Re: [PATCH net-next 1/3] net: tcp: send zero-window ACK when no memory

[Menglong Dong <menglong8.dong@gmail.com>]

On Fri, Jul 28, 2023 at 3:17 AM Eric Dumazet <edumazet@google.com> wrote:
>
> On Thu, Jul 27, 2023 at 2:51 PM <menglong8.dong@gmail.com> wrote:
> >
> > From: Menglong Dong <imagedong@tencent.com>
> >
> > For now, skb will be dropped when no memory, which makes client keep
> > retrans util timeout and it's not friendly to the users.
> >
> > In this patch, we reply an ACK with zero-window in this case to update
> > the snd_wnd of the sender to 0. Therefore, the sender won't timeout the
> > connection and will probe the zero-window with the retransmits.
> >
> > Signed-off-by: Menglong Dong <imagedong@tencent.com>
> > ---
> >  include/net/inet_connection_sock.h |  3 ++-
> >  net/ipv4/tcp_input.c               |  4 ++--
> >  net/ipv4/tcp_output.c              | 14 +++++++++++---
> >  3 files changed, 15 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
> > index c2b15f7e5516..be3c858a2ebb 100644
> > --- a/include/net/inet_connection_sock.h
> > +++ b/include/net/inet_connection_sock.h
> > @@ -164,7 +164,8 @@ enum inet_csk_ack_state_t {
> >         ICSK_ACK_TIMER  = 2,
> >         ICSK_ACK_PUSHED = 4,
> >         ICSK_ACK_PUSHED2 = 8,
> > -       ICSK_ACK_NOW = 16       /* Send the next ACK immediately (once) */
> > +       ICSK_ACK_NOW = 16,      /* Send the next ACK immediately (once) */
> > +       ICSK_ACK_NOMEM = 32,
> >  };
> >
> >  void inet_csk_init_xmit_timers(struct sock *sk,
> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index 3cd92035e090..03111af6115d 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -5061,7 +5061,8 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
> >                         reason = SKB_DROP_REASON_PROTO_MEM;
> >                         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVQDROP);
> >                         sk->sk_data_ready(sk);
> > -                       goto drop;
> > +                       inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOMEM;
>
> Also set ICSK_ACK_NOW ?
>
> We also need to make sure to send an immediate ACK WIN 0 in the case we queued
> the skb in an empty receive queue and we were under pressure.
>
> We do not want to have a delayed ACK sending a normal RWIN,
> then wait for another packet that we will probably drop.
>
> Look at the code :
>
> if (skb_queue_len(&sk->sk_receive_queue) == 0)
>      sk_forced_mem_schedule(sk, skb->truesize);
> else if (tcp_try_rmem_schedule(sk, skb, skb->truesize)) {
>
> and refactor it to make sure to set  ICSK_ACK_NOMEM even on the first packet
> that bypassed the rmem_schedule().
>

Ok, that makes sense.

>
>
> > +                       goto out_of_window;
>
> Why forcing quickack mode ? Please leave the "goto drop;"
>

Hmm...because I want to make it send an immediate
ACK. Obviously, a ICSK_ACK_NOW flag should be
the better choice here.

> >                 }
> >
> >                 eaten = tcp_queue_rcv(sk, skb, &fragstolen);
> > @@ -5102,7 +5103,6 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
> >  out_of_window:
> >                 tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
> >                 inet_csk_schedule_ack(sk);
> > -drop:
> >                 tcp_drop_reason(sk, skb, reason);
> >                 return;
> >         }
> >