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

[Andrea Righi <arighi@nvidia.com>]

Hi Prateek,

On Wed, Sep 09, 2026 at 01:39:07AM +0530, K Prateek Nayak wrote:
> 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@nvidia.com>
> 
> Feel free to include:
> 
> Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
> Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
> 
> 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.

Right, there's no plan to install this topology unconditionally.

Since arm64_asym_smt_topology is installed only after detecting an Olympus
system with SMT, the boolean doesn't provide any additional state and can be
removed. I'll do that in the next version.

Thanks!
-Andrea