[SRU][Zesty][PATCH 1/5] tcp_bbr: cut pacing rate only if filled pipe

[Vinson Lee]

From: Neal Cardwell <ncardwell at google.com>

BugLink: https://bugs.launchpad.net/bugs/1708604

In bbr_set_pacing_rate(), which decides whether to cut the pacing
rate, there was some code that considered exiting STARTUP to be
equivalent to the notion of filling the pipe (i.e.,
bbr_full_bw_reached()). Specifically, as the code was structured,
exiting STARTUP and going into PROBE_RTT could cause us to cut the
pacing rate down to something silly and low, based on whatever
bandwidth samples we've had so far, when it's possible that all of
them have been small app-limited bandwidth samples that are not
representative of the bandwidth available in the path. (The code was
correct at the time it was written, but the state machine changed
without this spot being adjusted correspondingly.)

Fixes: 0f8782ea1497 ("tcp_bbr: add BBR congestion control")
Signed-off-by: Neal Cardwell <ncardwell at google.com>
Signed-off-by: Yuchung Cheng <ycheng at google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil at google.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit 4aea287e90dd61a48268ff2994b56f9799441b62)
Signed-off-by: Vinson Lee <vlee at freedesktop.org>
---
 net/ipv4/tcp_bbr.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c
index b89bce4c721e..b152e1d3b754 100644
--- a/net/ipv4/tcp_bbr.c
+++ b/net/ipv4/tcp_bbr.c
@@ -221,12 +221,11 @@ static u64 bbr_rate_bytes_per_sec(struct sock *sk, u64 rate, int gain)
  */
 static void bbr_set_pacing_rate(struct sock *sk, u32 bw, int gain)
 {
-	struct bbr *bbr = inet_csk_ca(sk);
 	u64 rate = bw;
 
 	rate = bbr_rate_bytes_per_sec(sk, rate, gain);
 	rate = min_t(u64, rate, sk->sk_max_pacing_rate);
-	if (bbr->mode != BBR_STARTUP || rate > sk->sk_pacing_rate)
+	if (bbr_full_bw_reached(sk) || rate > sk->sk_pacing_rate)
 		sk->sk_pacing_rate = rate;
 }
 
-- 
2.11.0