Re: [PATCH] s390/topology: Enable ASYM_PACKING to establish cpu priority

[Heiko Carstens <hca@linux.ibm.com>]

On Tue, Sep 01, 2026 at 04:40:26PM +0200, Mete Durlu wrote:
> On s390 LPARs the hypervisor assigns varying CPU runtime capacities
> under vertical polarization, making CPUs unequal. Add SD_ASYM_PACKING
> into topology level flag callbacks so the scheduler keeps busier
> scheduling groups packed onto higher-capacity CPUs.
> Implement arch_asym_cpu_priority() using smp_cpu_get_capacity() as
> the decision factor.

...
> Subtract the CPU number so that the CPUs with lower ids are prioritized.

Why?

> Only enabled on LPAR level as other guest levels either do not have
> vertical polarization (which causes asymmetric CPUs), or machine backed
> topology.

...

> +static int s390_core_flags(void)
> +{
> +	int flags = cpu_core_flags();
> +
> +	if (machine_is_lpar())
> +		flags |= SD_ASYM_PACKING;
> +	return flags;
> +}

Why is this machine_is_lpar()? This should actually test if cpu
polarization is available to make this independent of some random
environment test.

Furthermore to me this reads like the above only makes sense in case of
vertical polarization. What happens in case of horizontal polarization?
Does SD_ASYM_PACKING have any negative impact for that case?

> +int arch_asym_cpu_priority(int cpu)
> +{
> +	return smp_cpu_get_capacity(cpu) - cpu;
> +}

As commented above. The rationale for this needs to documented.
Does this make sense for horizontal polarization?