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

[Peter Zijlstra]

On Fri, Feb 20, 2026 at 11:24:11AM -0800, Tim Chen wrote:

> +static int __sched_domains_alloc_llc_id(void)
> +{
> +	int lid, max_lid;
> +
> +	lockdep_assert_held(&sched_domains_mutex);
> +
> +	lid = cpumask_first_zero(sched_domains_llc_id_allocmask);
> +	/*
> +	 * llc_id space should never grow larger than the
> +	 * possible number of CPUs in the system.
> +	 */
> +	BUG_ON(lid >= nr_cpu_ids);

	__cpumask_set_cpu(lid, sched_domains_llc_is_allocmask);

> +	max_lid = cpumask_last(sched_domains_llc_id_allocmask);
> +	/* size is one more than max index */
> +	tl_max_llcs = max(lid, max_lid) + 1;
> +
> +	return lid;
> +}
> +
> +static void __sched_domains_free_llc_id(int cpu)
> +{
> +	int i, lid, last_lid;
> +
> +	lockdep_assert_held(&sched_domains_mutex);
> +
> +	lid = per_cpu(sd_llc_id, cpu);
> +	if (lid == -1)
> +		return;
> +
> +	BUG_ON(lid >= nr_cpu_ids);
> +	per_cpu(sd_llc_id, cpu) = -1;
> +
> +	for_each_cpu(i, cpu_coregroup_mask(cpu)) {
> +		/* An online CPU owns the llc_id. */
> +		if (per_cpu(sd_llc_id, i) == lid)
> +			return;
> +	}
> +
	__cpumask_clear_cpu(lid, sched_domains_llc_id_allocmask);
> +
> +	last_lid = cpumask_last(sched_domains_llc_id_allocmask);
> +	/* shrink max LLC size to save memory */
> +	if (last_lid < tl_max_llcs - 1)
> +		tl_max_llcs = last_lid + 1;
> +}

Might be simpler to just track max_lid, and do the +1 at the alloc site?