netdev - Re: [RFC PATCH 00/17] xdp: Add packet queueing and scheduling capabilities

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Date:   Thu, 14 Jul 2022 18:21:00 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jamal Hadi Salim <jhs@...atatu.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Mykola Lysenko <mykolal@...com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        Freysteinn Alfredsson <freysteinn.alfredsson@....se>,
        Cong Wang <xiyou.wangcong@...il.com>
Subject: Re: [RFC PATCH 00/17] xdp: Add packet queueing and scheduling
 capabilities

Jamal Hadi Salim <jhs@...atatu.com> writes:

> I think what would be really interesting is to see the performance numbers when
> you have multiple producers/consumers(translation multiple
> threads/softirqs) in play
> targeting the same queues. Does PIFO alleviate the synchronization challenge
> when you have multiple concurrent readers/writers? Or maybe for your use case
> this would not be a common occurrence or not something you care about?

Right, this is definitely one of the areas we want to flesh out some
more and benchmark. I think a PIFO-based algorithm *can* be an
improvement here because you can compute the priority without holding
any lock and only grab a lock for inserting the packet; which can be
made even better with a (partially) lockless data structure and/or
batching.

In any case we *have* to do a certain amount of re-inventing for XDP
because we can't reuse the qdisc infrastructure anyway. Ultimately, I
expect it will be possible to write both really well-performing
algorithms, and really badly-performing ones. Such is the power of BPF,
after all, and as long as we can provide an existence proof of the
former, that's fine with me :)

> As I mentioned previously, I think this is what Cong's approach gets
> for free.

Yes, but it also retains the global qdisc lock; my (naive, perhaps?)
hope is that since we have to do things differently in XDP land anyway,
that work can translate into something that is amenable to being
lockless in qdisc land as well...

-Toke