Re: [PATCH net-next 2/3] net: tcp: send zero-window when no memory

[Menglong Dong]

On Wed, May 17, 2023 at 10:45 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Wed, May 17, 2023 at 2:42 PM <menglong8.dong@xxxxxxxxx> wrote:
> >
> > From: Menglong Dong <imagedong@xxxxxxxxxxx>
> >
> > For now, skb will be dropped when no memory, which makes client keep
> > retrans util timeout and it's not friendly to the users.
>
> Yes, networking needs memory. Trying to deny it is recipe for OOM.
>
> >
> > Therefore, now we force to receive one packet on current socket when
> > the protocol memory is out of the limitation. Then, this socket will
> > stay in 'no mem' status, util protocol memory is available.
> >
>
> I think you missed one old patch.
>
> commit ba3bb0e76ccd464bb66665a1941fabe55dadb3ba    tcp: fix
> SO_RCVLOWAT possible hangs under high mem pressure
>
>
>
> > When a socket is in 'no mem' status, it's receive window will become
> > 0, which means window shrink happens. And the sender need to handle
> > such window shrink properly, which is done in the next commit.
> >
> > Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
> > ---
> >  include/net/sock.h    |  1 +
> >  net/ipv4/tcp_input.c  | 12 ++++++++++++
> >  net/ipv4/tcp_output.c |  7 +++++++
> >  3 files changed, 20 insertions(+)
> >
> > diff --git a/include/net/sock.h b/include/net/sock.h
> > index 5edf0038867c..90db8a1d7f31 100644
> > --- a/include/net/sock.h
> > +++ b/include/net/sock.h
> > @@ -957,6 +957,7 @@ enum sock_flags {
> >         SOCK_XDP, /* XDP is attached */
> >         SOCK_TSTAMP_NEW, /* Indicates 64 bit timestamps always */
> >         SOCK_RCVMARK, /* Receive SO_MARK  ancillary data with packet */
> > +       SOCK_NO_MEM, /* protocol memory limitation happened */
> >  };
> >
> >  #define SK_FLAGS_TIMESTAMP ((1UL << SOCK_TIMESTAMP) | (1UL << SOCK_TIMESTAMPING_RX_SOFTWARE))
> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index a057330d6f59..56e395cb4554 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -5047,10 +5047,22 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
> >                 if (skb_queue_len(&sk->sk_receive_queue) == 0)
> >                         sk_forced_mem_schedule(sk, skb->truesize);
>
> I think you missed this part : We accept at least one packet,
> regardless of memory pressure,
> if the queue is empty.
>
> So your changelog is misleading.
>
> >                 else if (tcp_try_rmem_schedule(sk, skb, skb->truesize)) {
> > +                       if (sysctl_tcp_wnd_shrink)
>
> We no longer add global sysctls for TCP. All new sysctls must per net-ns.
>
> > +                               goto do_wnd_shrink;
> > +
> >                         reason = SKB_DROP_REASON_PROTO_MEM;
> >                         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVQDROP);
> >                         sk->sk_data_ready(sk);
> >                         goto drop;
> > +do_wnd_shrink:
> > +                       if (sock_flag(sk, SOCK_NO_MEM)) {
> > +                               NET_INC_STATS(sock_net(sk),
> > +                                             LINUX_MIB_TCPRCVQDROP);
> > +                               sk->sk_data_ready(sk);
> > +                               goto out_of_window;
> > +                       }
> > +                       sk_forced_mem_schedule(sk, skb->truesize);
>
> So now we would accept two packets per TCP socket, and yet EPOLLIN
> will not be sent in time ?
>
> packets can consume about 45*4K each, I do not think it is wise to
> double receive queue sizes.
>
> What you want instead is simply to send EPOLLIN sooner (when the first
> packet is queued instead when the second packet is dropped)
> by changing sk_forced_mem_schedule() a bit.
>
> This might matter for applications using SO_RCVLOWAT, but not for
> other applications.

To be more clear, what I talk about here is not to send EPOLLIN
sooner, but try to make the TCP connection, which has a "hang"
receiver and in TCP protocol memory pressure, entry 0-probe
state. And this commit is the first step: make the receiver
shrink the window by sending a zero-window ack.

Thanks!
Menglong Dong