netdev - RE: [RFC PATCH bpf-next 0/8] XDP_REDIRECT_XSK and Batched AF

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Message-ID: <87mtm2g8r9.fsf@toke.dk>
Date:   Wed, 17 Nov 2021 15:24:42 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     "Loftus, Ciara" <ciara.loftus@...el.com>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Cc:     "brouer@...hat.com" <brouer@...hat.com>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "hawk@...nel.org" <hawk@...nel.org>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "bjorn@...nel.org" <bjorn@...nel.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "jonathan.lemon@...il.com" <jonathan.lemon@...il.com>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>
Subject: RE: [RFC PATCH bpf-next 0/8] XDP_REDIRECT_XSK and Batched AF_XDP Rx

"Loftus, Ciara" <ciara.loftus@...el.com> writes:

>> I'm fine with adding a new helper, but I don't like introducing a new
>> XDP_REDIRECT_XSK action, which requires updating ALL the drivers.
>> 
>> With XDP_REDIRECT infra we beleived we didn't need to add more
>> XDP-action code to drivers, as we multiplex/add new features by
>> extending the bpf_redirect_info.
>> In this extreme performance case, it seems the this_cpu_ptr "lookup" of
>> bpf_redirect_info is the performance issue itself.
>> 
>> Could you experiement with different approaches that modify
>> xdp_do_redirect() to handle if new helper bpf_redirect_xsk was called,
>> prior to this_cpu_ptr() call.
>> (Thus, avoiding to introduce a new XDP-action).
>
> Thanks for your feedback Jesper.
> I understand the hesitation of adding a new action. If we can achieve the same improvement without
> introducing a new action I would be very happy!
> Without new the action we'll need a new way to indicate that the bpf_redirect_xsk helper was
> called. Maybe another new field in the netdev alongside the xsk_refcnt. Or else extend
> bpf_redirect_info - if we find a new home for it that it's too costly to access.
> Thanks for your suggestions. I'll experiment as you suggested and
> report back.

I'll add a +1 to the "let's try to solve this without a new return code" :)

Also, I don't think we need a new helper either; the bpf_redirect()
helper takes a flags argument, so we could just use ifindex=0,
flags=DEV_XSK or something like that.

Also, I think the batching in the driver idea can be generalised: we
just need to generalise the idea of "are all these packets going to the
same place" and have a batched version of xdp_do_redirect(), no? The
other map types do batching internally already, though, so I'm wondering
why batching in the driver helps XSK?

-Toke