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

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Message-ID: <CADVnQykHXktzKY_jXa9GRwPciTf-1CsAegGRPyUc+F1mdMMmBw@mail.gmail.com>
Date:   Mon, 2 Jul 2018 11:17:50 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Lawrence Brakmo <brakmo@...com>
Cc:     Netdev <netdev@...r.kernel.org>, Kernel Team <kernel-team@...com>,
        bmatheny@...com, ast@...com, Yuchung Cheng <ycheng@...gle.com>,
        Steve Ibanez <sibanez@...nford.edu>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next 1/2] tcp: notify when a delayed ack is sent

On Fri, Jun 29, 2018 at 9:48 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.
>
> Signed-off-by: Lawrence Brakmo <brakmo@...com>
> ---
>  net/ipv4/tcp_output.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index f8f6129160dd..41f6ad7a21e4 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);
>  }

Thanks for this fix! Seems like this would work, but if I am reading
this correctly then it seems like this would cause a duplicate call to
tcp_ca_event(sk, CA_EVENT_NON_DELAYED_ACK) when we are sending a pure
ACK (delayed or non-delayed):

(1) once from tcp_send_ack() before we send the ACK:

tcp_send_ack(struct sock *sk)
 ->        tcp_ca_event(sk, CA_EVENT_NON_DELAYED_ACK);

(2) then again from tcp_event_ack_sent() after we have sent the ACK:

tcp_event_ack_sent()
    ->   if (inet_csk_ack_scheduled(sk))
                 tcp_ca_event(sk, CA_EVENT_NON_DELAYED_ACK);

What if we remove the original CA_EVENT_NON_DELAYED_ACK call and just
replace it with your new one? (not compiled, not tested):

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 3889dcd4868d4..bddb49617d9be 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -184,6 +184,8 @@ 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);
 }

@@ -3836,8 +3838,6 @@ 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.

Aside from lower CPU overhead, one nice benefit of that is that we
then only call tcp_ca_event(sk, CA_EVENT_NON_DELAYED_ACK) in one
place, which might be a little easier to reason about.

Does that work?

neal