Subject: Re: [PATCH bpf-next 2/4] xsk: allow AF_XDP sockets to receive packets directly from a queue

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 08 Oct 2019 11:04:00 +0200

Björn Töpel <[email-masked]> writes:

> On Tue, 8 Oct 2019 at 08:59, Toke Høiland-Jørgensen <[email-masked]> wrote:
>>
>> Sridhar Samudrala <[email-masked]> writes:
>>
>> >  int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
>> >                   struct bpf_prog *xdp_prog)
>> >  {
>> >       struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
>> >       struct bpf_map *map = READ_ONCE(ri->map);
>> > +     struct xdp_sock *xsk;
>> > +
>> > +     xsk = xdp_get_direct_xsk(ri);
>> > +     if (xsk)
>> > +             return xsk_rcv(xsk, xdp);
>>
>> This is a new branch and a read barrier in the XDP_REDIRECT fast path.
>> What's the performance impact of that for non-XSK redirect?
>>
>
> The dependent-read-barrier in READ_ONCE? Another branch -- leave that
> to the branch-predictor already! ;-) No, you're right, performance
> impact here is interesting. I guess the same static_branch could be
> used here as well...

In any case, some measurements to see the impact (or lack thereof) would
be useful :)

-Toke