Re: [PATCH v6 5/6] sched/fair: Allow load balancing between CPUs of identical capacity

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

On 7/21/26 03:43, 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 get_actual_cpu_capacity() to reflect architectural capacity as well
> as diminished capacity due to hardware or cpufreq 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>
> Tested-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
> Changes in v6:
>  * Switched to use get_actual_cpu_capacity() instead of
>    arch_scale_cpu_capacity(). The former considers rq->avg_hw.load_avg and
>    cpufreq_pressure and their impact on CPU capacity. (Vincent)
>  * Renamed the variable same_arch_cluster as cluster_equal_cap for
>    clarity. (Andrea)
>  * Added Tested-by tag from Andrea. Thanks!
> 
> 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 feea47e6abea..de4189b562ac 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -13104,13 +13104,20 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
>  		 */
>  		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
>  		    nr_running == 1) {
> +			bool cluster_equal_cap = static_branch_unlikely(&sched_cluster_active) &&
> +						 (get_actual_cpu_capacity(env->dst_cpu) ==
> +						  get_actual_cpu_capacity(i));


I guess it's extremely unlikely, but it _feels_ wrong to have to clusters of different
arch_scale_cpu_capacity() equal to true here because of system/thermal pressure (which is
obviously considered more transient).
Adding && arch_scale_cpu_capacity(env->dst_cpu) == arch_scale_cpu_capacity(i) might even
make the check cheaper because it's better for the branch predictor than get_actual_cpu_capacity().
Vincent, would you be fine with requiring both: equal get_actual_cpu_capacity() and
arch_scale_cpu_capacity()?

Given how unlikely it actually is in any case:
Reviewed-by: Christian Loehle <christian.loehle@arm.com>

>  			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, accounting for
> +			 * hardware and cpufreq pressure.
>  			 */
> -			if (!smt_degraded_cap &&
> +			if (!smt_degraded_cap && !cluster_equal_cap &&
>  			    !capacity_greater(capacity_of(env->dst_cpu), capacity))
>  				continue;
>  		}
>