Re: [PATCH v5 5/6] sched/fair: Allow load balancing between CPUs of identical capacity

[Andrea Righi <arighi@nvidia.com>]

Hi Ricardo,

On Mon, Jun 22, 2026 at 05:05:55PM -0700, 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.
> 
> Use arch_scale_cpu_capacity() to reflect architectural capacity, excluding
> runtime reductions due to side activity or thermal pressure. Guard this
> check with the sched_cluster_active static key so that systems without
> cluster topology are unaffected.
> 
> Tested-by: Christian Loehle <christian.loehle@arm.com>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
> Changes in v5:
>  * Optimized logic to identify same-arch clusters only when needed.
>  * Added Tested-by tag from Christian. Thanks!
> 
> Changes in v4:
>  * Implemented the check for cluster with a local variable for improved
>    readability.
> 
> Changes in v3:
>  * Reverted the inverted capacity check; the inverted form incorrectly
>    allows migrations to CPUs of slightly less capacity.
>  * Guarded the check for architectural capacity with the
>    sched_cluster_active static key.
> 
> Changes in v2:
>  * 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 | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e55eb019d2c9..f4eb55cad54d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12992,13 +12992,20 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
>  		 */
>  		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
>  		    nr_running == 1) {
> +			bool same_arch_cluster = static_branch_unlikely(&sched_cluster_active) &&
> +						 (arch_scale_cpu_capacity(env->dst_cpu) ==
> +						  arch_scale_cpu_capacity(i));

I find same_arch_cluster a bit misleading. It sounds like "these two CPUs belong
to the same cluster", while what it actually checks is whether a cluster
topology exists somewhere in the root domain and the two CPUs have exactly the
same architectural capacity. Am I understanding it correctly?

If so, would something like same_arch_capacity or cluster_equal_capacity be a
better name? I think either would make the intent of the code a bit clearer.

Thanks,
-Andrea

>  			bool smt_degraded_cap = sched_smt_active() && !is_core_idle(i);
>  
>  			/*
>  			 * Busy SMT siblings reduce the capacity of CPU @i. Do
>  			 * not skip it in this case.
> +			 *
> +			 * CONFIG_SCHED_CLUSTER requires balancing load across clusters
> +			 * of identical capacity. Use architectural capacity to ignore
> +			 * runtime variability.
>  			 */
> -			if (!smt_degraded_cap &&
> +			if (!smt_degraded_cap && !same_arch_cluster &&
>  			    !capacity_greater(capacity_of(env->dst_cpu), capacity))
>  				continue;
>  		}
> 
> -- 
> 2.43.0
>