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    Subject[PATCH 4.9 17/84] tcp: when rearming RTO, if RTO time is in past then fire RTO ASAP
    Date
    4.9-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Neal Cardwell <ncardwell@google.com>


    [ Upstream commit cdbeb633ca71a02b7b63bfeb94994bf4e1a0b894 ]

    In some situations tcp_send_loss_probe() can realize that it's unable
    to send a loss probe (TLP), and falls back to calling tcp_rearm_rto()
    to schedule an RTO timer. In such cases, sometimes tcp_rearm_rto()
    realizes that the RTO was eligible to fire immediately or at some
    point in the past (delta_us <= 0). Previously in such cases
    tcp_rearm_rto() was scheduling such "overdue" RTOs to happen at now +
    icsk_rto, which caused needless delays of hundreds of milliseconds
    (and non-linear behavior that made reproducible testing
    difficult). This commit changes the logic to schedule "overdue" RTOs
    ASAP, rather than at now + icsk_rto.

    Fixes: 6ba8a3b19e76 ("tcp: Tail loss probe (TLP)")
    Suggested-by: Yuchung Cheng <ycheng@google.com>
    Signed-off-by: Neal Cardwell <ncardwell@google.com>
    Signed-off-by: Yuchung Cheng <ycheng@google.com>
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/tcp_input.c | 3 +--
    1 file changed, 1 insertion(+), 2 deletions(-)

    --- a/net/ipv4/tcp_input.c
    +++ b/net/ipv4/tcp_input.c
    @@ -3036,8 +3036,7 @@ void tcp_rearm_rto(struct sock *sk)
    /* delta may not be positive if the socket is locked
    * when the retrans timer fires and is rescheduled.
    */
    - if (delta > 0)
    - rto = delta;
    + rto = max(delta, 1);
    }
    inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
    TCP_RTO_MAX);

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