Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230720-upstream-net-next-20230720-mptcp-fix-rcv-buffer-auto-tuning-v1-1-175ef12b8380@tessares.net>
In-Reply-To: <20230720-upstream-net-next-20230720-mptcp-fix-rcv-buffer-auto-tuning-v1-1-175ef12b8380@tessares.net>
From:   Eric Dumazet <edumazet@google.com>
Date:   Thu, 20 Jul 2023 21:07:39 +0200
Message-ID: <CANn89iL-tLZwjYPqmXx9-DbSHV9=epEK9iahqjwu=nsyW_tVrg@mail.gmail.com>
Subject: Re: [PATCH net-next] mptcp: fix rcv buffer auto-tuning
To:     Matthieu Baerts <matthieu.baerts@tessares.net>
Cc:     mptcp@lists.linux.dev, "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Mat Martineau <martineau@kernel.org>,
        Soheil Hassas Yeganeh <soheil@google.com>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Thu, Jul 20, 2023 at 8:48=E2=80=AFPM Matthieu Baerts
<matthieu.baerts@tessares.net> wrote:
>
> From: Paolo Abeni <pabeni@redhat.com>
>
> The MPTCP code uses the assumption that the tcp_win_from_space() helper
> does not use any TCP-specific field, and thus works correctly operating
> on an MPTCP socket.
>
> The commit dfa2f0483360 ("tcp: get rid of sysctl_tcp_adv_win_scale")
> broke such assumption, and as a consequence most MPTCP connections stall
> on zero-window event due to auto-tuning changing the rcv buffer size
> quite randomly.
>
> Address the issue syncing again the MPTCP auto-tuning code with the TCP
> one. To achieve that, factor out the windows size logic in socket
> independent helpers, and reuse them in mptcp_rcv_space_adjust(). The
> MPTCP level scaling_ratio is selected as the minimum one from the all
> the subflows, as a worst-case estimate.
>
> Fixes: dfa2f0483360 ("tcp: get rid of sysctl_tcp_adv_win_scale")
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> Co-developed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
> Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
> ---
>  include/net/tcp.h    | 20 +++++++++++++++-----
>  net/mptcp/protocol.c | 15 +++++++--------
>  net/mptcp/protocol.h |  8 +++++++-
>  net/mptcp/subflow.c  |  2 +-
>  4 files changed, 30 insertions(+), 15 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index c5fb90079920..794642fbd724 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -1430,22 +1430,32 @@ void tcp_select_initial_window(const struct sock =
*sk, int __space,
>                                __u32 *window_clamp, int wscale_ok,
>                                __u8 *rcv_wscale, __u32 init_rcv_wnd);
>
> -static inline int tcp_win_from_space(const struct sock *sk, int space)
> +static inline int __tcp_win_from_space(u8 scaling_ratio, int space)
>  {
> -       s64 scaled_space =3D (s64)space * tcp_sk(sk)->scaling_ratio;
> +       s64 scaled_space =3D (s64)space * scaling_ratio;
>
>         return scaled_space >> TCP_RMEM_TO_WIN_SCALE;
>  }
>
> -/* inverse of tcp_win_from_space() */
> -static inline int tcp_space_from_win(const struct sock *sk, int win)
> +static inline int tcp_win_from_space(const struct sock *sk, int space)

Maybe in a follow up patch we could change the prototype of this helper
to avoid future mis use :)

static inline int tcp_win_from_space(const struct tcp_sock *tp, int space)
{
}

Reviewed-by: Eric Dumazet <edumazet@google.com>


> +{
> +       return __tcp_win_from_space(tcp_sk(sk)->scaling_ratio, space);
> +}
> +
> +/* inverse of __tcp_win_from_space() */
> +static inline int __tcp_space_from_win(u8 scaling_ratio, int win)
>  {
>         u64 val =3D (u64)win << TCP_RMEM_TO_WIN_SCALE;
>
> -       do_div(val, tcp_sk(sk)->scaling_ratio);
> +       do_div(val, scaling_ratio);
>         return val;
>  }
>
> +static inline int tcp_space_from_win(const struct sock *sk, int win

Same remark here.

Thanks for the fix !