netdev - Re: [PATCH net-next] mptcp: fix rcv buffer auto-tuning

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Date: Thu, 20 Jul 2023 15:14:20 -0400
From: Soheil Hassas Yeganeh <soheil@...gle.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Matthieu Baerts <matthieu.baerts@...sares.net>, mptcp@...ts.linux.dev, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Mat Martineau <martineau@...nel.org>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] mptcp: fix rcv buffer auto-tuning

On Thu, Jul 20, 2023 at 3:07 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Thu, Jul 20, 2023 at 8:48 PM Matthieu Baerts
> <matthieu.baerts@...sares.net> wrote:
> >
> > From: Paolo Abeni <pabeni@...hat.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@...hat.com>
> > Co-developed-by: Matthieu Baerts <matthieu.baerts@...sares.net>
> > Signed-off-by: Matthieu Baerts <matthieu.baerts@...sares.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 = (s64)space * tcp_sk(sk)->scaling_ratio;
> > +       s64 scaled_space = (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@...gle.com>

Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Thank you for the fix!

> > +{
> > +       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 = (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 !