Jakub Kicinski [off-list ref] writes:

On Tue, 04 Feb 2020 12:00:40 +0100, Toke Høiland-Jørgensen wrote:

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Jesper Dangaard Brouer [off-list ref] writes:

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On Mon, 03 Feb 2020 21:13:24 +0100
Toke Høiland-Jørgensen [off-list ref] wrote:
 

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Oops, I see I forgot to reply to this bit:
  

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Yeah, but having the low-level details available to the XDP program
(such as HW queue occupancy for the egress hook) is one of the benefits
of XDP, isn't it?    

I think I glossed over the hope for having access to HW queue occupancy
- what exactly are you after? 

I don't think one can get anything beyond a BQL type granularity.
Reading over PCIe is out of question, device write back on high
granularity would burn through way too much bus throughput.    

This was Jesper's idea originally, so maybe he can explain better; but
as I understood it, he basically wanted to expose the same information
that BQL has to eBPF. Making it possible for an eBPF program to either
(re-)implement BQL with its own custom policy, or react to HWQ pressure
in some other way, such as by load balancing to another interface.  

Yes, and I also have plans that goes beyond BQL. But let me start with
explaining the BQL part, and answer Toke's question below.

On Mon, 03 Feb 2020 20:56:03 +0100 Toke wrote:  

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[...] Hmm, I wonder if a TX driver hook is enough?  

Short answer is no, a TX driver hook is not enough.  The queue state
info the TX driver hook have access to, needs to be updated once the
hardware have "confirmed" the TX-DMA operation have completed.  For
BQL/DQL this update happens during TX-DMA completion/cleanup (code
see call sites for netdev_tx_completed_queue()).  (As Jakub wisely
point out we cannot query the device directly due to performance
implications).  It doesn't need to be a new BPF hook, just something
that update the queue state info (we could piggy back on the
netdev_tx_completed_queue() call or give TX hook access to
dev_queue->dql).  

Interesting, that model does make sense to me.

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The question is whether this can't simply be done through bpf helpers?
bpf_get_txq_occupancy(ifindex, txqno)?

Helper vs dev_queue->dql field access seems like a technicality.
The usual flexibility of implementation vs performance and simplicity
consideration applies.. I guess?

Yeah, that was my point: We can do whatever makes the most sense for
that; it doesn't necessarily have to be something that's baked into the
TX hook point.

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Regarding "where is the queue": For me the XDP-TX queue is the NIC
hardware queue, that this BPF hook have some visibility into and can do
filtering on. (Imagine that my TX queue is bandwidth limited, then I
can shrink the packet size and still send a "congestion" packet to my
receiver).  

I'm not sure the hardware queues will be enough, though. Unless I'm
misunderstanding something, hardware queues are (1) fairly short and (2)
FIFO. So, say we wanted to implement fq_codel for XDP forwarding: we'd
still need a software queueing layer on top of the hardware queue.

Jesper makes a very interesting point tho. If all the implementation
wants is FIFO queues which are services in some simple manner (that is
something that can be offloaded) we should support that.

That means REDIRECT can target multiple TX queues, and we need an API
to control the queue allocation..

Yes. I think that should be part of the "hwq API" that we presented at
LPC last year[0].

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If the hardware is EDT-aware this may change, I suppose, but I'm not
sure if we can design the XDP queueing primitives with this assumption? :)

But I agree with you as well. I think both HW and SW feeding needs to
be supported. The HW implementations are always necessarily behind
ideas people implemented and tested in SW..

Exactly. And even though we mostly think about 10-100G network
interfaces, I believe there is also potential for this to be useful at
the low end (think small embedded routers that are very CPU
constrained). In these sorts of environments software queueing is more
feasible.

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The bigger picture is that I envision the XDP-TX/egress hook can
open-up for taking advantage of NIC hardware TX queue features. This
also ties into the queue abstraction work by Björn+Magnus. Today NIC
hardware can do a million TX-queues, and hardware can also do rate
limiting per queue. Thus, I also envision that the XDP-TX/egress hook
can choose/change the TX queue the packet is queue/sent on (we can
likely just overload the XDP_REDIRECT and have a new bpf map type for
this).  

I wonder what that does to our HW offload model which is based on
TC Qdisc offload today :S Do we use TC API to control configuration 
of XDP queues? :S

My thought was that it could be baked into the same API[0]. I.e., add a
way to say not just "I want 3 hardware queues for this", but also "I
want 3 hardware queues, with fq_codel (or whatever) on top".

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Yes, I think I mentioned in another email that putting all the queueing
smarts into the redirect map was also something I'd considered (well, I
do think we've discussed this in the past, so maybe not so surprising if
we're thinking along the same lines) :)

But the implication of this is also that an actual TX hook in the driver
need not necessarily incorporate a lot of new functionality, as it can
control the queueing through a combination of BPF helpers and map
updates?

True, it's the dequeuing that's on the TX side, so we could go as far
as putting all the enqueuing logic in the RX prog..

To answer your question from the other email Toke, my basic model was
kind of similar to TC Qdiscs. XDP redirect selects a device, then that
device has an enqueue and dequeue programs. Enqueue program can be run
in the XDP_REDIRECT context, dequeue is run every time NAPI cleaned up
some space on the TX descriptor ring. There is a "queue state" but the
FIFOs etc are sort of internal detail that the enqueue and dequeue
programs only share between each other. To be clear this is not a
suggestion of how things should be, it's what sprung to my mind without
thinking.

Ah, that's interesting. I do agree that it may be better to bake this
into the existing hooks rather than have an additional set of
(en/de)queue hooks.

-Toke

[0] https://linuxplumbersconf.org/event/4/contributions/462/