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

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Message-ID: <3c785042-0b9a-69cd-9415-e8a10ea0baf7@iogearbox.net>
Date:   Tue, 24 Jul 2018 04:23:27 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Neal Cardwell <ncardwell@...gle.com>,
        Lawrence Brakmo <brakmo@...com>
Cc:     Netdev <netdev@...r.kernel.org>, Kernel Team <kernel-team@...com>,
        ast@...com, Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next] tcp: ack immediately when a cwr packet arrives

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?

Thanks,
Daniel