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

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Message-ID: <87o7nrzeww.fsf@toke.dk>
Date:   Thu, 13 Apr 2023 22:49:03 +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, you mentioned yourself that:
>>
>> > The disadvantage of this patchset is requiring the user to allocate
>> > HugeTLB pages which is an extra complication.
>
> It's a small extra complication *for the user*. However, users that
> need this feature are willing to allocate hugepages. We are one such
> user. For us, having to deal with packets split into disjoint buffers
> (from the XDP multi-buffer paradigm) is a significantly more annoying
> complication than allocating hugepages (particularly on the RX side).

"More annoying" is not a great argument, though. You're basically saying
"please complicate your code so I don't have to complicate mine". And
since kernel API is essentially frozen forever, adding more of them
carries a pretty high cost, which is why kernel developers tend not to
be easily swayed by convenience arguments (if all you want is a more
convenient API, just build one on top of the kernel primitives and wrap
it into a library).

So you'll need to come up with either (1) a use case that you *can't*
solve without this new API (with specifics as to why that is the case),
or (2) a compelling performance benchmark showing the complexity is
worth it. Magnus indicated he would be able to produce the latter, in
which case I'm happy to be persuaded by the numbers.

In any case, however, the behaviour needs to be consistent wrt the rest
of XDP, so it's not as simple as just increasing the limit (as I
mentioned in my previous email).

-Toke