Subject: Re: net-shapers plan

From: Jakub Kicinski <[email-masked]>
Date: Mon, 14 Apr 2025 09:27:00 -0700

On Mon, 14 Apr 2025 11:27:00 +0300 Carolina Jubran wrote:
> > I hope you understand my concern, tho. Since you're providing the first
> > implementation, if the users can grow dependent on such behavior we'd
> > be in no position to explain later that it's just a quirk of mlx5 and
> > not how the API is intended to operate.  
> 
> Thanks for bringing this up. I want to make it clear that traffic 
> classes must be properly matched to queues. We don’t rely on the 
> hardware fallback behavior in mlx5. If the driver or firmware isn’t 
> configured correctly, traffic class bandwidth control won’t work as 
> expected — the user will suffer from constant switching of the TX queue 
> between scheduling queues and head-of-line blocking. As a result, users 
> shouldn’t expect reliable performance or correct bandwidth allocation.
> We don’t encourage configuring this without proper TX queue mapping, so 
> users won’t grow dependent on behavior that only happens to work without it.
> We tried to highlight this in the plan section discussing queue 
> selection and head-of-line blocking: To make traffic class shaping work, 
> we must keep traffic classes separate for each transmit queue.

Right, my concern is more that there is no requirement for explicit
configuration of the queues, as long as traffic arrives silo'ed WRT
DSCP markings. As long as a VF sorts the traffic it does not have
to explicitly say (or even know) that queue A will land in TC N.

BTW the classification is before all rewrites? IOW flower or any other
forwarding rules cannot affect scheduling?