Re: [PATCH net-next] tcp: optimise receiver buffer autotuning initialisation for high latency connections

From: Eric Dumazet <edumazet@google.com>
To: Hazem Mohamed Abuelfotoh <abuehaze@amazon.com>
Cc: netdev <netdev@vger.kernel.org>,
	stable@vger.kernel.org, Yuchung Cheng <ycheng@google.com>,
	Neal Cardwell <ncardwell@google.com>,
	Wei Wang <weiwan@google.com>,
	"Strohman, Andy" <astroh@amazon.com>,
	Benjamin Herrenschmidt <benh@amazon.com>
Subject: Re: [PATCH net-next] tcp: optimise receiver buffer autotuning initialisation for high latency connections
Date: Fri, 4 Dec 2020 20:10:04 +0100	[thread overview]
Message-ID: <CANn89iL1BkSyE=iSigZxvVB4_59QjWBY_5GuSoH8rcAaZ84EUg@mail.gmail.com> (raw)
In-Reply-To: <20201204180622.14285-1-abuehaze@amazon.com>

On Fri, Dec 4, 2020 at 7:08 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) configured
>     specially within environments where RTT is high.
>     To address this issue this patch is relying on RCV_MSS
>     so auto-tuning can start early regardless
>     the receiver 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);

So are you claiming icsk->icsk_ack.rcv_mss is related to MTU 9000 ?

RCV_MSS is not known until we receive actual packets... The initial
value is somthing like 536 if I am not mistaken.

I think your patch does not match the changelog.

>         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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