netdev - Re: [PATCH net] net: tcp: don't allocate fast clones for fastopen SYN

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Date:   Thu, 4 Mar 2021 20:41:45 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        kernel-team <kernel-team@...com>, Neil Spring <ntspring@...com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net] net: tcp: don't allocate fast clones for fastopen SYN

On Thu, Mar 4, 2021 at 8:06 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu, 4 Mar 2021 13:51:15 +0100 Eric Dumazet wrote:
> > I think we are over thinking this really (especially if the fix needs
> > a change in core networking or drivers)
> >
> > We can reuse TSQ logic to have a chance to recover when the clone is
> > eventually freed.
> > This will be more generic, not only for the SYN+data of FastOpen.
> >
> > Can you please test the following patch ?
>
> #7 - Eric comes up with something much better :)
>
>
> But so far doesn't seem to quite do it, I'm looking but maybe you'll
> know right away (FWIW testing a v5.6 backport but I don't think TSQ
> changed?):
>
> On __tcp_retransmit_skb kretprobe:
>
> ==> Hit TFO case ret:-16 ca_state:0 skb:ffff888fdb4bac00!
>
> First hit:
>         __tcp_retransmit_skb+1
>         tcp_rcv_state_process+2488
>         tcp_v6_do_rcv+405
>         tcp_v6_rcv+2984
>         ip6_protocol_deliver_rcu+180
>         ip6_input_finish+17
>
> Successful hit:
>         __tcp_retransmit_skb+1
>         tcp_retransmit_skb+18
>         tcp_retransmit_timer+716
>         tcp_write_timer_handler+136
>         tcp_write_timer+141
>         call_timer_fn+43
>
>  skb:ffff888fdb4bac00 --- delay:51642us bytes_acked:1


Humm maybe one of the conditions used in tcp_tsq_write() does not hold...

if (tp->lost_out > tp->retrans_out &&
    tp->snd_cwnd > tcp_packets_in_flight(tp)) {
    tcp_mstamp_refresh(tp);
    tcp_xmit_retransmit_queue(sk);
}

Maybe FastOpen case is 'special' and tp->lost_out is wrong.