netdev - Re: Spurious TCP retransmissions on ack vs kfree

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Message-ID: <CANn89iL8KO5KLqCRdGbGJg5cZj7zVBUjrStFv7A_wqnLusQQ_Q@mail.gmail.com>
Date:   Fri, 26 Feb 2021 17:35:11 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: Spurious TCP retransmissions on ack vs kfree_skb reordering

On Fri, Feb 26, 2021 at 5:09 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Fri, 26 Feb 2021 11:41:22 +0100 Eric Dumazet wrote:
> > > > Seems like I'm pretty lost here and the tcp:tcp_retransmit_skb events
> > > > are less spurious than I thought. Looking at some tcpdump traces we see:
> > > >
> > > > 0.045277 IP6 A > B: Flags [SEW], seq 2248382925:2248383296, win 61920, options [mss 1440,sackOK,TS val 658870494 ecr 0,nop,wscale 11], length 371
> > > >
> > > > 0.045348 IP6 B > A: Flags [S.E], seq 961169456, ack 2248382926, win 65535, options [mss 1440,sackOK,TS val 883864022 ecr 658870494,nop,wscale 9], length 0
> > >
> > > The SYNACK does not include the prior payload.
> > >
> > > > 0.045369 IP6 A > B: Flags [P.], seq 1:372, ack 1, win 31, options [nop,nop,TS val 658870494 ecr 883864022], length 371
> > >
> > > So this rtx is not spurious.
> > >
> > > However in your prior email you wrote :
> > >
> > > bytes_in:      0
> > > bytes_out:   742
> > > bytes_acked: 742
> > >
> > > Are you sure that at the time of the retransmit, bytes_acked was 742 ?
> > > I do not see how this could happen.
> >
> > Yes, this packetdrill test confirms TCP INFO stuff seems correct .
>
> Looks like it's TcpExtTCPSpuriousRtxHostQueues - the TFO fails as it
> might, but at the time the syn is still not kfree_skb()d because of
> the IRQ coalescing settings, so __tcp_retransmit_skb() returns -EBUSY
> and we have to wait for a timeout.
>
> Credit to Neil Spring @FB for figuring it out.

Yes, this makes sense.

Presumably tcp_send_syn_data() could allocate a regular (non fclone)
skb, to avoid this.

But if skb_still_in_host_queue() returns true, __tcp_retransmit_skb()
should return -EBUSY
and your tracepoint should not be called ?

In anycase, the bytes_acked should not be 742 as mentioned in your
email, if only the SYN was acked ?