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

[Yuchung Cheng <ycheng@google.com>]

On Tue, Mar 2, 2021 at 11:58 AM Eric Dumazet <edumazet@google.com> wrote:
>
> On Tue, Mar 2, 2021 at 7:08 AM Jakub Kicinski <kuba@kernel.org> wrote:
> >
> > When receiver does not accept TCP Fast Open it will only ack
> > the SYN, and not the data. We detect this and immediately queue
> > the data for (re)transmission in tcp_rcv_fastopen_synack().
> >
> > In DC networks with very low RTT and without RFS the SYN-ACK
> > may arrive before NIC driver reported Tx completion on
> > the original SYN. In which case skb_still_in_host_queue()
> > returns true and sender will need to wait for the retransmission
> > timer to fire milliseconds later.
> >
> > Revert back to non-fast clone skbs, this way
> > skb_still_in_host_queue() won't prevent the recovery flow
> > from completing.
> >
> > Suggested-by: Eric Dumazet <edumazet@google.com>
> > Fixes: 355a901e6cf1 ("tcp: make connect() mem charging friendly")
>
> Hmmm, not sure if this Fixes: tag makes sense.
>
> Really, if we delay TX completions by say 10 ms, other parts of the
> stack will misbehave anyway.
>
> Also, backporting this patch up to linux-3.19 is going to be tricky.
>
> The real issue here is that skb_still_in_host_queue() can give a false positive.
>
> I have mixed feelings here, as you can read my answer :/
>
> Maybe skb_still_in_host_queue() signal should not be used when a part
> of the SKB has been received/acknowledged by the remote peer
> (in this case the SYN part).
Thank you Eric and Jakub for working on the TFO issue.

I like this option the most because it's more generic and easy to understand. Is
it easy to implement by checking snd_una etc?




>
> Alternative is that drivers unable to TX complete their skbs in a
> reasonable time should call skb_orphan()
>  to avoid skb_unclone() penalties (and this skb_still_in_host_queue() issue)
>
> If you really want to play and delay TX completions, maybe provide a
> way to disable skb_still_in_host_queue() globally,
> using a static key ?
>
> My personal WIP/hack was something like :
>
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 69a545db80d2ead47ffcf2f3819a6d066e95f35d..666f6f0a6a06fece204199e07a79e21d1faf8f92
> 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5995,7 +5995,8 @@ 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) {
> -                       if (__tcp_retransmit_skb(sk, data, 1))
> +                       /* segs = -1 to bypass
> skb_still_in_host_queue() check */
> +                       if (__tcp_retransmit_skb(sk, data, -1))
>                                 break;
>                 }
>                 tcp_rearm_rto(sk);
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index fbf140a770d8e21b936369b79abbe9857537acd8..1d1489e596976e352fe7d5ccee7a6eae55fdbcce
> 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -3155,8 +3155,12 @@ int __tcp_retransmit_skb(struct sock *sk,
> struct sk_buff *skb, int segs)
>                   sk->sk_sndbuf))
>                 return -EAGAIN;
>
> -       if (skb_still_in_host_queue(sk, skb))
> -               return -EBUSY;
> +       if (segs > 0) {
> +               if (skb_still_in_host_queue(sk, skb))
> +                       return -EBUSY;
> +       } else {
> +               segs = -segs;
> +       }
>
>         if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
>                 if (unlikely(before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))) {
>
>
> > Signed-off-by: Neil Spring <ntspring@fb.com>
> > Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> > ---
> >  net/ipv4/tcp_output.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> > index fbf140a770d8..cd9461588539 100644
> > --- a/net/ipv4/tcp_output.c
> > +++ b/net/ipv4/tcp_output.c
> > @@ -3759,9 +3759,16 @@ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
> >         /* limit to order-0 allocations */
> >         space = min_t(size_t, space, SKB_MAX_HEAD(MAX_TCP_HEADER));
> >
> > -       syn_data = sk_stream_alloc_skb(sk, space, sk->sk_allocation, false);
> > +       syn_data = alloc_skb(MAX_TCP_HEADER + space, sk->sk_allocation);
> >         if (!syn_data)
> >                 goto fallback;
> > +       if (!sk_wmem_schedule(sk, syn_data->truesize)) {
> > +               __kfree_skb(syn_data);
> > +               goto fallback;
> > +       }
> > +       skb_reserve(syn_data, MAX_TCP_HEADER);
> > +       INIT_LIST_HEAD(&syn_data->tcp_tsorted_anchor);
> > +
> >         syn_data->ip_summed = CHECKSUM_PARTIAL;
> >         memcpy(syn_data->cb, syn->cb, sizeof(syn->cb));
> >         if (space) {
> > --
> > 2.26.2
> >