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

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Date:   Thu, 10 Dec 2020 09:50:56 +0000
From:   Ingemar Johansson S <ingemar.s.johansson@...csson.com>
To:     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

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 😐. 

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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