netdev - Re: [PATCH net-next v2 1/2] tcp: notify when a delayed ack is sent

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Message-ID: <CAK6E8=f2n9PKwne5COvT0sFyFymfJDDeE9Vxk6PeTpOEwg7z8w@mail.gmail.com>
Date:   Mon, 2 Jul 2018 16:48:56 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Lawrence Brakmo <brakmo@...com>
Cc:     netdev <netdev@...r.kernel.org>, Kernel Team <kernel-team@...com>,
        Blake Matheny <bmatheny@...com>,
        Alexei Starovoitov <ast@...com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Steve Ibanez <sibanez@...nford.edu>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next v2 1/2] tcp: notify when a delayed ack is sent

On Mon, Jul 2, 2018 at 2:39 PM, Lawrence Brakmo <brakmo@...com> wrote:
>
> DCTCP depends on the CA_EVENT_NON_DELAYED_ACK and CA_EVENT_DELAYED_ACK
> notifications to keep track if it needs to send an ACK for packets that
> were received with a particular ECN state but whose ACK was delayed.
>
> Under some circumstances, for example when a delayed ACK is sent with a
> data packet, DCTCP state was not being updated due to a lack of
> notification that the previously delayed ACK was sent. As a result, it
> would sometimes send a duplicate ACK when a new data packet arrived.
>
> This patch insures that DCTCP's state is correctly updated so it will
> not send the duplicate ACK.
Sorry to chime-in late here (lame excuse: IETF deadline)

IIRC this issue would exist prior to 4.11 kernel. While it'd be good
to fix that, it's not clear which patch introduced the regression
between 4.11 and 4.16? I assume you tested Eric's most recent quickack
fix.

In terms of the fix itself, it seems odd the tcp_send_ack() call in
DCTCP generates NON_DELAYED_ACK event to toggle DCTCP's
delayed_ack_reserved bit. Shouldn't the fix to have DCTCP send the
"prior" ACK w/o cancelling delayed-ACK and mis-recording that it's
cancelled, because that prior-ACK really is a supplementary old ACK.

But it's still unclear how this bug introduces the regression 4.11 - 4.16


>
> Improved based on comments from Neal Cardwell <ncardwell@...gle.com>.
>
> Signed-off-by: Lawrence Brakmo <brakmo@...com>
> ---
>  net/ipv4/tcp_output.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index f8f6129160dd..acefb64e8280 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -172,6 +172,8 @@ static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts)
>                         __sock_put(sk);
>         }
>         tcp_dec_quickack_mode(sk, pkts);
> +       if (inet_csk_ack_scheduled(sk))
> +               tcp_ca_event(sk, CA_EVENT_NON_DELAYED_ACK);
>         inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
>  }
>
> @@ -3567,8 +3569,6 @@ void tcp_send_ack(struct sock *sk)
>         if (sk->sk_state == TCP_CLOSE)
>                 return;
>
> -       tcp_ca_event(sk, CA_EVENT_NON_DELAYED_ACK);
> -
>         /* We are not putting this on the write queue, so
>          * tcp_transmit_skb() will set the ownership to this
>          * sock.
> --
> 2.17.1
>