Re: [PATCH v3 3/4] sched/fair: Allow load balancing between CPUs of identical capacity

["Chen, Yu C" <yu.c.chen@intel.com>]

On 5/15/2026 2:34 AM, 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.
> 
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
> 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 | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index dcc02ceb44b5..d2a4c529f67f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11846,8 +11846,14 @@ 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.
> +		 *
> +		 * CONFIG_SCHED_CLUSTER requires balancing load across clusters
> +		 * of identical capacity. Use architectural capacity to ignore
> +		 * runtime variability.
>   		 */
>   		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
> +		    (!static_branch_unlikely(&sched_cluster_active) ||
> +		     arch_scale_cpu_capacity(env->dst_cpu) != arch_scale_cpu_capacity(i)) &&
>   		    !capacity_greater(capacity_of(env->dst_cpu), capacity) &&

As stated in the commit log, the existing logic blocks task migrations 
between CPUs
with identical capacity, which is based on capacity_of() comparison 
rather than
arch_scale_cpu_capacity. Could I kindly ask why replacing
!capacity_greater(capacity_of(env->dst_cpu), capacity)
with
capacity_greater(capacity, capacity_of(env->dst_cpu))
does not achieve the expected effect?
This would theoretically enable migration among equal-capacity CPUs, and
in most cases e-cores in different clusters should return 0 thus
load balance is allowed.

thanks,
Chenyu