Re: [PATCH v2 net-next] selftests: devmem: configure HDS threshold

[Jakub Kicinski <kuba@kernel.org>]

On Mon, 7 Jul 2025 11:48:53 -0700 Mina Almasry wrote:
> > On Wed,  2 Jul 2025 10:42:49 +0000 Taehee Yoo wrote:  
> > > The devmem TCP requires the hds-thresh value to be 0, but it doesn't
> > > change it automatically.
> > > Therefore, make configure_headersplit() sets hds-thresh value to 0.  
> >
> > I don't see any undoing of the configuration :(
> > The selftest should leave the system in the state that it started.
> > We should either add some code to undo at shutdown or (preferably)
> > move the logic to the Python script where we can handle this more
> > cleanly with defer().  
> 
> I'm sure you're aware but this test in general doesn't aim to undo any
> of it's configuration AFAIR :( that includes ethtool tcp-data-split,
> -X, -N and -L. Sorry about that.
> 
> I wonder if you want this series to clean that up completely such that
> all configurations are cleaned up, or if you're asking Taehee to only
> clean up the hds-thres configuration for now.

Just the hds-thrs config for now. Avoid adding more problems.

> Also, sorry for the late reply, but FWIW, I prefer the configuration
> cleanup to be in ncdevmem itself. We use it outside of the ksft to run
> stress tests, and folks are going to copy-paste ncdevmem for their
> applications, so having it be as nice as possible is a plus. But if
> you feel strongly about doing this outside of ncdevmem.c itself I
> don't mind that much.

Stan & Bobby added devmem support to kperf I think that's a better
choice for stress testing: https://github.com/facebookexperimental/kperf
selftests are for testing the kernel, in the context of upstream CIs.
The experience with netdevsim teaches me that having "out of tree"
use cases as an explicit goal is a road to nowhere.