netdev - Re: [PATCH -next] tcp: make undo_cwnd mandatory for congestion modules

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Date:   Fri, 18 Nov 2016 19:54:34 +0100
From:   Florian Westphal <fw@...len.de>
To:     David Miller <davem@...emloft.net>
Cc:     fw@...len.de, netdev@...r.kernel.org, edumazet@...gle.com,
        ycheng@...gle.com, ncardwell@...gle.com
Subject: Re: [PATCH -next] tcp: make undo_cwnd mandatory for congestion
 modules

David Miller <davem@...emloft.net> wrote:
> From: Florian Westphal <fw@...len.de>
> Date: Thu, 17 Nov 2016 13:56:51 +0100
> 
> > The undo_cwnd fallback in the stack doubles cwnd based on ssthresh,
> > which un-does reno halving behaviour.
> > 
> > It seems more appropriate to let congctl algorithms pair .ssthresh
> > and .undo_cwnd properly. Add a 'tcp_reno_undo_cwnd' function and wire it
> > up for all congestion algorithms that used to rely on the fallback.
> > 
> > highspeed, illinois, scalable, veno and yeah use 'reno undo' while their
> > .ssthresh implementation doesn't halve the slowstart threshold, this
> > might point to similar issue as the one fixed for dctcp in
> > ce6dd23329b1e ("dctcp: avoid bogus doubling of cwnd after loss").
> > 
> > Cc: Eric Dumazet <edumazet@...gle.com>
> > Cc: Yuchung Cheng <ycheng@...gle.com>
> > Cc: Neal Cardwell <ncardwell@...gle.com>
> > Signed-off-by: Florian Westphal <fw@...len.de>
> 
> If you really suspect that highspeed et al. need to implement their own
> undo_cwnd instead of using the default reno fallback, I would really
> rather that this gets either fixed or explicitly marked as likely wrong
> (in an "XXX" comment or similar).

Ok, fair enough.  I am not familiar with these algorithms, I will check
what they're doing in more detail and if absolutely needed resubmit this
patch with XXX/FIXME/TODO comments added.

> Otherwise nobody is going to remember this down the road.

Agreed.