netdev - Re: [PATCH net] tcp_bbr: fix pacing_gain to always be unity when using lt

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Date:   Thu, 01 Feb 2018 09:44:22 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     ncardwell@...gle.com
Cc:     netdev@...r.kernel.org, ycheng@...gle.com, soheil@...gle.com
Subject: Re: [PATCH net] tcp_bbr: fix pacing_gain to always be unity when
 using lt_bw

From: Neal Cardwell <ncardwell@...gle.com>
Date: Wed, 31 Jan 2018 15:43:05 -0500

> This commit fixes the pacing_gain to remain at BBR_UNIT (1.0) when
> using lt_bw and returning from the PROBE_RTT state to PROBE_BW.
> 
> Previously, when using lt_bw, upon exiting PROBE_RTT and entering
> PROBE_BW the bbr_reset_probe_bw_mode() code could sometimes randomly
> end up with a cycle_idx of 0 and hence have bbr_advance_cycle_phase()
> set a pacing gain above 1.0. In such cases this would result in a
> pacing rate that is 1.25x higher than intended, potentially resulting
> in a high loss rate for a little while until we stop using the lt_bw a
> bit later.
> 
> This commit is a stable candidate for kernels back as far as 4.9.
> 
> Fixes: 0f8782ea1497 ("tcp_bbr: add BBR congestion control")
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> Signed-off-by: Soheil Hassas Yeganeh <soheil@...gle.com>
> Reported-by: Beyers Cronje <bcronje@...il.com>

Applied and queued up for -stable, thanks Neal.