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

[K Prateek Nayak]

Hello Andrea,

On 9/8/2026 1:53 PM, Andrea Righi wrote:
> NVIDIA Olympus implements spatial SMT with symmetric steady-state PE
> capacity but two different resource modes. One-Thread Active mode gives
> one PE the full core, while waking the other PE restores Two-Thread
> Active mode and partitions decode, issue, cache, TLB, and vector
> resources. Returning to full-resource mode requires the sibling to
> remain in WFI for 10 Ki cycles.
> 
> Measurements show that pinned workloads perform equally on either PE,
> but freely migratable workloads lose substantial throughput when they
> alternate between PE identities. Consistently selecting PE0 keeps PE1
> idle, avoids repeated SMT repartitioning, and restores
> one-thread-per-core performance.
> 
> Describe this scheduling preference with SD_ASYM_PACKING and give PE0,
> identified by MPIDR_EL1.Aff0, the higher arch_asym_cpu_priority(). This
> is independent of SD_ASYM_CPUCAPACITY: SMT siblings retain equal
> capacity, while physical cores with different maximum frequencies are
> handled by a higher scheduling domain.
> 
> Firmware currently provides no interface for describing the preferred
> SMT sibling. Detect Olympus by MIDR until such an interface is
> available.
> 
> Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>

Feel free to include:

Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>

Just one nit below ...
> +/*
> + * Switching the active PE on an NVIDIA Olympus SMT core can keep the core in
> + * two-thread active mode, with resources partitioned between the PEs.
> + *
> + * Prefer PE0 so PE1 can remain idle and the core can stay in full-resource
> + * mode. Firmware does not currently describe this preference, so detect
> + * Olympus by MIDR until a firmware interface is available.
> + */
> +static bool olympus_prefer_pe0 __ro_after_init;

nit. What is the purpose of "olympus_prefer_pe0"?

> +
> +#ifdef CONFIG_SCHED_SMT
> +static int arm64_smt_flags(void)
> +{
> +	int flags = cpu_smt_flags();
> +
> +	if (olympus_prefer_pe0)
> +		flags |= SD_ASYM_PACKING;

If this flag function is being used, olympus_prefer_pe0 is always true,
and the scheduler will never call into arch_asym_cpu_priority() without
SD_ASYM_PACKING being set so olympus_prefer_pe0 is always true there
too.

Seems redundant unless you were planning to do set_sched_topology()
unconditionally for whole of arm64.

> +
> +	return flags;
> +}

-- 
Thanks and Regards,
Prateek