Patch "tcp_bbr: record "full bw reached" decision in new full_bw_reached bit" has been added to the 4.9-stable tree

[<gregkh@linuxfoundation.org>]

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;a=summary

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