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authorNeal Cardwell <ncardwell@google.com>2018-07-27 17:19:12 -0400
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2018-08-03 07:50:44 +0200
commit7309441887f64f6ed942408a569bd629386cec71 (patch)
tree74ce39188484021370f639219852a1237bb7ba17
parent7f36a065963aebecc94d2be1e37f30162e5fe229 (diff)
tcp_bbr: fix bw probing to raise in-flight data for very small BDPs
[ Upstream commit 383d470936c05554219094a4d364d964cb324827 ] 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@google.com> Acked-by: Yuchung Cheng <ycheng@google.com> Acked-by: Soheil Hassas Yeganeh <soheil@google.com> Acked-by: Priyaranjan Jha <priyarjha@google.com> Reviewed-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--net/ipv4/tcp_bbr.c4
1 files changed, 4 insertions, 0 deletions
diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c
index 9a0b952dd09b..06f247ca9197 100644
--- a/net/ipv4/tcp_bbr.c
+++ b/net/ipv4/tcp_bbr.c
@@ -353,6 +353,10 @@ static u32 bbr_target_cwnd(struct sock *sk, u32 bw, int gain)
/* Reduce delayed ACKs by rounding up cwnd to the next even number. */
cwnd = (cwnd + 1) & ~1U;
+ /* Ensure gain cycling gets inflight above BDP even for small BDPs. */
+ if (bbr->mode == BBR_PROBE_BW && gain > BBR_UNIT)
+ cwnd += 2;
+
return cwnd;
}