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[ Upstream commit1b9e2a8c99] 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:0f8782ea14("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> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>