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    Subject[PATCH 4.9 16/47] tcp: only postpone PROBE_RTT if RTT is < current min_rtt estimate
    Date
    From: Ryan Sharpelletti <sharpelletti@google.com>

    [ 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: 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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/tcp_bbr.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/net/ipv4/tcp_bbr.c
    +++ b/net/ipv4/tcp_bbr.c
    @@ -740,7 +740,7 @@ static void bbr_update_min_rtt(struct so
    filter_expired = after(tcp_time_stamp,
    bbr->min_rtt_stamp + bbr_min_rtt_win_sec * HZ);
    if (rs->rtt_us >= 0 &&
    - (rs->rtt_us <= bbr->min_rtt_us || filter_expired)) {
    + (rs->rtt_us < bbr->min_rtt_us || filter_expired)) {
    bbr->min_rtt_us = rs->rtt_us;
    bbr->min_rtt_stamp = tcp_time_stamp;
    }

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