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

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Message-Id: <20180725.162138.394671961252485113.davem@davemloft.net>
Date:   Wed, 25 Jul 2018 16:21:38 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     ncardwell@...gle.com
Cc:     brakmo@...com, ycheng@...gle.com, daniel@...earbox.net,
        netdev@...r.kernel.org, Kernel-team@...com, ast@...com,
        eric.dumazet@...il.com
Subject: Re: [PATCH net-next] tcp: ack immediately when a cwr packet arrives

From: Neal Cardwell <ncardwell@...gle.com>
Date: Tue, 24 Jul 2018 21:57:27 -0400

> On Tue, Jul 24, 2018 at 1:42 PM Lawrence Brakmo <brakmo@...com> wrote:
>>
>> Note that without this fix the 99% latencies when doing 10KB RPCs
>> in a congested network using DCTCP are 40ms vs. 190us with the patch.
>> Also note that these 40ms high tail latencies started after commit
>> 3759824da87b30ce7a35b4873b62b0ba38905ef5 in Jul 2015,
>> which triggered the bugs/features we are fixing/adding. I agree it is a
>> debatable whether it is a bug fix or a feature improvement and I am
>> fine either way.
> 
> Good point. The fact that this greatly mitigates a regression in DCTCP
> performance resulting from 3759824da87b30ce7a35b4873b62b0ba38905ef5
> ("tcp: PRR uses CRB mode by default and SS mode conditionally") IMHO
> seems to be a good argument for putting this patch ("tcp: ack
> immediately when a cwr packet arrives") in the "net" branch and stable
> releases.

Thus, applied to 'net' and queued up for -stable.