Re: [PATCH net] tcp_bbr: fix pacing_gain to always be unity when using lt_bw

[David Miller <davem@davemloft.net>]

From: Neal Cardwell <ncardwell@google.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@google.com>
> Signed-off-by: Yuchung Cheng <ycheng@google.com>
> Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
> Reported-by: Beyers Cronje <bcronje@gmail.com>

Applied and queued up for -stable, thanks Neal.