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 4:37 PM Eric Dumazet <edumazet@google.com> wrote:
>
> 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.

0 will not work, since we use a do_div(grow, tp->rcvq_space.space)

>
> 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.