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

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Date:   Thu, 14 Sep 2017 12:41:46 -0400
From:   Soheil Hassas Yeganeh <soheil.kdev@...il.com>
To:     Yuchung Cheng <ycheng@...gle.com>
Cc:     Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        liujian <liujian56@...wei.com>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...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 Thu, Sep 14, 2017 at 12:32 PM, Yuchung Cheng <ycheng@...gle.com> wrote:
> On Thu, Sep 14, 2017 at 6:57 AM, Neal Cardwell <ncardwell@...gle.com> wrote:
>> 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>
> Acked-by: Yuchung Cheng <ycheng@...gle.com>

Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>
Very nice! Thank you, Eric!