netdev - Re: [PATCH net] tcp: update skb->skb

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Date:   Thu, 14 Sep 2017 09:57:47 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     liujian <liujian56@...wei.com>, David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Jerry Chu <hkchu@...gle.com>, Netdev <netdev@...r.kernel.org>,
        "weiyongjun (A)" <weiyongjun1@...wei.com>,
        wangkefeng 00227729 <wangkefeng.wang@...wei.com>
Subject: Re: [PATCH net] tcp: update skb->skb_mstamp more carefully

On Wed, Sep 13, 2017 at 11:30 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gl.com>
>
> liujian reported a problem in TCP_USER_TIMEOUT processing with a patch
> in tcp_probe_timer() :
>       https://www.spinics.net/lists/netdev/msg454496.html
>
> After investigations, the root cause of the problem is that we update
> skb->skb_mstamp of skbs in write queue, even if the attempt to send a
> clone or copy of it failed. One reason being a routing problem.
>
> This patch prevents this, solving liujian issue.
>
> It also removes a potential RTT miscalculation, since
> __tcp_retransmit_skb() is not OR-ing TCP_SKB_CB(skb)->sacked with
> TCPCB_EVER_RETRANS if a failure happens, but skb->skb_mstamp has
> been changed.
>
> A future ACK would then lead to a very small RTT sample and min_rtt
> would then be lowered to this too small value.
...
>
> Signed-off-by: Eric Dumazet <edumazet@...gl.com>
> Reported-by: liujian <liujian56@...wei.com>
> ---
>  net/ipv4/tcp_output.c |   19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)

Acked-by: Neal Cardwell <ncardwell@...gle.com>

Thanks, Eric!

neal