Re: [PATCH v2 09/23] sched/cache: Count tasks prefering destination LLC in a sched group

[Peter Zijlstra]

On Wed, Dec 03, 2025 at 03:07:28PM -0800, Tim Chen wrote:
> During LLC load balancing, tabulate the number of tasks on each runqueue
> that prefer the LLC contains the env->dst_cpu in a sched group.
> 
> For example, consider a system with 4 LLC sched groups (LLC0 to LLC3)
> balancing towards LLC3. LLC0 has 3 tasks preferring LLC3, LLC1 has
> 2, and LLC2 has 1. LLC0, having the most tasks preferring LLC3, is
> selected as the busiest source to pick tasks from.
> 
> Within a source LLC, the total number of tasks preferring a destination
> LLC is computed by summing counts across all CPUs in that LLC. For
> instance, if LLC0 has CPU0 with 2 tasks and CPU1 with 1 task preferring
> LLC3, the total for LLC0 is 3.
> 
> These statistics allow the load balancer to choose tasks from source
> sched groups that best match their preferred LLCs.
> 
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> ---
> 
> Notes:
>     v1->v2:
>         Convert nr_pref_llc array in sg_lb_stats to a single
>         variable as only the dst LLC stat is needed.
>         (K Prateek Nayak)
> 
>  kernel/sched/fair.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b0e87616e377..4d7803f69a74 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10445,6 +10445,9 @@ struct sg_lb_stats {
>  	unsigned int nr_numa_running;
>  	unsigned int nr_preferred_running;
>  #endif
> +#ifdef CONFIG_SCHED_CACHE
> +	unsigned int nr_pref_llc;
> +#endif

At this point I have to note that rq->nr_pref_llc seems like a horrible
misnomer, for it being an array, and not an actual number like the
naming suggests.