Re: [PATCH iwl-net 0/3] Fix XDP loading on machines with many CPUs

[Michal Kubiak <michal.kubiak@intel.com>]

On Tue, May 06, 2025 at 10:31:59PM -0700, Jesse Brandeburg wrote:
> On 4/22/25 8:36 AM, Michal Kubiak wrote:
> > Hi,
> > 
> > Some of our customers have reported a crash problem when trying to load
> > the XDP program on machines with a large number of CPU cores. After
> > extensive debugging, it became clear that the root cause of the problem
> > lies in the Tx scheduler implementation, which does not seem to be able
> > to handle the creation of a large number of Tx queues (even though this
> > number does not exceed the number of available queues reported by the
> > FW).
> > This series addresses this problem.
> 
> 
> Hi Michal,
> 
> Unfortunately this version of the series seems to reintroduce the original
> problem error: -22.

Hi Jesse,

Thanks for testing and reporting!

I will take a look at the problem and try to reproduce it locally. I would also
have a few questions inline.

First, was your original problem not the failure with error: -5? Or did you have
both (-5 and -22), depending on the scenario/environment?
I am asking because it seems that these two errors occurred at different
initialization stages of the tx scheduler. Of course, the series
was intended to address both of these issues.

> 
> I double checked the patches, they looked like they were applied in our test
> version 2025.5.8 build which contained a 6.12.26 kernel with this series
> applied (all 3)
> 
> Our setup is saying max 252 combined queues, but running 384 CPUs by
> default, loads an XDP program, then reduces the number of queues using
> ethtool, to 192. After that we get the error -22 and link is down.
> 

To be honest, I did not test the scenario in which the number of queues is
reduced while the XDP program is running. This is the first thing I will check.

Can you please confirm that you did that step on both the current
and the draft version of the series?
It would also be interesting to check what happens if the queue number is reduced
before loading the XDP program.

> Sorry to bring some bad news, and I know it took a while, it is a bit of a
> process to test this in our lab.
> 
> The original version you had sent us was working fine when we tested it, so
> the problem seems to be between those two versions. I suppose it could be
> possible (but unlikely because I used git to apply the patches) that there
> was something wrong with the source code, but I sincerely doubt it as the
> patches had applied cleanly.

The current series contains mostly some cosmetic fixes, but the potential
regression is still possible, so I will take a look at the diff.

> 
> We are only able to test 6.12.y or 6.6.y stable variants of the kernel if
> you want to make a test version of a fixed series for us to try.
> 
> Thanks,
> 
> Jesse
> 

I will keep you updated on my findings.

Thanks,
Michal