Re: [PATCH 1/2] arm64: topology: Prefer PE0 on NVIDIA Olympus SMT cores

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 09.09.26 18:17, Andrea Righi wrote:
> On Wed, Sep 09, 2026 at 05:19:08PM +0200, Dietmar Eggemann wrote:
>> On 09.09.26 08:26, Andrea Righi wrote:
>>
>> [...]
>>
>>> +void __init arm64_init_sched_topology(void)
>>> +{
>>> +	if (!IS_ENABLED(CONFIG_SCHED_SMT))
>>> +		return;
>>> +
>>> +	if ((read_cpuid_id() & MIDR_CPU_MODEL_MASK) != MIDR_NVIDIA_OLYMPUS)
>>> +		return;
>>> +
>>> +	if (!topology_core_has_smt(smp_processor_id()))
>>> +		return;
>>> +
>>> +	set_sched_topology(arm64_asym_smt_topology);
>>> +	pr_info("Enabling PE0 SMT preference for NVIDIA Olympus\n");
>>
>> I'm not really a big fan of using this arm64 setup for NVIDIA Olympus
>> alone here.
>>
>>> +}
>>> +
>>> +int arch_asym_cpu_priority(int cpu)
>>> +{
>>> +	return MPIDR_AFFINITY_LEVEL(cpu_logical_map(cpu), 0) == 0;
>>> +}
>>> +
>>
>> This is done since it will only be called for NVIDIA Olympus since it's
>> the only CPU model enabling this. I think it will work but it's still
>> ugly :-)
>>
>> [...]
>>
> 
> Agreed on both points. As mentioned in the other email, I'll coordinate with the
> firmware team on a proper ABI for describing the PE0 preference. For the systems
> with the current firmware, I don't have many ideas on how to enable this... a
> special scheduler facility enabled via kernel boot option? Other ideas?
With the explicit kernel boot time option discussed in the other thread,
how do you want to code arch_asym_cpu_priority()?

AFAIK, Arm architecture does not require the first/primary HW thread of
an SMT core to have MPIDR_EL1.Aff0 == 0.

The closest I came to you setup in Arm64 is a ThunderX2 SMT-4 machine on
which I can run 'OpenBLAS benchmark/sgemm.goto'. This machine has
symmetric CPU capacity though like Power7.
It has MPIDR_EL1.Aff0 == {0,1,2,3} for HW threads but '== 0' wouldn't
work here.

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