Re: [PATCH v2 3/4] sched/fair: Allow load balancing between CPUs of identical capacity

[Christian Loehle <christian.loehle@arm.com>]

On 4/29/26 22:19, Ricardo Neri wrote:
> sched_balance_find_src_rq() avoids selecting a runqueue with a single
> running task as busiest if doing so results in migrating the task to a
> CPU with less than ~5% of extra capacity. It also unintentionally
> prevents migrations between CPUs of identical capacity.
> 
> When CONFIG_SCHED_CLUSTER is enabled, load should be balanced across
> clusters of CPUs with the same capacity. Allowing migration between CPUs
> of identical capacity is necessary to meet this goal.
> 
> We are interested in the architectural capacity of the involved CPUs,
> excluding any reductions due to side activity or thermal pressure. Use
> arch_scale_cpu_capacity().
> 
> While here, invert the check for runtime capacity for clarity.
> 
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
> Changes since v1:
>  * Used arch_scale_cpu_capacity() instead of capacity_of() to ignore
>    runtime variability.
>  * Inverted the check for runtime capacity. (Christian)
>  * Reworded patch description for clarity.
> ---
>  kernel/sched/fair.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 166a5b109e0e..4105717e64fe 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11816,9 +11816,14 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
>  		 * eventually lead to active_balancing high->low capacity.
>  		 * Higher per-CPU capacity is considered better than balancing
>  		 * average load.
> +		 *
> +		 * Cluster scheduling requires balancing load across clusters
> +		 * of identical capacity. Use architectural capacity to ignore
> +		 * runtime variability.
>  		 */
>  		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
> -		    !capacity_greater(capacity_of(env->dst_cpu), capacity) &&
> +		    arch_scale_cpu_capacity(env->dst_cpu) != arch_scale_cpu_capacity(i) &&
> +		    capacity_greater(capacity, capacity_of(env->dst_cpu)) &&
>  		    nr_running == 1)
>  			continue;
>  
> 

I wonder if we shouldn't use capacity_greater() margin for both, i.e.
capacity_greater(arch_scale_cpu_capacity(i), arch_scale_cpu_capacity(env->dst_cpu)) &&

For example the orion o6 has a cluster with 1024 and one with 984, If we allow balancing
984->984 I think it's only consistent to also allow 984->1024.