Re: [PATCH 2/2] sched/fair: Honor asymmetric SMT priority in idle selection

[Srikar Dronamraju <srikar@linux.ibm.com>]

* Andrea Righi <arighi@nvidia.com> [2026-09-04 11:18:05]:

Hi Andrea,

> POWER7 and NVIDIA Olympus use SD_ASYM_PACKING at the shared-capacity
> SMT level to order hardware threads. Idle CPU selection does not consult
> that order, so a task can wake on an arbitrary sibling and remain there
> until load balancing corrects the placement. On these systems, that
> initial choice can prevent the core from entering its preferred
> lower-thread resource mode and cause a large and persistent performance
> loss.
> 

> When idle selection finds an available CPU in an SMT core, choose the
> highest-priority available sibling. On SMT2 Olympus this only changes
> selection on fully idle cores. A partially idle core has only one
> available CPU. On wider SMT systems such as POWER7, it also fills
> available siblings in priority order while the core is partially busy.
> 

Don't we need changes in the slow path too?
Something like this?
https://lore.kernel.org/all/20251204175405.1511340-2-srikar@linux.ibm.com/T/#u

> Apply the preference to idle-core and idle-CPU scans,
> asymmetric-capacity scans, and the target, previous, and recently-used
> CPU fast paths. Inspect the lowest scheduling domain directly, but
> require both CPUs to share its span because isolcpus can split hardware
> siblings across scheduling domains.
> 
> Keep physical-core capacity selection independent from SMT sibling
> ordering. SD_ASYM_CPUCAPACITY first selects among cores with different
> maximum capacities, then SD_ASYM_PACKING selects the preferred available
> sibling inside the chosen core, whose siblings continue to share equal
> capacity.
> 
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  kernel/sched/fair.c     | 79 ++++++++++++++++++++++++++++++++++++-----
>  kernel/sched/sched.h    |  6 ++++
>  kernel/sched/topology.c | 36 +++++++++++++++++++
>  3 files changed, 112 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b8bd308c2d5b1..ff9a7b1fcbe7f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8587,6 +8587,63 @@ static inline bool test_idle_cores(int cpu)
>  	return false;
>  }
>  
> +/*
> + * Return true when @cpu has a higher asymmetric-packing priority than
> + * @other in their shared SMT scheduling domain.
> + */
> +static bool sched_smt_asym_prefer(int cpu, int other)
> +{
> +	struct sched_domain *sd = rcu_dereference_all(cpu_rq(cpu)->sd);
> +
> +	if (!sd)
> +		return false;
> +
> +	if (!(sd->flags & SD_SHARE_CPUCAPACITY) ||
> +	    !(sd->flags & SD_ASYM_PACKING))
> +		return false;
> +
> +	if (!cpumask_test_cpu(other, sched_domain_span(sd)))
> +		return false;
> +
> +	return sched_asym_prefer(cpu, other);
> +}
> +
> +/*
> + * Return the highest-priority available CPU in @cpu's SMT core that is also in @cpus.
> + */
> +static int __select_idle_smt_cpu(struct task_struct *p, int cpu, const struct cpumask *cpus)
> +{
> +	int best = cpu;
> +	int sibling;
> +
> +	for_each_cpu_and(sibling, cpu_smt_mask(cpu), cpus) {
> +		if (sibling == best || !choose_idle_cpu(sibling, p))
> +			continue;
> +
> +		if (sched_smt_asym_prefer(sibling, best))
> +			best = sibling;
> +	}
> +
> +	return best;
> +}
> +
> +static inline int
> +select_idle_smt_cpu(struct task_struct *p, int cpu, const struct cpumask *cpus)
> +{
> +	if (!sched_smt_asym_active())
> +		return cpu;
> +
> +	return __select_idle_smt_cpu(p, cpu, cpus);

Nit: I see __select_idle_smt_cpu called only here.
Cant we fold __select_idle_smt_cpu() here itself.

> +}
> +
> +/*
> + * Redirect an available SMT CPU to a higher-priority available sibling allowed by task affinity.
> + */
> +static inline int select_idle_smt_priority(struct task_struct *p, int cpu)
> +{
> +	return select_idle_smt_cpu(p, cpu, p->cpus_ptr);
> +}
> +

nit: Can we also replace select_idle_smt_priority with select_idle_smt_cpu()
itself.

Otherwise looks good to me

Reviewed-by: Srikar Dronamraju <srikar@linux.ibm.com>

-- 
Thanks and Regards
Srikar Dronamraju