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

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Message-ID: <CANn89i+Sf66QknMO7+1gxowhV6g+Bs-DMhnvsvFx8vaqPfBVug@mail.gmail.com>
Date:   Thu, 4 Mar 2021 22:20:51 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        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 10:08 PM Jonathan Lemon <jonathan.lemon@...il.com> wrote:
>
> On Thu, Mar 04, 2021 at 08:41:45PM +0100, Eric Dumazet wrote:
> > 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.
>
>
> Something like this?  (completely untested)
> --
> Jonathan
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 69a545db80d2..92bc9b0f4955 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5995,8 +5995,10 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
>                 else
>                         tp->fastopen_client_fail = TFO_DATA_NOT_ACKED;
>                 skb_rbtree_walk_from(data) {
> +                       tcp_mark_skb_lost(sk, data);
>                         if (__tcp_retransmit_skb(sk, data, 1))
>                                 break;
> +                       tp->retrans_out += tcp_skb_pcount(data);
>                 }
>                 tcp_rearm_rto(sk);
>                 NET_INC_STATS(sock_net(sk),
>

Yes, but the hard part is testing this ;)

Once we properly mark the skb lost, we can call regular
tcp_xmit_retransmit_queue() to not have to care of tp->retrans_out

Not that we really need to make sure tcp_xmit_retransmit_queue() can
be called anyway from TSQ handler if TX completion is delayed.