No Linux logs when doing `ppc64_cpu --smt=off/8`

[Michal Suchánek]

On Mon, Feb 14, 2022 at 01:33:24PM +0100, Paul Menzel wrote:
> Dear Michal,
> 
> 
> Thank you for your reply.
> 
> Am 14.02.22 um 10:43 schrieb Michal Suchánek:
> 
> > On Mon, Feb 14, 2022 at 07:08:07AM +0100, Paul Menzel wrote:
> > > Dear PPC folks,
> > > 
> > > 
> > > On the POWER8 server IBM S822LC running `ppc64_cpu --smt=off` or `ppc64_cpu
> > > --smt=8`, Linux 5.17-rc4 does not log anything. I would have expected a
> > > message about the change in number of processing units.
> > 
> > IIRC it was considered too noisy for systems with many CPUs and the
> > message was dropped. You can always check the resulting state with
> > ppc64_cpu or examining sysfs.
> 
> Yes, simple `nproc` suffice, but I was more thinking about, that the Linux
> log is often used for debugging and the changes of amount of processing
> units might be good to have. `ppc64_cpu --smt=off` or `=8` seems to block
> for quite some time, and each thread/processing unit seems to powered
> down/on sequentially, so it takes quite some time and it blocks. So 140
> messages would indeed be quite noise. No idea how `ppc64_cpu` works, and if
> it could log a message at the beginning and end.

Yes, it enables/disables threads one by one. AFAICT the kernel cannot know that
ppc64_cpu will enable/disable more threads later, it can either log each
or none. Rate limiting would not show the whole picture so it's not
great solution either.

Thanks

Michal