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

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Date:   Sun, 17 Jul 2022 20:41:10 +0200
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        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>, 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>, netdev@...r.kernel.org,
        bpf@...r.kernel.org,
        Freysteinn Alfredsson <freysteinn.alfredsson@....se>
Subject: Re: [RFC PATCH 00/17] xdp: Add packet queueing and scheduling capabilities

On Sun, 17 Jul 2022 at 20:17, Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> On Wed, Jul 13, 2022 at 11:52:07PM +0200, Toke Høiland-Jørgensen wrote:
> > Stanislav Fomichev <sdf@...gle.com> writes:
> >
> > > On Wed, Jul 13, 2022 at 4:14 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> > >>
> > >> Packet forwarding is an important use case for XDP, which offers
> > >> significant performance improvements compared to forwarding using the
> > >> regular networking stack. However, XDP currently offers no mechanism to
> > >> delay, queue or schedule packets, which limits the practical uses for
> > >> XDP-based forwarding to those where the capacity of input and output links
> > >> always match each other (i.e., no rate transitions or many-to-one
> > >> forwarding). It also prevents an XDP-based router from doing any kind of
> > >> traffic shaping or reordering to enforce policy.
> > >>
> > >> This series represents a first RFC of our attempt to remedy this lack. The
> > >> code in these patches is functional, but needs additional testing and
> > >> polishing before being considered for merging. I'm posting it here as an
> > >> RFC to get some early feedback on the API and overall design of the
> > >> feature.
> > >>
> > >> DESIGN
> > >>
> > >> The design consists of three components: A new map type for storing XDP
> > >> frames, a new 'dequeue' program type that will run in the TX softirq to
> > >> provide the stack with packets to transmit, and a set of helpers to dequeue
> > >> packets from the map, optionally drop them, and to schedule an interface
> > >> for transmission.
> > >>
> > >> The new map type is modelled on the PIFO data structure proposed in the
> > >> literature[0][1]. It represents a priority queue where packets can be
> > >> enqueued in any priority, but is always dequeued from the head. From the
> > >> XDP side, the map is simply used as a target for the bpf_redirect_map()
> > >> helper, where the target index is the desired priority.
> > >
> > > I have the same question I asked on the series from Cong:
> > > Any considerations for existing carousel/edt-like models?
> >
> > Well, the reason for the addition in patch 5 (continuously increasing
> > priorities) is exactly to be able to implement EDT-like behaviour, where
> > the priority is used as time units to clock out packets.
>
> Are you sure? I seriouly doubt your patch can do this at all...
>
> Since your patch relies on bpf_map_push_elem(), which has no room for
> 'key' hence you reuse 'flags' but you also reserve 4 bits there... How
> could tstamp be packed with 4 reserved bits??
>
> To answer Stanislav's question, this is how my code could handle EDT:
>
> // BPF_CALL_3(bpf_skb_map_push, struct bpf_map *, map, struct sk_buff *, skb, u64, key)
> skb->tstamp = XXX;
> bpf_skb_map_push(map, skb, skb->tstamp);

It is also possible here, if we could not push into the map with a
certain key it wouldn't be a PIFO.
Please look at patch 16/17 for an example (test_xdp_pifo.c), it's just
that the interface is different (bpf_redirect_map),
the key has been expanded to 64 bits to accommodate such use cases. It
is also possible in a future version of the patch to amortize the cost
of taking the lock for each enqueue by doing batching, similar to what
cpumap/devmap implementations do.

>
> (Please refer another reply from me for how to get the min when poping,
> which is essentially just a popular interview coding problem.)
>
> Actually, if we look into the in-kernel EDT implementation (net/sched/sch_etf.c),
> it is also based on rbtree rather than PIFO. ;-)
>
> Thanks.