netdev - Re: [PATCH net-next] tcp: ack immediately when a cwr packet arrives

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Date:   Tue, 24 Jul 2018 10:06:40 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Neal Cardwell <ncardwell@...gle.com>,
        Lawrence Brakmo <brakmo@...com>,
        Netdev <netdev@...r.kernel.org>,
        Kernel Team <kernel-team@...com>,
        Alexei Starovoitov <ast@...com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next] tcp: ack immediately when a cwr packet arrives

On Mon, Jul 23, 2018 at 7:23 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 07/24/2018 04:15 AM, Neal Cardwell wrote:
> > On Mon, Jul 23, 2018 at 8:49 PM Lawrence Brakmo <brakmo@...com> wrote:
> >>
> >> We observed high 99 and 99.9% latencies when doing RPCs with DCTCP. The
> >> problem is triggered when the last packet of a request arrives CE
> >> marked. The reply will carry the ECE mark causing TCP to shrink its cwnd
> >> to 1 (because there are no packets in flight). When the 1st packet of
> >> the next request arrives, the ACK was sometimes delayed even though it
> >> is CWR marked, adding up to 40ms to the RPC latency.
> >>
> >> This patch insures that CWR marked data packets arriving will be acked
> >> immediately.
> > ...
> >> Modified based on comments by Neal Cardwell <ncardwell@...gle.com>
> >>
> >> Signed-off-by: Lawrence Brakmo <brakmo@...com>
> >> ---
> >>  net/ipv4/tcp_input.c | 9 ++++++++-
> >>  1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > Seems like a nice mechanism to have, IMHO.
> >
> > Acked-by: Neal Cardwell <ncardwell@...gle.com>
>
> Should this go to net tree instead where all the other fixes went?
I am neutral but this feels more like a feature improvement

Acked-by: Yuchung Cheng <ycheng@...gle.com>

btw this should also help the classic ECN case upon timeout that
triggers one packet retransmission.
>
> Thanks,
> Daniel