Subject: Re: [RFC net] Fixes: b63c5478e9cb ("ipv6: ioam: Support for Queue depth data field")

From: Jakub Kicinski <[email-masked]>
Date: Wed, 7 Dec 2022 16:04:18 -0800

On Wed, 7 Dec 2022 13:07:18 +0100 Justin Iurman wrote:
> > Can you say more about the use? What signal do you derive from it?
> > I do track qlen on Meta's servers but haven't found a strong use
> > for it yet (I did for backlog drops but not the qlen itself).  
> 
> The specification goal of the queue depth was initially to be able to 
> track the entire path with a detailed view for packets or flows (kind of 
> a zoom on the interface to have details about its queues). With the 
> current definition/implementation of the queue depth, if only one queue 
> is congested, you're able to know it. Which doesn't necessarily mean 
> that all queues are full, but this one is and there might be something 
> going on. And this is something operators might want to be able to 
> detect precisely, for a lot of use cases depending on the situation. On 
> the contrary, if all queues are full, then you could deduce that as well 
> for each queue separately, as soon as a packet is assigned to it. So I 
> think that with "queue depth = sum(queues)", you don't have details and 
> you're not able to detect a single queue congestion, while with "queue 
> depth = queue" you could detect both. One might argue that it's fine to 
> only have the aggregation in some situation. I'd say that we might need 
> both, actually. Which is technically possible (even though expensive, as 
> Eric mentioned) thanks to the way it is specified by the RFC, where some 
> freedom was intentionally given. I could come up with a solution for that.

Understood. My hope was that by now there was some in-field experience
which could help us judge how much signal can one derive from a single
queue. Or a user that could attest.

> > Because it measures the length of a single queue not the device.  
> 
> Yep, I figured that out after the off-list discussion we've had with Eric.
> 
> So my plan would be, if you all agree with, to correct and repost this 
> patch to fix the NULL qdisc issue. Then, I'd come with a solution to 
> allow both (with and without aggregation of queues) and post it on 
> net-next. But again, if the consensus is to revert this patch (which I 
> think would bring no benefit IMHO), then so be it. Thoughts?

To summarize - we have reservations about correctness and about 
breaking layering (ip6 calling down to net/sched).

You can stick to your approach, respost and see if any of the other
maintainer is willing to pick this up (i.e. missed this nack).
If you ask for my option I'll side with Eric.