Patch "tcp_bbr: init pacing rate on first RTT sample" has been added to the 4.12-stable tree

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

tcp_bbr: init pacing rate on first RTT sample

to the 4.12-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-init-pacing-rate-on-first-rtt-sample.patch
and it can be found in the queue-4.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Neal Cardwell <[email protected]>
Date: Fri, 14 Jul 2017 17:49:25 -0400
Subject: tcp_bbr: init pacing rate on first RTT sample

From: Neal Cardwell <[email protected]>

[ Upstream commit 32984565574da7ed3afa10647bb4020d7a9e6c93 ]

Fixes the following behavior: for connections that had no RTT sample at the time of initializing congestion control, BBR was initializing the pacing rate to a high nominal rate (based an a guess of RTT=1ms, in case this is LAN traffic). Then BBR never adjusted the pacing rate downward upon obtaining an actual RTT sample, if the connection never filled the pipe (e.g. all sends were small app-limited writes()).

This fix adjusts the pacing rate upon obtaining the first RTT sample.

---
 net/ipv4/tcp_bbr.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

@@ -113,7 +113,8 @@ struct bbr {
 		cwnd_gain:10,	/* current gain for setting cwnd */
 		full_bw_cnt:3,	/* number of rounds without large bw gains */
 		cycle_idx:3,	/* current index in pacing_gain cycle array */
-		unused_b:6;
+		has_seen_rtt:1, /* have we seen an RTT sample yet? */
+		unused_b:5;
 	u32	prior_cwnd;	/* prior cwnd upon entering loss recovery */
 	u32	full_bw;	/* recent bw, to estimate if pipe is full */
 };

@@ -226,11 +227,13 @@ static u32 bbr_bw_to_pacing_rate(struct
 static void bbr_init_pacing_rate_from_rtt(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
 	u64 bw;
 	u32 rtt_us;
 
 	if (tp->srtt_us) {		/* any RTT sample yet? */
 		rtt_us = max(tp->srtt_us >> 3, 1U);
+		bbr->has_seen_rtt = 1;
 	} else {			 /* no RTT sample yet */
 		rtt_us = USEC_PER_MSEC;	 /* use nominal default RTT */
 	}

@@ -248,8 +251,12 @@ static void bbr_init_pacing_rate_from_rt
  */
 static void bbr_set_pacing_rate(struct sock *sk, u32 bw, int gain)
 {
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
 	u32 rate = bbr_bw_to_pacing_rate(sk, bw, gain);
 
+	if (unlikely(!bbr->has_seen_rtt && tp->srtt_us))
+		bbr_init_pacing_rate_from_rtt(sk);
 	if (bbr_full_bw_reached(sk) || rate > sk->sk_pacing_rate)
 		sk->sk_pacing_rate = rate;
 }

@@ -838,6 +845,7 @@ static void bbr_init(struct sock *sk)
 
 	minmax_reset(&bbr->bw, bbr->rtt_cnt, 0);  /* init max bw to 0 */
 
+	bbr->has_seen_rtt = 0;
 	bbr_init_pacing_rate_from_rtt(sk);
 
 	bbr->restore_cwnd = 0;

Patches currently in stable-queue which might be from [email protected] are

queue-4.12/tcp_bbr-remove-sk_pacing_rate-0-transient-during-init.patch queue-4.12/tcp_bbr-introduce-bbr_bw_to_pacing_rate-helper.patch queue-4.12/tcp_bbr-introduce-bbr_init_pacing_rate_from_rtt-helper.patch queue-4.12/tcp_bbr-init-pacing-rate-on-first-rtt-sample.patch queue-4.12/tcp_bbr-cut-pacing-rate-only-if-filled-pipe.patch