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

[Tim Chen]

On Thu, 2026-02-19 at 23:20 +0800, Chen, Yu C wrote:
> 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.
> 

That's correct.  One example is Meteor Lake where some Atom CPUs don't have
L3 but have only L2.  And some Ampere CPUs also have no shared L3.

https://www.spinics.net/lists/kernel/msg5863118.html?utm_source=chatgpt.com

This also reminded me that if we rely on cpu_coregroup_mask for LLC id
assignment, we may be missing out such platforms which need to treat 
L2 as the last level cache.  So we may need to fallback to cpu_clustergroup_mask
or cpu_smt_mask where applicable.

Tim 

> thanks,
> Chenyu
>