Re: [Patch v4 18/22] sched/cache: Enable cache aware scheduling for multi LLCs NUMA node

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 01, 2026 at 02:52:30PM -0700, Tim Chen wrote:

> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 995a42cb4697..0b1fc1b0709d 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -809,6 +809,7 @@ enum s_alloc {
>  };
>  
>  #ifdef CONFIG_SCHED_CACHE
> +DEFINE_STATIC_KEY_FALSE(sched_cache_present);
>  static bool alloc_sd_llc(const struct cpumask *cpu_map,
>  			 struct s_data *d)
>  {
> @@ -2674,6 +2675,7 @@ static int
>  build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *attr)
>  {
>  	enum s_alloc alloc_state = sa_none;
> +	bool has_multi_llcs = false;
>  	struct sched_domain *sd;
>  	struct s_data d;
>  	struct rq *rq = NULL;
> @@ -2784,10 +2786,12 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>  				 * between LLCs and memory channels.
>  				 */
>  				nr_llcs = sd->span_weight / child->span_weight;
> -				if (nr_llcs == 1)
> +				if (nr_llcs == 1) {
>  					imb = sd->span_weight >> 3;
> -				else
> +				} else {
>  					imb = nr_llcs;
> +					has_multi_llcs = true;
> +				}
>  				imb = max(1U, imb);
>  				sd->imb_numa_nr = imb;
>  

Well that all don't apply :/

Also, this seems sub-optimal, we can have multi-llc without NUMA.

Doesn't the LLC domain having a parent more or less imply there being
multiple and we should be doing this, irrespective of having NUMA?