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

[Chen, Yu C]

On 2/19/2026 10:59 PM, Peter Zijlstra wrote:
> 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?

Yes, for some hybrid platforms, some CPUs on that platforms might not 
have L3,
Tim might correct me if I’m wrong. Above code is derived from the 
update_top_cache_domain(),
 if there is no sched domain with SD_SHARE_LLC, per_cpu(sd_llc_id, cpu) 
is set to the
CPU number directly.

thanks,
Chenyu