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

[Eric Dumazet <edumazet@google.com>]

On Mon, Dec 7, 2020 at 12:41 PM Hazem Mohamed Abuelfotoh
<abuehaze@amazon.com> wrote:
>
>     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);

I find using icsk->icsk_ack.rcv_mss misleading.

I would either use TCP_MSS_DEFAULT , or maybe simply 0, since we had
no samples yet, there is little point to use a magic value.

Note that if a driver uses 16KB of memory to hold a 1500 bytes packet,
then a 10 MSS GRO packet is consuming 160 KB of memory,
which is bigger than tcp_rmem[1]. TCP could decide to drop these fat packets.

I wonder if your patch does not work around a more fundamental issue,
I am still unable to reproduce the issue.