This is a note to let you know that I've just added the patch titled

tcp_bbr: fix pacing_gain to always be unity when using lt_bw

to the 4.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: tcp_bbr-fix-pacing_gain-to-always-be-unity-when-using-lt_bw.patch and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From foo@baz Wed Feb 7 11:29:33 PST 2018

From: Neal Cardwell ncardwell@google.com Date: Wed, 31 Jan 2018 15:43:05 -0500 Subject: tcp_bbr: fix pacing_gain to always be unity when using lt_bw

From: Neal Cardwell ncardwell@google.com

[ Upstream commit 3aff3b4b986e51bcf4ab249e5d48d39596e0df6a ]

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 Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/ipv4/tcp_bbr.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/net/ipv4/tcp_bbr.c +++ b/net/ipv4/tcp_bbr.c @@ -481,7 +481,8 @@ static void bbr_advance_cycle_phase(stru

bbr->cycle_idx = (bbr->cycle_idx + 1) & (CYCLE_LEN - 1); bbr->cycle_mstamp = tp->delivered_mstamp; - bbr->pacing_gain = bbr_pacing_gain[bbr->cycle_idx]; + bbr->pacing_gain = bbr->lt_use_bw ? BBR_UNIT : + bbr_pacing_gain[bbr->cycle_idx]; }

/* Gain cycling: cycle pacing gain to converge to fair share of available bw. */ @@ -490,8 +491,7 @@ static void bbr_update_cycle_phase(struc { struct bbr *bbr = inet_csk_ca(sk);

- if ((bbr->mode == BBR_PROBE_BW) && !bbr->lt_use_bw && - bbr_is_next_cycle_phase(sk, rs)) + if (bbr->mode == BBR_PROBE_BW && bbr_is_next_cycle_phase(sk, rs)) bbr_advance_cycle_phase(sk); }

Patches currently in stable-queue which might be from ncardwell@google.com are

queue-4.15/tcp_bbr-fix-pacing_gain-to-always-be-unity-when-using-lt_bw.patch