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

On Wed, Jul 13, 2022 at 2:52 PM Toke Høiland-Jørgensen [off-list ref] wrote:

Stanislav Fomichev [off-list ref] writes:

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On Wed, Jul 13, 2022 at 4:14 AM Toke Høiland-Jørgensen [off-list ref] wrote:

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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.

Ah, ok, I didn't read the patches closely enough. I saw some limits
for the ranges and assumed that it wasn't capable of efficiently
storing 64-bit timestamps..

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Can we make the map flexible enough to implement different qdisc
policies?

That's one of the things we want to be absolutely sure about. We are
starting out with the PIFO map type because the literature makes a good
case that it is flexible enough to implement all conceivable policies.
The goal of the test harness linked as note [4] is to actually examine
this; Frey is our PhD student working on this bit.

Thus far we haven't hit any limitations on this, but we'll need to add
more policies before we are done with this. Another consideration is
performance, of course, so we're also planning to do a comparison with a
more traditional "bunch of FIFO queues" type data structure for at least
a subset of the algorithms. Kartikeya also had an idea for an
alternative way to implement a priority queue using (semi-)lockless
skiplists, which may turn out to perform better.

If there's any particular policy/algorithm you'd like to see included in
this evaluation, please do let us know, BTW! :)

I honestly am not sure what the bar for accepting this should be. But
on the Cong's series I mentioned Martin's CC bpf work as a great
example of what we should be trying to do for qdisc-like maps. Having
a bpf version of fq/fq_codel/whatever_other_complex_qdisc might be
very convincing :-)

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The dequeue program type is a new BPF program type that is attached to an
interface; when an interface is scheduled for transmission, the stack will
execute the attached dequeue program and, if it returns a packet to
transmit, that packet will be transmitted using the existing ndo_xdp_xmit()
driver function.

The dequeue program can obtain packets by pulling them out of a PIFO map
using the new bpf_packet_dequeue() helper. This returns a pointer to an
xdp_md structure, which can be dereferenced to obtain packet data and
data_meta pointers like in an XDP program. The returned packets are also
reference counted, meaning the verifier enforces that the dequeue program
either drops the packet (with the bpf_packet_drop() helper), or returns it
for transmission. Finally, a helper is added that can be used to actually
schedule an interface for transmission using the dequeue program type; this
helper can be called from both XDP and dequeue programs.

PERFORMANCE

Preliminary performance tests indicate about 50ns overhead of adding
queueing to the xdp_fwd example (last patch), which translates to a 20% PPS
overhead (but still 2x the forwarding performance of the netstack):

xdp_fwd :     4.7 Mpps  (213 ns /pkt)
xdp_fwd -Q:   3.8 Mpps  (263 ns /pkt)
netstack:       2 Mpps  (500 ns /pkt)

RELATION TO BPF QDISC

Cong Wang's BPF qdisc patches[2] share some aspects of this series, in
particular the use of a map to store packets. This is no accident, as we've
had ongoing discussions for a while now. I have no great hope that we can
completely converge the two efforts into a single BPF-based queueing
API (as has been discussed before[3], consolidating the SKB and XDP paths
is challenging). Rather, I'm hoping that we can converge the designs enough
that we can share BPF code between XDP and qdisc layers using common
functions, like it's possible to do with XDP and TC-BPF today. This would
imply agreeing on the map type and API, and possibly on the set of helpers
available to the BPF programs.

What would be the big difference for the map wrt xdp_frame vs sk_buff
excluding all obvious stuff like locking/refcnt?

I expect it would be quite straight-forward to just add a second subtype
of the PIFO map in this series that holds skbs. In fact, I think that
from the BPF side, the whole model implemented here would be possible to
carry over to the qdisc layer more or less wholesale. Some other
features of the qdisc layer, like locking, classes, and
multi-CPU/multi-queue management may be trickier, but I'm not sure how
much of that we should expose in a BPF qdisc anyway (as you may have
noticed I commented on Cong's series to this effect regarding the
classful qdiscs).

Maybe a related question here: with the way you do
BPF_MAP_TYPE_PIFO_GENERIC vs BPF_MAP_TYPE_PIFO_XDP, how hard it would
be have support for storing xdp_frames/skb in any map? Let's say we
have generic BPF_MAP_TYPE_RBTREE, where the key is
priority/timestamp/whatever, can we, based on the value's btf_id,
figure out the rest? (that the value is kernel structure and needs
special care and more constraints - can't be looked up from user space
and so on)

Seems like we really need to have two special cases: where we transfer
ownership of xdp_frame/skb to/from the map, any other big
complications?

That way we can maybe untangle the series a bit: we can talk about
efficient data structures for storing frames/skbs independently of
some generic support for storing them in the maps. Any major
complications with that approach?

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PATCH STRUCTURE

This series consists of a total of 17 patches, as follows:

Patches 1-3 are smaller preparatory refactoring patches used by subsequent
patches.

Seems like these can go separately without holding the rest?

Yeah, guess so? They don't really provide much benefit without the users
alter in the series, though, so not sure there's much point in sending
them separately?

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Patches 4-5 introduce the PIFO map type, and patch 6 introduces the dequeue
program type.

[...]

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Patches 7-10 adds the dequeue helpers and the verifier features needed to
recognise packet pointers, reference count them, and allow dereferencing
them to obtain packet data pointers.

Have you considered using kfuncs for these instead of introducing new
hooks/contexts/etc?

I did, but I'm not sure it's such a good fit? In particular, the way the
direct packet access is implemented for dequeue programs (where you can
get an xdp_md pointer and deref that to get data and data_end pointers)
is done this way so programs can share utility functions between XDP and
dequeue programs. And having a new program type for the dequeue progs
seem like the obvious thing to do since they're doing something new?

Maybe I'm missing something, though; could you elaborate on how you'd
use kfuncs instead?

I was thinking about the approach in general. In networking bpf, we've
been adding new program types, new contexts and new explicit hooks.
This all requires a ton of boiler plate (converting from uapi ctx to
the kernel, exposing hook points, etc, etc). And looking at Benjamin's
HID series, it's so much more elegant: there is no uapi, just kernel
function that allows it to be overridden and a bunch of kfuncs
exposed. No uapi, no helpers, no fake contexts.

For networking and xdp the ship might have sailed, but I was wondering
whether we should be still stuck in that 'old' boilerplate world or we
have a chance to use new nice shiny things :-)

(but it might be all moot if we'd like to have stable upis?)