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

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

Hello,

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.
> >
> > To work around this possibility, and assuming that the LPAR run with the
> > same number of threads for each CPU, which is the common case,
> 
> I am skeptical at best of baking that assumption into this code. Mixed
> SMT modes within a partition doesn't strike me as an unreasonable
> possibility for some use cases. And if that's wrong, then we should just
> add a global smt value instead of using heuristics.
> 
> > the number
> > of active threads of the CPU doing the hot-plug operation is computed. Only
> > that number of threads will be activated for the newly added CPU.
> >
> > This way on a LPAR running in SMT=4, newly added CPU will be running 4
> > threads, which is what a end user would expect.
> 
> I could see why most users would prefer this new behavior. But surely
> some users have come to expect the existing behavior, which has been in
> place for years, and developed workarounds that might be broken by this
> change?
> 
> I would suggest that to handle this well, we need to give user space
> more ability to tell the kernel what actions to take on added cores, on
> an opt-in basis.
> 
> This could take the form of extending the DLPAR sysfs command set:
> 
> Option 1 - Add a flag that tells the kernel not to online any threads at
> all; user space will online the desired threads later.
> 
> Option 2 - Add an option that tells the kernel which SMT mode to apply.

powerpc-utils grew some drmgr hooks recently so maybe the policy can be
moved to userspace?

Thanks

Michal