Re: [PATCH v2 2/6] powerpc/smp: Enable Asym packing for cores on shared processor

Peter Zijlstra [off-list ref] writes:

On Thu, Oct 19, 2023 at 03:38:40PM +1100, Michael Ellerman wrote:

quoted

Srikar Dronamraju [off-list ref] writes:

quoted

If there are shared processor LPARs, underlying Hypervisor can have more
virtual cores to handle than actual physical cores.

Starting with Power 9, a core has 2 nearly independent thread groups.

You need to be clearer here that you're talking about "big cores", not
SMT4 cores as seen on bare metal systems.

What is a 'big core' ? I'm thinking big.LITTLE, but I didn't think Power
went that route (yet?).. help?

No it's not big.LITTLE :)

It means we have two SMT4 cores glued together that behave as a single
SMT8 core, a system is either in "big core" mode or it's not, it's never
heterogeneous.

If you grep for "big_core" there's some code in the kernel for dealing
with it, though it's probably not very illuminating.

Possibly we should switch to using the "thread group" terminology, so
that it doesn't confuse folks about big.LITTLE. Also the device tree
property that we use to discover if we're using big cores is called
ibm,thread-groups.

cheers