(struct sock *sk)
 {
 	int tcp_app_win = sock_net(sk)->ipv4.sysctl_tcp_app_win;
+	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	int maxwin;
 
 	if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK))
 		tcp_sndbuf_expand(sk);
 
-	tp->rcvq_space.space = min_t(u32, tp->rcv_wnd, TCP_INIT_CWND * tp->advmss);
+	tp->rcvq_space.space = min_t(u32, tp->rcv_wnd, TCP_INIT_CWND * icsk->icsk_ack.rcv_mss);
 	tcp_mstamp_refresh(tp);
 	tp->rcvq_space.time = tp->tcp_mstamp;
 	tp->rcvq_space.seq = tp->copied_seq;
-- 
2.16.6




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