linux-kernel - [PATCH 4.9 36/93] tcp_bbr: cut pacing rate only if filled pipe

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Date:   Wed,  9 Aug 2017 11:13:29 -0700
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: [PATCH 4.9 36/93] tcp_bbr: cut pacing rate only if filled pipe

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Neal Cardwell <ncardwell@...gle.com>

[ Upstream commit 4aea287e90dd61a48268ff2994b56f9799441b62 ]

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>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 net/ipv4/tcp_bbr.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/net/ipv4/tcp_bbr.c
+++ b/net/ipv4/tcp_bbr.c
@@ -191,12 +191,11 @@ static u64 bbr_rate_bytes_per_sec(struct
  */
 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;
 }