Re: [PATCH RESEND 2/4] sched/fair: Ignore misfit load if the destination CPU cannot help

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

On 3/30/26 23:20, Ricardo Neri wrote:
> There is no point in identifying scheduling groups with misfit tasks if the
> destination CPU cannot help (i.e., it has less than 20% greater capacity
> than the most performant CPU in the group).

There's a mismatch here because capacity_greater() is 5%?
You could use 20% fits fits_capacity() I'd say it's too strict here.

> 
> Since migrating misfit tasks takes precedence over relieving fully_busy
> groups, identifying a group with misfit tasks causes a destination CPU of
> smaller maximum capacity to back off (see capacity checks in update_sd_
> pick_busiest()) even if it can help: it could help a group of equally small
> maximum capacity if classified as fully_busy or has_spare.
> 
> The described situation can happen if a scheduling domain has groups of
> big CPUs alongside two or more clusters of smaller CPUs that share L2
> cache. Load should be balanced between these sets of smaller CPUs when
> CONFIG_SCHED_CLUSTER is enabled.
> 
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
>  kernel/sched/fair.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9da5014f8387..3c50ecffa4c7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10302,7 +10302,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>  		if (local_group)
>  			continue;
>  
> -		if (sd_flags & SD_ASYM_CPUCAPACITY) {
> +		/* Only look for misfit load if dst_cpu can help */
> +		if (sd_flags & SD_ASYM_CPUCAPACITY &&
> +		    capacity_greater(capacity_of(env->dst_cpu), group->sgc->max_capacity)) {
>  			/* 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;
> 

Apart from the above nit:

Reviewed-by: Christian Loehle <christian.loehle@arm.com>