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

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Date:   Tue, 8 Oct 2019 10:47:09 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Sridhar Samudrala <sridhar.samudrala@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        maciej.fijalkowski@...el.com, tom.herbert@...el.com
Subject: Re: [PATCH bpf-next 2/4] xsk: allow AF_XDP sockets to receive packets
 directly from a queue

On Tue, 8 Oct 2019 at 08:59, Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Sridhar Samudrala <sridhar.samudrala@...el.com> 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...


> -Toke
>