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

tcp_bbr: record "full bw reached" decision in new full_bw_reached bit

to the 4.9-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-record-full-bw-reached-decision-in-new-full_bw_reached-bit.patch and it can be found in the queue-4.9 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 Sun Dec 31 11:13:15 CET 2017

From: Neal Cardwell ncardwell@google.com Date: Thu, 7 Dec 2017 12:43:30 -0500 Subject: tcp_bbr: record "full bw reached" decision in new full_bw_reached bit

From: Neal Cardwell ncardwell@google.com

[ Upstream commit c589e69b508d29ed8e644dfecda453f71c02ec27 ]

This commit records the "full bw reached" decision in a new full_bw_reached bit. This is a pure refactor that does not change the current behavior, but enables subsequent fixes and improvements.

In particular, this enables simple and clean fixes because the full_bw and full_bw_cnt can be unconditionally zeroed without worrying about forgetting that we estimated we filled the pipe in Startup. And it enables future improvements because multiple code paths can be used for estimating that we filled the pipe in Startup; any new code paths only need to set this bit when they think the pipe is full.

Note that this fix intentionally reduces the width of the full_bw_cnt counter, since we have never used the most significant bit.

Signed-off-by: Neal Cardwell ncardwell@google.com Reviewed-by: Yuchung Cheng ycheng@google.com Acked-by: Soheil Hassas Yeganeh soheil@google.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/ipv4/tcp_bbr.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/net/ipv4/tcp_bbr.c +++ b/net/ipv4/tcp_bbr.c @@ -81,7 +81,8 @@ struct bbr { u32 lt_last_lost; /* LT intvl start: tp->lost */ u32 pacing_gain:10, /* current gain for setting pacing rate */ cwnd_gain:10, /* current gain for setting cwnd */ - full_bw_cnt:3, /* number of rounds without large bw gains */ + full_bw_reached:1, /* reached full bw in Startup? */ + full_bw_cnt:2, /* number of rounds without large bw gains */ cycle_idx:3, /* current index in pacing_gain cycle array */ has_seen_rtt:1, /* have we seen an RTT sample yet? */ unused_b:5; @@ -151,7 +152,7 @@ static bool bbr_full_bw_reached(const st { const struct bbr *bbr = inet_csk_ca(sk);

- return bbr->full_bw_cnt >= bbr_full_bw_cnt; + return bbr->full_bw_reached; }

/* Return the windowed max recent bandwidth sample, in pkts/uS << BW_SCALE. */ @@ -688,6 +689,7 @@ static void bbr_check_full_bw_reached(st return; } ++bbr->full_bw_cnt; + bbr->full_bw_reached = bbr->full_bw_cnt >= bbr_full_bw_cnt; }

/* If pipe is probably full, drain the queue and then enter steady-state. */ @@ -821,6 +823,7 @@ static void bbr_init(struct sock *sk) bbr->restore_cwnd = 0; bbr->round_start = 0; bbr->idle_restart = 0; + bbr->full_bw_reached = 0; bbr->full_bw = 0; bbr->full_bw_cnt = 0; bbr->cycle_mstamp.v64 = 0;

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

queue-4.9/tcp-invalidate-rate-samples-during-sack-reneging.patch queue-4.9/tcp_bbr-record-full-bw-reached-decision-in-new-full_bw_reached-bit.patch