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From:   Magnus Karlsson <magnus.karlsson@gmail.com>
Date:   Mon, 17 Apr 2023 14:13:06 +0200
Message-ID: <CAJ8uoz3Rts2Xfhqq+0cm3GES=dMb2hTqPzGm515oG_nmt=-Nbg@mail.gmail.com>
Subject: Re: [PATCH bpf-next v3 1/3] xsk: Support UMEM chunk_size > PAGE_SIZE
To:     =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>
Cc:     Kal Cutter Conley <kal.conley@dectris.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
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On Thu, 13 Apr 2023 at 22:52, Toke H=C3=B8iland-J=C3=B8rgensen <toke@redhat=
.com> wrote:
>
> Kal Cutter Conley <kal.conley@dectris.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
>