Re: [PATCH v3 2/4] sched/fair: Skip misfit load accounting when the destination CPU cannot help

[Tim Chen <tim.c.chen@linux.intel.com>]

On Thu, 2026-05-14 at 11:34 -0700, Ricardo Neri wrote:
> In domains with asymmetric capacity, identifying misfit load in a
> scheduling group is not useful when the destination CPU cannot help (i.e.,
> its capacity exceeds the group's maximum CPU capacity by less than ~5%). In
> such cases, it also prevents load balance among clusters of equal capacity
> when CONFIG_SCHED_CLUSTER is enabled. This happens because
> update_sd_pick_busiest() skips candidate groups of type misfit_task if the
> destination CPU has similar capacity.
> 
> Skipping misfit load accounting in this situation allows the group to be
> classified as has_spare or fully_busy and lets load balancing proceed. Keep
> marking scheduling groups as overloaded when misfit tasks are present. The
> sg_overloaded flag propagates to the root domain and allows bigger CPUs in
> it to help via newly idle balance.
> 
> Reviewed-by: Christian Loehle <christian.loehle@arm.com>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
> Changes in v3:
>  * Added Reviewed-by tag from Christian. Thanks!
> 
> Changes in v2:
>  * Moved the check of the destination CPU capacity inside the code block
>    used for SD_ASYM_CPUCAPACITY. v1 inadvertently broke the mutual
>    exclusion of the sched_reduced_capacity() path.
>  * Keep marking the root domain as overloaded to allow bigger CPUs to
>    help. (sashiko)
>  * Fixed patch description to clarify that the capacity_greater() looks
>    for differences of 5% or more. (Christian)
>  * Reworded the patch description for clarity.
>  * I did not include the Reviewed-by tag from Christian since the patch
>    changed functionally.
> ---
>  kernel/sched/fair.c | 20 +++++++++++++++++---
>  1 file changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e06e74d9ce0e..dcc02ceb44b5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10749,10 +10749,24 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>  			continue;
>  
>  		if (sd_flags & SD_ASYM_CPUCAPACITY) {
> -			/* Check for a misfit task on the cpu */
> -			if (sgs->group_misfit_task_load < rq->misfit_task_load) {
> -				sgs->group_misfit_task_load = rq->misfit_task_load;
> +			if (rq->misfit_task_load) {
> +				/*
> +				 * Always mark the domain overloaded so big CPUs
> +				 * can pick up misfit tasks via newly idle
> +				 * balance.
> +				 */
>  				*sg_overloaded = 1;
> +
> +				/*
> +				 * Only account misfit load if @dst_cpu can
> +				 * help; otherwise, the group may be classified
> +				 * as misfit_task and update_sd_pick_busiest()
> +				 * will skip it.

You mean "sd_pick_busiest() will pick it" instead of "skip it" for misfit task
load balancing in the above comment?

Tim

> +				 */
> +				if (capacity_greater(capacity_of(env->dst_cpu),
> +						     group->sgc->max_capacity) &&
> +				    (sgs->group_misfit_task_load < rq->misfit_task_load))
> +					sgs->group_misfit_task_load = rq->misfit_task_load;
>  			}
>  		} else if (env->idle && sched_reduced_capacity(rq, env->sd)) {
>  			/* Check for a task running on a CPU with reduced capacity */