Re: [PATCH v2 2/4] sched/fair: Skip misfit load accounting when the destination CPU cannot help

[Ricardo Neri]

On Wed, May 06, 2026 at 12:39:23PM +0100, Christian Loehle wrote:
> On 4/29/26 22:19, 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. This
> > flag propagates to the root domain and allows bigger CPUs in it to help
> > via newly idle balance.
> > 
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > ---
> > Changes since v1:
> >  * Moved the check of the destination CPU capacity inside the code block
> >    used for SD_ASYM_CPUCAPACITY. v1 inadvertedly 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
> >    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 0dbed82aa63f..166a5b109e0e 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -10719,10 +10719,24 @@ 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 */
> > 
> 
> Reviewed-by: Christian Loehle <christian.loehle@xxxxxxx>

Thank you for your review!