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    Subject[PATCH 4.19 51/95] tcp: always set retrans_stamp on recovery
    Date
    From: Yuchung Cheng <ycheng@google.com>

    commit 7ae189759cc48cf8b54beebff566e9fd2d4e7d7c upstream.

    Previously TCP socket's retrans_stamp is not set if the
    retransmission has failed to send. As a result if a socket is
    experiencing local issues to retransmit packets, determining when
    to abort a socket is complicated w/o knowning the starting time of
    the recovery since retrans_stamp may remain zero.

    This complication causes sub-optimal behavior that TCP may use the
    latest, instead of the first, retransmission time to compute the
    elapsed time of a stalling connection due to local issues. Then TCP
    may disrecard TCP retries settings and keep retrying until it finally
    succeed: not a good idea when the local host is already strained.

    The simple fix is to always timestamp the start of a recovery.
    It's worth noting that retrans_stamp is also used to compare echo
    timestamp values to detect spurious recovery. This patch does
    not break that because retrans_stamp is still later than when the
    original packet was sent.

    Signed-off-by: Yuchung Cheng <ycheng@google.com>
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Reviewed-by: Neal Cardwell <ncardwell@google.com>
    Reviewed-by: Soheil Hassas Yeganeh <soheil@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Qiumiao Zhang <zhangqiumiao1@huawei.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/tcp_output.c | 9 ++++-----
    net/ipv4/tcp_timer.c | 23 +++--------------------
    2 files changed, 7 insertions(+), 25 deletions(-)

    --- a/net/ipv4/tcp_output.c
    +++ b/net/ipv4/tcp_output.c
    @@ -2958,13 +2958,12 @@ int tcp_retransmit_skb(struct sock *sk,
    #endif
    TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;
    tp->retrans_out += tcp_skb_pcount(skb);
    -
    - /* Save stamp of the first retransmit. */
    - if (!tp->retrans_stamp)
    - tp->retrans_stamp = tcp_skb_timestamp(skb);
    -
    }

    + /* Save stamp of the first (attempted) retransmit. */
    + if (!tp->retrans_stamp)
    + tp->retrans_stamp = tcp_skb_timestamp(skb);
    +
    if (tp->undo_retrans < 0)
    tp->undo_retrans = 0;
    tp->undo_retrans += tcp_skb_pcount(skb);
    --- a/net/ipv4/tcp_timer.c
    +++ b/net/ipv4/tcp_timer.c
    @@ -22,28 +22,14 @@
    #include <linux/gfp.h>
    #include <net/tcp.h>

    -static u32 tcp_retransmit_stamp(const struct sock *sk)
    -{
    - u32 start_ts = tcp_sk(sk)->retrans_stamp;
    -
    - if (unlikely(!start_ts)) {
    - struct sk_buff *head = tcp_rtx_queue_head(sk);
    -
    - if (!head)
    - return 0;
    - start_ts = tcp_skb_timestamp(head);
    - }
    - return start_ts;
    -}
    -
    static u32 tcp_clamp_rto_to_user_timeout(const struct sock *sk)
    {
    struct inet_connection_sock *icsk = inet_csk(sk);
    u32 elapsed, start_ts;
    s32 remaining;

    - start_ts = tcp_retransmit_stamp(sk);
    - if (!icsk->icsk_user_timeout || !start_ts)
    + start_ts = tcp_sk(sk)->retrans_stamp;
    + if (!icsk->icsk_user_timeout)
    return icsk->icsk_rto;
    elapsed = tcp_time_stamp(tcp_sk(sk)) - start_ts;
    remaining = icsk->icsk_user_timeout - elapsed;
    @@ -198,10 +184,7 @@ static bool retransmits_timed_out(struct
    if (!inet_csk(sk)->icsk_retransmits)
    return false;

    - start_ts = tcp_retransmit_stamp(sk);
    - if (!start_ts)
    - return false;
    -
    + start_ts = tcp_sk(sk)->retrans_stamp;
    if (likely(timeout == 0)) {
    linear_backoff_thresh = ilog2(TCP_RTO_MAX/rto_base);


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