netdev - Re: [PATCH bpf-next v3 1/3] xsk: Support UMEM chunk_size > PAGE

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Date:   Thu, 13 Apr 2023 13:08:44 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Kal Cutter Conley <kal.conley@...tris.com>
Cc:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 1/3] xsk: Support UMEM chunk_size > PAGE_SIZE

Kal Cutter Conley <kal.conley@...tris.com> writes:

>>
>> Well, I'm mostly concerned with having two different operation and
>> configuration modes for the same thing. We'll probably need to support
>> multibuf for AF_XDP anyway for the non-ZC path, which means we'll need
>> to create a UAPI for that in any case. And having two APIs is just going
>> to be more complexity to handle at both the documentation and
>> maintenance level.
>
> I don't know if I would call this another "API". This patchset doesn't
> change the semantics of anything. It only lifts the chunk size
> restriction when hugepages are used. Furthermore, the changes here are
> quite small and easy to understand. The four sentences added to the
> documentation shouldn't be too concerning either. :-)

Well, you mentioned yourself that:

> The disadvantage of this patchset is requiring the user to allocate
> HugeTLB pages which is an extra complication.

In addition, presumably when using this mode, the other XDP actions
(XDP_PASS, XDP_REDIRECT to other targets) would stop working unless we
add special handling for that in the kernel? We'll definitely need to
handle that somehow...

> In 30 years when everyone finally migrates to page sizes >= 64K the
> maintenance burden will drop to zero. Knock wood. :-)

Haha, right, but let's make sure we have something that is consistent in
the intervening decades, shall we? ;)

>> It *might* be worth it to do this if the performance benefit is really
>> compelling, but, well, you'd need to implement both and compare directly
>> to know that for sure :)
>
> What about use-cases that require incoming packet data to be
> contiguous? Without larger chunk sizes, the user is forced to allocate
> extra space per packet and copy the data. This defeats the purpose of
> ZC.

What use cases would that be, exactly? Presumably this is also a
performance issue? Which goes back to me asking for benchmarks :)

-Toke