netdev - Re: [RFC bpf-next 1/2] net: bridge: add unstable br_fdb_find_port_from

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Message-ID: <87ee4u3dtb.fsf@toke.dk>
Date:   Wed, 26 Jan 2022 13:02:24 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Nikolay Aleksandrov <nikolay@...dia.com>,
        bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Ahern <dsahern@...nel.org>,
        Yoshiki Komachi <komachi.yoshiki@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Roopa Prabhu <roopa@...dia.com>,
        "bridge@...ts.linux-foundation.org" 
        <bridge@...ts.linux-foundation.org>,
        Ido Schimmel <idosch@...sch.org>
Subject: Re: [RFC bpf-next 1/2] net: bridge: add unstable
 br_fdb_find_port_from_ifindex helper

Lorenzo Bianconi <lorenzo@...nel.org> writes:

>> On Mon, Jan 24, 2022 at 10:32 AM Nikolay Aleksandrov <nikolay@...dia.com> wrote:
>> > >
>> > > +int br_fdb_find_port_from_ifindex(struct xdp_md *xdp_ctx,
>> > > +                               struct bpf_fdb_lookup *opt,
>> > > +                               u32 opt__sz)
>> > > +{
>> > > +     struct xdp_buff *ctx = (struct xdp_buff *)xdp_ctx;
>> > > +     struct net_bridge_port *port;
>> > > +     struct net_device *dev;
>> > > +     int ret = -ENODEV;
>> > > +
>> > > +     BUILD_BUG_ON(sizeof(struct bpf_fdb_lookup) != NF_BPF_FDB_OPTS_SZ);
>> > > +     if (!opt || opt__sz != sizeof(struct bpf_fdb_lookup))
>> > > +             return -ENODEV;
>> > > +
>> > > +     rcu_read_lock();
>> > > +
>> > > +     dev = dev_get_by_index_rcu(dev_net(ctx->rxq->dev), opt->ifindex);
>> > > +     if (!dev)
>> > > +             goto out;
>> 
>> imo that is way too much wrapping for an unstable helper.
>> The dev lookup is not cheap.
>> 
>> With all the extra checks the XDP acceleration gets reduced.
>> I think it would be better to use kprobe/fentry on bridge
>> functions that operate on fdb and replicate necessary
>> data into bpf map.
>> Then xdp prog would do a single cheap lookup from that map
>> to figure out 'port'.
>
> ack, right. This is a very interesting approach. I will investigate
> it. Thanks.

I think it would be interesting to try both, and compare their
performance. I'm a bit sceptical about Alexei's assertion that
dev_get_by_index_rcu() is that expensive: we do such a lookup in the XDP
redirect code when using the non-map bpf_redirect() helper, and I have
not been able to measure a significant performance difference between
the map and non-map variants (after we added bulking to the latter).

If looking up devices by ifindex does turn out to be too expensive,
maybe what we really need is a way to pass around 'struct net_device'
pointers to BPF helpers, so a given BPF program only has to do the
lookup once if it's calling multiple dev-based helpers? I think this
should be doable with BTF, no?

-Toke