netdev - [PATCH net 1/5] tcp_bbr: cut pacing rate only if filled pipe

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Message-Id: <20170714214925.30720-1-ncardwell@google.com>
Date:   Fri, 14 Jul 2017 17:49:21 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: [PATCH net 1/5] tcp_bbr: cut pacing rate only if filled pipe

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@...gle.com>
Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@...gle.com>
---
 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 dbcc9352a48f..743e97511dc8 100644
--- a/net/ipv4/tcp_bbr.c
+++ b/net/ipv4/tcp_bbr.c
@@ -220,12 +220,11 @@ 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;
 }

-- 
2.13.2.932.g7449e964c-goog