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

[Vincent Guittot]

On Thu, 14 May 2026 at 20:24, Ricardo Neri
<ricardo.neri-calderon@xxxxxxxxxxxxxxx> 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@xxxxxxx>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>

Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

> ---
> 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.
> +                                */
> +                               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 */
>
> --
> 2.43.0
>