netdev - [PATCH net-next] tcp_bbr: update comments to reflect pacing_margin

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Message-Id: <20181109025400.38021-1-ncardwell@google.com>
Date:   Thu,  8 Nov 2018 21:54:00 -0500
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>,
        Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next] tcp_bbr: update comments to reflect pacing_margin_percent

Recently, in commit ab408b6dc744 ("tcp: switch tcp and sch_fq to new
earliest departure time model"), the TCP BBR code switched to a new
approach of using an explicit bbr_pacing_margin_percent for shaving a
pacing rate "haircut", rather than the previous implict
approach. Update an old comment to reflect the new approach.

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: Eric Dumazet <edumazet@...gle.com>
---
 net/ipv4/tcp_bbr.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c
index 9277abdd822a0..0f497fc49c3fe 100644
--- a/net/ipv4/tcp_bbr.c
+++ b/net/ipv4/tcp_bbr.c
@@ -128,7 +128,12 @@ static const u32 bbr_probe_rtt_mode_ms = 200;
 /* Skip TSO below the following bandwidth (bits/sec): */
 static const int bbr_min_tso_rate = 1200000;
 
-/* Pace at ~1% below estimated bw, on average, to reduce queue at bottleneck. */
+/* Pace at ~1% below estimated bw, on average, to reduce queue at bottleneck.
+ * In order to help drive the network toward lower queues and low latency while
+ * maintaining high utilization, the average pacing rate aims to be slightly
+ * lower than the estimated bandwidth. This is an important aspect of the
+ * design.
+ */
 static const int bbr_pacing_margin_percent = 1;
 
 /* We use a high_gain value of 2/ln(2) because it's the smallest pacing gain
@@ -247,13 +252,7 @@ static void bbr_init_pacing_rate_from_rtt(struct sock *sk)
 	sk->sk_pacing_rate = bbr_bw_to_pacing_rate(sk, bw, bbr_high_gain);
 }
 
-/* Pace using current bw estimate and a gain factor. In order to help drive the
- * network toward lower queues while maintaining high utilization and low
- * latency, the average pacing rate aims to be slightly (~1%) lower than the
- * estimated bandwidth. This is an important aspect of the design. In this
- * implementation this slightly lower pacing rate is achieved implicitly by not
- * including link-layer headers in the packet size used for the pacing rate.
- */
+/* Pace using current bw estimate and a gain factor. */
 static void bbr_set_pacing_rate(struct sock *sk, u32 bw, int gain)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
-- 
2.19.1.930.g4563a0d9d0-goog