Re: [PATCH 3/6] sched/fair: Fix busiest group selection for asym groups

[Peter Zijlstra]

On Thu, May 04, 2023 at 09:09:53AM -0700, Tim Chen wrote:
> From: Tim C Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> 
> For two groups that have spare capacity and are partially busy, the
> busier group should be the group with pure CPUs rather than the group
> with SMT CPUs.  We do not want to make the SMT group the busiest one to
> pull task off the SMT and makes the whole core empty.
> 
> Otherwise suppose in the search for busiest group,
> we first encounter an SMT group with 1 task and set it as the busiest.
> The local group is an atom cluster with 1 task and we then encounter an atom
> cluster group with 3 tasks, we will not pick this atom cluster group over the
> SMT group, even though we should.  As a result, we do not load balance
> the busier Atom cluster (with 3 tasks) towards the local Atom cluster
> (with 1 task).  And it doesn't make sense to pick the 1 task SMT group
> as the busier group as we also should not pull task off the SMT towards
> the 1 task atom cluster and make the SMT core completely empty.
> 
> Fix this case.
> 
> Reviewed-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bde962aa160a..8a325db34b02 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9548,6 +9548,18 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>  		break;
>  
>  	case group_has_spare:
> +		/*
> +		 * Do not pick sg with SMT CPUs over sg with pure CPUs,
> +		 * as we do not want to pull task off half empty SMT core
> +		 * and make the core idle.
> +		 */

Comment says what the code does; not why.

> +		if (asymmetric_groups(sds->busiest, sg)) {
> +			if (sds->busiest->flags & SD_SHARE_CPUCAPACITY)
> +				return true;
> +			else
> +				return false;

			return (sds->busiest->flags & SD_SHARE_CPUCAPACITY)
> +		}

Also, should this not be part of the previous patch?