Subject: Re: [RFC Patch net-next] net_sched: introduce eBPF based Qdisc

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Mon, 06 Sep 2021 13:45:38 +0200

Cong Wang <[email-masked]> writes:

> On Wed, Sep 1, 2021 at 3:42 AM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>
>> John Fastabend <[email-masked]> writes:
>>
>> > Cong Wang wrote:
>> >> On Tue, Aug 24, 2021 at 4:47 PM Martin KaFai Lau <[email-masked]> wrote:
>> >> > Please explain more on this.  What is currently missing
>> >> > to make qdisc in struct_ops possible?
>> >>
>> >> I think you misunderstand this point. The reason why I avoid it is
>> >> _not_ anything is missing, quite oppositely, it is because it requires
>> >> a lot of work to implement a Qdisc with struct_ops approach, literally
>> >> all those struct Qdisc_ops (not to mention struct Qdisc_class_ops).
>> >> WIth current approach, programmers only need to implement two
>> >> eBPF programs (enqueue and dequeue).
>> >>
>> >> Thanks.
>> >
>> > Another idea. Rather than work with qdisc objects which creates all
>> > these issues with how to work with existing interfaces, filters, etc.
>> > Why not create an sk_buff map? Then this can be used from the existing
>> > egress/ingress hooks independent of the actual qdisc being used.
>>
>> I agree. In fact, I'm working on doing just this for XDP, and I see no
>> reason why the map type couldn't be reused for skbs as well. Doing it
>> this way has a couple of benefits:
>
> I do see a lot of reasons, for starters, struct skb_buff is very different
> from struct xdp_buff, any specialized map can not be reused. I guess you
> are using a generic one, how do you handle the refcnt at least for skb?

Well, you can't keep XDP frames and skbs in the same map instance, but
you can create a map type that can be instantiated to hold either type
and otherwise keep the same semantics. The map can just inc/dec the
refcnt as skbs are added/removed from it.

>> - It leaves more flexibility to BPF: want a simple FIFO queue? just
>>   implement that with a single queue map. Or do you want to build a full
>>   hierarchical queueing structure? Just instantiate as many queue maps
>>   as you need to achieve this. Etc.
>
> Please give an example without a queue. ;) Queue is too simple, show us
> something more useful please. How do you plan to re-implement EDT with
> just queues?

I'm using 'queue' as a shorthand for any queueing/scheduling algorithm
implementable by a qdisc. We need to cover them all, obviously, not just
FIFO queues (in fact I think we should actively be discouraging those,
but that's a different story :) )

For EDT it would be something like:

- On enqueue, stick frames into the map with a rank corresponding to
  their transmission time (the map implements the PIFO queue, just like
  your patch).

- (re-)arm a BPF timer to fire at the time of the next transmission
  event, and have that timer trigger interface TX.

The first bit is straight-forward, and that last bit needs a new helper
or something like it. For qdiscs I guess we could just expose
qdisc_watchdog()? For XDP we'd need something new...

>> - The behaviour is defined entirely by BPF program behaviour, and does
>>   not require setting up a qdisc hierarchy in addition to writing BPF
>>   code.
>
> I have no idea why you call this a benefit, because my goal is to
> replace Qdisc's, not to replace any other things. You know there are
> plenty of Qdisc's which are not implemented in Linux kernel.

It's a benefit because it means you can keep everything together. I.e.,
you don't need to *both* write BPF code implementing your qdisc, *and* a
setup script to build the qdisc hierarchy. That simplifies deployment.

I suppose we could support inserting BPF qdiscs into a qdisc hierarchy
as well if needed. I don't personally see much use for that, but if
there's a use case, sure, why not?

>> - It should be possible to structure the hooks in a way that allows
>>   reusing queueing algorithm implementations between the qdisc and XDP
>>   layers.
>
> XDP has no skb but xdp_buff, no? And again, why only queues?

See above :)

-Toke