Re: [PATCH] powerpc/pseries/cpuhp: respect current SMT when adding new CPU

["Michal Suchánek" <msuchanek@suse.de>]

On Thu, Mar 30, 2023 at 05:51:57PM +0200, Laurent Dufour wrote:
> On 13/02/2023 16:40:50, Nathan Lynch wrote:
> > Michal Suchánek <msuchanek@suse.de> writes:
> >> On Mon, Feb 13, 2023 at 08:46:50AM -0600, Nathan Lynch wrote:
> >>> Laurent Dufour <ldufour@linux.ibm.com> writes:
> >>>> When a new CPU is added, the kernel is activating all its threads. This
> >>>> leads to weird, but functional, result when adding CPU on a SMT 4 system
> >>>> for instance.
> >>>>
> >>>> Here the newly added CPU 1 has 8 threads while the other one has 4 threads
> >>>> active (system has been booted with the 'smt-enabled=4' kernel option):
> >>>>
> >>>> ltcden3-lp12:~ # ppc64_cpu --info
> >>>> Core   0:    0*    1*    2*    3*    4     5     6     7
> >>>> Core   1:    8*    9*   10*   11*   12*   13*   14*   15*
> >>>>
> >>>> There is no SMT value in the kernel. It is possible to run unbalanced LPAR
> >>>> with 2 threads for a CPU, 4 for another one, and 5 on the latest.

> Indeed, that's not so easy. There are multiple ways for the SMT level to be
> impacted:
>  - smt-enabled kernel option
>  - smtstate systemctl service (if activated), saving SMT level at shutdown
> time to restore it a boot time
>  - pseries-energyd daemon (if activated) could turn off threads
>  - ppc64_cpu --smt=x user command
>  - sysfs direct writing to turn off/on specific threads.
> 
> There is no SMT level saved, on "disk" or in the kernel, and any of these
> options can interact in parallel. So from the user space point of view, the
> best we could do is looking for the SMT current values, there could be
> multiple values in the case of a mixed SMT state, peek one value and apply it.
> 
> Extending the drmgr's hook is still valid, and I sent a patch series on the
> powerpc-utils mailing list to achieve that. However, changing the SMT level
> in that hook means that newly added CPU will be first turn on and there is
> a window where this threads could be seen active. Not a big deal but not
> turning on these extra threads looks better to me.

Which means

1) add an option to not onlince hotplugged CPUs by default

2) when a tool that wants to manage CPU onlining is active it can set
the option so that no threads are onlined automatically, and online the
desired threads

3) when no such tool is active the default should be to online all
threeads to preserve compatibility with existing behavior

> That's being said, I can't see any benefit of a user space implementation
> compared to the option I'm proposing in that patch.

The userspace implementation can implement arbitrily complex policy,
that's not something that belongs into the kernel.

Thanks

Michal