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

[Michael Ellerman]

Michal Suchánek <msuchanek at suse.de> writes:
> On Mon, Feb 14, 2022 at 01:33:24PM +0100, Paul Menzel wrote:
>> Am 14.02.22 um 10:43 schrieb Michal Suchánek:
>> > On Mon, Feb 14, 2022 at 07:08:07AM +0100, Paul Menzel wrote:
>> > > 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.

One of the messages was removed because it was potentially buggy:

  ed8029d7b472 ("powerpc/pseries: Stop calling printk in rtas_stop_self()")

We may have removed some other messages, but my grepping skills can't find
anything relevant at the moment.

But in general yes, it used to be far too verbose on large systems.
 
>> 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.

Right, ppc64_cpu just uses the sysfs online files, so it's doing them one at a
time. The kernel has no knowledge that ppc64_cpu is turning all
secondaries on/off so there's no easy way for the kernel to do a summary
message.

An easy solution would be for ppc64_cpu to log something via syslog(3).

cheers