netdev - RE: [PATCH net] tcp: fix cwnd-limited bug for TSO deferral where we send nothing

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Message-ID: <HE1PR0701MB22992D932301DC559B8A4D46C2CB0@HE1PR0701MB2299.eurprd07.prod.outlook.com>
Date:   Thu, 10 Dec 2020 10:52:26 +0000
From:   Ingemar Johansson S <ingemar.s.johansson@...csson.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Neal Cardwell <ncardwell.kernel@...il.com>
CC:     David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Ingemar Johansson S <ingemar.s.johansson@...csson.com>
Subject: RE: [PATCH net] tcp: fix cwnd-limited bug for TSO deferral where we
 send nothing


Obviously my memory needs a backup 😊

Thanks
/Ingemar

> -----Original Message-----
> From: Eric Dumazet <eric.dumazet@...il.com>
> Sent: den 10 december 2020 11:44
> To: Ingemar Johansson S <ingemar.s.johansson@...csson.com>; Jakub Kicinski
> <kuba@...nel.org>; Neal Cardwell <ncardwell.kernel@...il.com>
> Cc: David Miller <davem@...emloft.net>; netdev@...r.kernel.org; Neal
> Cardwell <ncardwell@...gle.com>; Yuchung Cheng <ycheng@...gle.com>;
> Soheil Hassas Yeganeh <soheil@...gle.com>; Eric Dumazet
> <edumazet@...gle.com>
> Subject: Re: [PATCH net] tcp: fix cwnd-limited bug for TSO deferral where we
> send nothing
> 
> 
> 
> On 12/10/20 10:50 AM, Ingemar Johansson S wrote:
> > Hi
> > Slighty off topic
> > It is a smaller mystery why I am listed as having reported this artifact ?.
> > I don't have any memory that I did so.. strange 😐.
> >
> 
> I think this was your report :
> 
> https://mailarchive.ietf.org/arch/msg/tcpm/3U--r1vC81blOfZ5JwAYWIbm4vE/
> 
> Have fun !
> 
> > Regards
> > Ingemar
> >
> >> -----Original Message-----
> >> From: Jakub Kicinski <kuba@...nel.org>
> >> Sent: den 10 december 2020 01:14
> >> To: Neal Cardwell <ncardwell.kernel@...il.com>
> >> Cc: David Miller <davem@...emloft.net>; netdev@...r.kernel.org; Neal
> >> Cardwell <ncardwell@...gle.com>; Ingemar Johansson S
> >> <ingemar.s.johansson@...csson.com>; Yuchung Cheng
> >> <ycheng@...gle.com>; Soheil Hassas Yeganeh <soheil@...gle.com>; Eric
> >> Dumazet <edumazet@...gle.com>
> >> Subject: Re: [PATCH net] tcp: fix cwnd-limited bug for TSO deferral where
> we
> >> send nothing
> >>
> >> On Tue,  8 Dec 2020 22:57:59 -0500 Neal Cardwell wrote:
> >>> From: Neal Cardwell <ncardwell@...gle.com>
> >>>
> >>> When cwnd is not a multiple of the TSO skb size of N*MSS, we can get
> >>> into persistent scenarios where we have the following sequence:
> >>>
> >>> (1) ACK for full-sized skb of N*MSS arrives
> >>>   -> tcp_write_xmit() transmit full-sized skb with N*MSS
> >>>   -> move pacing release time forward
> >>>   -> exit tcp_write_xmit() because pacing time is in the future
> >>>
> >>> (2) TSQ callback or TCP internal pacing timer fires
> >>>   -> try to transmit next skb, but TSO deferral finds remainder of
> >>>      available cwnd is not big enough to trigger an immediate send
> >>>      now, so we defer sending until the next ACK.
> >>>
> >>> (3) repeat...
> >>>
> >>> So we can get into a case where we never mark ourselves as
> >>> cwnd-limited for many seconds at a time, even with
> >>> bulk/infinite-backlog senders, because:
> >>>
> >>> o In case (1) above, every time in tcp_write_xmit() we have enough
> >>> cwnd to send a full-sized skb, we are not fully using the cwnd
> >>> (because cwnd is not a multiple of the TSO skb size). So every time we
> >>> send data, we are not cwnd limited, and so in the cwnd-limited
> >>> tracking code in tcp_cwnd_validate() we mark ourselves as not
> >>> cwnd-limited.
> >>>
> >>> o In case (2) above, every time in tcp_write_xmit() that we try to
> >>> transmit the "remainder" of the cwnd but defer, we set the local
> >>> variable is_cwnd_limited to true, but we do not send any packets, so
> >>> sent_pkts is zero, so we don't call the cwnd-limited logic to update
> >>> tp->is_cwnd_limited.
> >>>
> >>> Fixes: ca8a22634381 ("tcp: make cwnd-limited checks measurement-
> based,
> >>> and gentler")
> >>> Reported-by: Ingemar Johansson <ingemar.s.johansson@...csson.com>
> >>> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> >>> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> >>> Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>
> >>> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> >>
> >> Applied, thank you!

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