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

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Date:   Mon, 17 Apr 2023 14:13:06 +0200
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Kal Cutter Conley <kal.conley@...tris.com>,
        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

On Thu, 13 Apr 2023 at 22:52, Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> 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.

We will measure it and get back to you. Would be good with some 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
>