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authorLuke Hsiao <lukehsiao@google.com>2019-08-29 10:02:44 -0400
committerDavid S. Miller <davem@davemloft.net>2019-08-30 15:08:15 -0700
commitde8e1beb191912f94bcab46e72f247daaf664d15 (patch)
tree3768bab6151d6f43e9f0d05f3d86c285b62ef5f9 /net/ipv4/tcp_bbr.c
parentfa730a3bb9d35fa2289a2b6a0a4846154873f6f2 (diff)
tcp_bbr: clarify that bbr_bdp() rounds up in comments
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@google.com> Acked-by: Soheil Hassas Yeganeh <soheil@google.com> Acked-by: Neal Cardwell <ncardwell@google.com> Acked-by: Priyaranjan Jha <priyarjha@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/ipv4/tcp_bbr.c')
-rw-r--r--net/ipv4/tcp_bbr.c6
1 files 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;