netdev - Re: [PATCH net] tcp_bbr: fix bw probing to raise in-flight data for very small BDPs

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Date:   Sat, 28 Jul 2018 22:46:49 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     ncardwell@...gle.com
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net] tcp_bbr: fix bw probing to raise in-flight data
 for very small BDPs

From: Neal Cardwell <ncardwell@...gle.com>
Date: Fri, 27 Jul 2018 17:19:12 -0400

> For some very small BDPs (with just a few packets) there was a
> quantization effect where the target number of packets in flight
> during the super-unity-gain (1.25x) phase of gain cycling was
> implicitly truncated to a number of packets no larger than the normal
> unity-gain (1.0x) phase of gain cycling. This meant that in multi-flow
> scenarios some flows could get stuck with a lower bandwidth, because
> they did not push enough packets inflight to discover that there was
> more bandwidth available. This was really only an issue in multi-flow
> LAN scenarios, where RTTs and BDPs are low enough for this to be an
> issue.
> 
> This fix ensures that gain cycling can raise inflight for small BDPs
> by ensuring that in PROBE_BW mode target inflight values with a
> super-unity gain are always greater than inflight values with a gain
> <= 1. Importantly, this applies whether the inflight value is
> calculated for use as a cwnd value, or as a target inflight value for
> the end of the super-unity phase in bbr_is_next_cycle_phase() (both
> need to be bigger to ensure we can probe with more packets in flight
> reliably).
> 
> This is a candidate fix for stable releases.
> 
> Fixes: 0f8782ea1497 ("tcp_bbr: add BBR congestion control")
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> Acked-by: Yuchung Cheng <ycheng@...gle.com>
> Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>
> Acked-by: Priyaranjan Jha <priyarjha@...gle.com>
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Applied and queued up for -stable, thank you.