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

[Andrea Righi <arighi@nvidia.com>]

Hi Christian,

On Mon, Aug 31, 2026 at 10:13:42PM +0100, Christian Loehle wrote:
> On 8/31/26 19:10, 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.
> 
> But why? This asympacking + CAS interaction is a bit hard to comprehend IMV.

The two mehcanisms describe different preferences at different scheduling domain
levels: SD_ASYM_CPUCAPACITY selects among physical cores with different max
capacities, SD_ASYM_PACKING is set only on the SMT domain and selects the
canonical PE within the core chose by the existing placement and capacity logic.

> Why can't we encode both preferences in the asym-packing priority, e.g.
> 
> priority(cpu) = is_primary(cpu) ? 2 * highest_perf(cpu)
>                                 : highest_perf(cpu)
> 
> so that all primary PEs are preferred over all sibling PEs, while still
> preserving the highest_perf ordering within each group, and do away with
> SD_ASYM_CPUCAPACITY on Vera altogether?

I can experiment with this combined priority, but I think removing
SD_ASYM_CPUCAPACITY is a separate policy change rather than an alternative
implementation of this fix.

A static asym-packing priority does not preserve the capacity-aware semantics
used for task fitting, uclamp, misfit handling and migration. The current
approach keeps those semantics when selecting a physical core, then applies the
PE preference only within that core.

Also, encoding the combined priority alone would not fix the problem addressed
by patch 2: the idle-selection paths currently do not consult asymmetric SMT
priority. They can still return an arbitrary idle sibling regardless of how
arch_asym_cpu_priority() is defined. Patch 2 adds that missing behavior and
scopes it to the shared-capacity SMT domain.

> I had suggested this a while ago, did you have a stab at that by any chance,
> too?

I tested your CPPC-based asym-packing series, but not this particular
combined-priority variant. IIUC the earlier proposal was replacing
capacity-aware scheduling for minor physical-core capacity differences, SMT
sibling ordering looks like an orthogonal problem.

And at the time, the combined SMT-aware SD_ASYM_CPUCAPACITY approach also gave
the best Vera results of the alternatives I tested, which is another reason I
kept physical-core capacity selection separate here.

> Am I missing something altogether?

Combining the priorities is a valid experiment, but I'm not sure if it
completely solves the problem by itself, I'll give it a try and share the
results.

Thanks,
-Andrea

> 
> > 
> > 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>
> > ---
> >  arch/arm64/include/asm/topology.h |  1 +
> >  arch/arm64/kernel/smp.c           |  1 +
> >  arch/arm64/kernel/topology.c      | 62 +++++++++++++++++++++++++++++++
> >  3 files changed, 64 insertions(+)
> > > 
> > diff --git a/arch/arm64/include/asm/topology.h b/arch/arm64/include/asm/topology.h
> > index b9eaf4ad70850..edc1c59b3448d 100644
> > --- a/arch/arm64/include/asm/topology.h
> > +++ b/arch/arm64/include/asm/topology.h
> > @@ -18,6 +18,7 @@ int pcibus_to_node(struct pci_bus *bus);
> >  #include <linux/arch_topology.h>
> >  
> >  void update_freq_counters_refs(void);
> > +void arm64_init_sched_topology(void);
> >  
> >  /* Replace task scheduler's default frequency-invariant accounting */
> >  #define arch_scale_freq_tick topology_scale_freq_tick
> > diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> > index a61dc3016a117..0135ac4eea8bd 100644
> > --- a/arch/arm64/kernel/smp.c
> > +++ b/arch/arm64/kernel/smp.c
> > @@ -443,6 +443,7 @@ void __init smp_cpus_done(unsigned int max_cpus)
> >  	hyp_mode_check();
> >  	setup_system_features();
> >  	setup_user_features();
> > +	arm64_init_sched_topology();
> >  	mark_linear_text_alias_ro();
> >  }
> >  
> > diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> > index d28438f8b83f1..0dd9eec1c4946 100644
> > --- a/arch/arm64/kernel/topology.c
> > +++ b/arch/arm64/kernel/topology.c
> > @@ -19,6 +19,8 @@
> >  #include <linux/init.h>
> >  #include <linux/percpu.h>
> >  #include <linux/sched/isolation.h>
> > +#include <linux/sched/topology.h>
> > +#include <linux/smp.h>
> >  #include <linux/xarray.h>
> >  
> >  #include <asm/cpu.h>
> > @@ -44,6 +46,66 @@
> >  static DEFINE_PER_CPU_READ_MOSTLY(unsigned long, arch_max_freq_scale) =  1UL << (2 * SCHED_CAPACITY_SHIFT);
> >  static cpumask_var_t amu_fie_cpus;
> >  
> > +/*
> > + * 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;
> > +
> > +#ifdef CONFIG_SCHED_SMT
> > +static int arm64_smt_flags(void)
> > +{
> > +	int flags = cpu_smt_flags();
> > +
> > +	if (olympus_prefer_pe0)
> > +		flags |= SD_ASYM_PACKING;
> > +
> > +	return flags;
> > +}
> > +#endif
> > +
> > +static struct sched_domain_topology_level arm64_asym_smt_topology[] = {
> > +#ifdef CONFIG_SCHED_SMT
> > +	SDTL_INIT(tl_smt_mask, arm64_smt_flags, SMT),
> > +#endif
> > +#ifdef CONFIG_SCHED_CLUSTER
> > +	SDTL_INIT(tl_cls_mask, cpu_cluster_flags, CLS),
> > +#endif
> > +#ifdef CONFIG_SCHED_MC
> > +	SDTL_INIT(tl_mc_mask, cpu_core_flags, MC),
> > +#endif
> > +	SDTL_INIT(tl_pkg_mask, NULL, PKG),
> > +	{ NULL, },
> > +};
> > +
> > +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;
> > +
> > +	olympus_prefer_pe0 = true;
> > +	set_sched_topology(arm64_asym_smt_topology);
> > +	pr_info("Enabling PE0 SMT preference for NVIDIA Olympus\n");
> > +}
> > +
> > +int arch_asym_cpu_priority(int cpu)
> > +{
> > +	if (!olympus_prefer_pe0)
> > +		return 0;
> > +
> > +	return MPIDR_AFFINITY_LEVEL(cpu_logical_map(cpu), 0) == 0;
> > +}
> > +
> >  struct amu_cntr_sample {
> >  	u64		arch_const_cycles_prev;
> >  	u64		arch_core_cycles_prev;
>