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Commit 88c2cfa3 authored by Ryan Sharpelletti's avatar Ryan Sharpelletti Committed by Greg Kroah-Hartman
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tcp: only postpone PROBE_RTT if RTT is < current min_rtt estimate 



[ Upstream commit 1b9e2a8c99a5c021041bfb2d512dc3ed92a94ffd ]

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: 0f8782ea ("tcp_bbr: add BBR congestion control")
Signed-off-by: default avatarRyan Sharpelletti <sharpelletti@google.com>
Signed-off-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarSoheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: default avatarYuchung Cheng <ycheng@google.com>
Link: https://lore.kernel.org/r/20201116174412.1433277-1-sharpelletti.kdev@gmail.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent c962aae4

net/ipv4/tcp_bbr.c

+1 −1
Original line number Diff line number Diff line
@@ -917,7 +917,7 @@ 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;