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.

Sorry that I didn't describe the problem clearly enough. The problem is
for two cases.

Case 1:

tcp_mem[2] limitation causes packet drop. In some cases, applications
may not read the data in the socket receiving queue quickly enough.
In my case, it will call recv() every 5 minutes. And there are a lot of such
sockets. tcp_mem[2] limitation can happen easily in such a case, and once
this happens, skb will be dropped (the receive queue is not empty) and
the send retrans the skb until timeout and the connection break.

Case 2:

The sender keeps sending small packets and makes the rec_buf full.
Meanwhile, the window is not zero, and the sender will keep retrans
until timeout, as the skb is dropped by the receiver.

>
> >                 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 we want to do here is to send a ack with zero window. It
may be not necessary to force receive new data here, but to stay
the same with the logic of 'tcp_may_update_window()', only
newer 'ack' in a ack packet can shrink the window.

If we don't receive new data and send a zero-window ack directly
here, it will be weird, as the previous ack with the same 'seq' and 'ack'
has non-zero window.

Thanks!
Menglong Dong

> 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.