Re: [PATCH v3 04/21] sched/cache: Make LLC id continuous

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Feb 10, 2026 at 02:18:44PM -0800, Tim Chen wrote:

> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index cf643a5ddedd..ca46b5cf7f78 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -20,6 +20,7 @@ void sched_domains_mutex_unlock(void)
>  /* Protected by sched_domains_mutex: */
>  static cpumask_var_t sched_domains_tmpmask;
>  static cpumask_var_t sched_domains_tmpmask2;
> +static int tl_max_llcs;
>  
>  static int __init sched_debug_setup(char *str)
>  {
> @@ -658,7 +659,7 @@ static void destroy_sched_domains(struct sched_domain *sd)
>   */
>  DEFINE_PER_CPU(struct sched_domain __rcu *, sd_llc);
>  DEFINE_PER_CPU(int, sd_llc_size);
> -DEFINE_PER_CPU(int, sd_llc_id);
> +DEFINE_PER_CPU(int, sd_llc_id) = -1;
>  DEFINE_PER_CPU(int, sd_share_id);
>  DEFINE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
>  DEFINE_PER_CPU(struct sched_domain __rcu *, sd_numa);
> @@ -684,7 +685,6 @@ static void update_top_cache_domain(int cpu)
>  
>  	rcu_assign_pointer(per_cpu(sd_llc, cpu), sd);
>  	per_cpu(sd_llc_size, cpu) = size;
> -	per_cpu(sd_llc_id, cpu) = id;
>  	rcu_assign_pointer(per_cpu(sd_llc_shared, cpu), sds);
>  
>  	sd = lowest_flag_domain(cpu, SD_CLUSTER);
> @@ -2567,10 +2567,18 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>  
>  	/* Set up domains for CPUs specified by the cpu_map: */
>  	for_each_cpu(i, cpu_map) {
> -		struct sched_domain_topology_level *tl;
> +		struct sched_domain_topology_level *tl, *tl_llc = NULL;
> +		int lid;
>  
>  		sd = NULL;
>  		for_each_sd_topology(tl) {
> +			int flags = 0;
> +
> +			if (tl->sd_flags)
> +				flags = (*tl->sd_flags)();
> +
> +			if (flags & SD_SHARE_LLC)
> +				tl_llc = tl;
>  
>  			sd = build_sched_domain(tl, cpu_map, attr, sd, i);
>  
> @@ -2581,6 +2589,39 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>  			if (cpumask_equal(cpu_map, sched_domain_span(sd)))
>  				break;
>  		}
> +
> +		lid = per_cpu(sd_llc_id, i);
> +		if (lid == -1) {
> +			int j;
> +
> +			/*
> +			 * Assign the llc_id to the CPUs that do not
> +			 * have an LLC.
> +			 */

Where does this happen? Is this for things like Atom that don't have an
L3 and so we don't set up a LLC domain?

> +			if (!tl_llc) {
> +				per_cpu(sd_llc_id, i) = tl_max_llcs++;
> +
> +				continue;
> +			}
> +
> +			/* try to reuse the llc_id of its siblings */
> +			for_each_cpu(j, tl_llc->mask(tl_llc, i)) {
> +				if (i == j)
> +					continue;
> +
> +				lid = per_cpu(sd_llc_id, j);
> +
> +				if (lid != -1) {
> +					per_cpu(sd_llc_id, i) = lid;
> +
> +					break;
> +				}
> +			}
> +
> +			/* a new LLC is detected */
> +			if (lid == -1)
> +				per_cpu(sd_llc_id, i) = tl_max_llcs++;
> +		}
>  	}
>  
>  	if (WARN_ON(!topology_span_sane(cpu_map)))
> -- 
> 2.32.0
>