netdev - [PATCH net-next] tcp_bbr: clarify that bbr

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Message-Id: <20190829140244.195954-1-luke.w.hsiao@gmail.com>
Date:   Thu, 29 Aug 2019 10:02:44 -0400
From:   Luke Hsiao <luke.w.hsiao@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, Luke Hsiao <lukehsiao@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Priyaranjan Jha <priyarjha@...gle.com>
Subject: [PATCH net-next] tcp_bbr: clarify that bbr_bdp() rounds up in comments

From: Luke Hsiao <lukehsiao@...gle.com>

This explicitly clarifies that bbr_bdp() returns the rounded-up value of
the bandwidth-delay product and why in the comments.

Signed-off-by: Luke Hsiao <lukehsiao@...gle.com>
Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>
Acked-by: Neal Cardwell <ncardwell@...gle.com>
Acked-by: Priyaranjan Jha <priyarjha@...gle.com>
---
 net/ipv4/tcp_bbr.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c
index 56be7d27f208..95b59540eee1 100644
--- a/net/ipv4/tcp_bbr.c
+++ b/net/ipv4/tcp_bbr.c
@@ -346,7 +346,7 @@ static void bbr_cwnd_event(struct sock *sk, enum tcp_ca_event event)
 
 /* Calculate bdp based on min RTT and the estimated bottleneck bandwidth:
  *
- * bdp = bw * min_rtt * gain
+ * bdp = ceil(bw * min_rtt * gain)
  *
  * The key factor, gain, controls the amount of queue. While a small gain
  * builds a smaller queue, it becomes more vulnerable to noise in RTT
@@ -370,7 +370,9 @@ static u32 bbr_bdp(struct sock *sk, u32 bw, int gain)
 
 	w = (u64)bw * bbr->min_rtt_us;
 
-	/* Apply a gain to the given value, then remove the BW_SCALE shift. */
+	/* Apply a gain to the given value, remove the BW_SCALE shift, and
+	 * round the value up to avoid a negative feedback loop.
+	 */
 	bdp = (((w * gain) >> BBR_SCALE) + BW_UNIT - 1) / BW_UNIT;
 
 	return bdp;
-- 
2.23.0.187.g17f5b7556c-goog