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authorRyan Sharpelletti <sharpelletti@google.com>2020-11-16 20:44:13 +0300
committerJakub Kicinski <kuba@kernel.org>2020-11-17 22:03:22 +0300
commit1b9e2a8c99a5c021041bfb2d512dc3ed92a94ffd (patch)
tree93791d0c260842e895e449e4406b7f8b49728a70 /net
parent3d5179458d22dc0b4fdc724e4bed4231a655112a (diff)
downloadlinux-1b9e2a8c99a5c021041bfb2d512dc3ed92a94ffd.tar.xz
tcp: only postpone PROBE_RTT if RTT is < current min_rtt estimate
During loss recovery, retransmitted packets are forced to use TCP timestamps to calculate the RTT samples, which have a millisecond granularity. BBR is designed using a microsecond granularity. As a result, multiple RTT samples could be truncated to the same RTT value during loss recovery. This is problematic, as BBR will not enter PROBE_RTT if the RTT sample is <= the current min_rtt sample, meaning that if there are persistent losses, PROBE_RTT will constantly be pushed off and potentially never re-entered. This patch makes sure that BBR enters PROBE_RTT by checking if RTT sample is < the current min_rtt sample, rather than <=. The Netflix transport/TCP team discovered this bug in the Linux TCP BBR code during lab tests. Fixes: 0f8782ea1497 ("tcp_bbr: add BBR congestion control") Signed-off-by: Ryan Sharpelletti <sharpelletti@google.com> Signed-off-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com> Signed-off-by: Yuchung Cheng <ycheng@google.com> Link: https://lore.kernel.org/r/20201116174412.1433277-1-sharpelletti.kdev@gmail.com Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Diffstat (limited to 'net')
-rw-r--r--net/ipv4/tcp_bbr.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c
index 6c4d79baff26..6ea3dc2e4219 100644
--- a/net/ipv4/tcp_bbr.c
+++ b/net/ipv4/tcp_bbr.c
@@ -945,7 +945,7 @@ static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs)
filter_expired = after(tcp_jiffies32,
bbr->min_rtt_stamp + bbr_min_rtt_win_sec * HZ);
if (rs->rtt_us >= 0 &&
- (rs->rtt_us <= bbr->min_rtt_us ||
+ (rs->rtt_us < bbr->min_rtt_us ||
(filter_expired && !rs->is_ack_delayed))) {
bbr->min_rtt_us = rs->rtt_us;
bbr->min_rtt_stamp = tcp_jiffies32;