netdev - Re: [PATCH net] tcp_cubic: do not set epoch

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Date:	Thu, 17 Sep 2015 22:35:35 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, ncardwell@...gle.com, ycheng@...gle.com,
	jri@...gle.com
Subject: Re: [PATCH net] tcp_cubic: do not set epoch_start in the future

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Thu, 17 Sep 2015 08:38:00 -0700

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Tracking idle time in bictcp_cwnd_event() is imprecise, as epoch_start
> is normally set at ACK processing time, not at send time.
> 
> Doing a proper fix would need to add an additional state variable,
> and does not seem worth the trouble, given CUBIC bug has been there
> forever before Jana noticed it.
> 
> Let's simply not set epoch_start in the future, otherwise
> bictcp_update() could overflow and CUBIC would again
> grow cwnd too fast.
> 
> This was detected thanks to a packetdrill test Neal wrote that was flaky
> before applying this fix.
> 
> Fixes: 30927520dbae ("tcp_cubic: better follow cubic curve after idle period")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> Cc: Jana Iyengar <jri@...gle.com>

Applied, thanks.
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