Re: [PATCH v2 04/23] sched/cache: Make LLC id continuous

[Tim Chen]

On Tue, 2025-12-09 at 12:58 +0100, Peter Zijlstra wrote:
> On Wed, Dec 03, 2025 at 03:07:23PM -0800, Tim Chen wrote:
> 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 710ed9943d27..0a3918269906 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -1210,10 +1210,17 @@ __read_mostly unsigned int llc_imb_pct            = 20;
> >  
> >  static int llc_id(int cpu)
> >  {
> > +	int llc;
> > +
> >  	if (cpu < 0)
> >  		return -1;
> >  
> > +	llc = per_cpu(sd_llc_id, cpu);
> > +	/* avoid race with cpu hotplug */
> > +	if (unlikely(llc >= max_llcs))
> > +		return -1;
> > +
> > +	return llc;
> >  }
> >  
> >  void mm_init_sched(struct mm_struct *mm, struct mm_sched __percpu *_pcpu_sched)
> 
> > @@ -668,6 +670,55 @@ DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_cpucapacity);
> >  DEFINE_STATIC_KEY_FALSE(sched_asym_cpucapacity);
> >  DEFINE_STATIC_KEY_FALSE(sched_cluster_active);
> >  
> > +/*
> > + * Assign continuous llc id for the CPU, and return
> > + * the assigned llc id.
> > + */
> > +static int update_llc_id(struct sched_domain *sd,
> > +			 int cpu)
> > +{
> > +	int id = per_cpu(sd_llc_id, cpu), i;
> > +
> > +	if (id >= 0)
> > +		return id;
> > +
> > +	if (sd) {
> > +		/* Look for any assigned id and reuse it.*/
> > +		for_each_cpu(i, sched_domain_span(sd)) {
> > +			id = per_cpu(sd_llc_id, i);
> > +
> > +			if (id >= 0) {
> > +				per_cpu(sd_llc_id, cpu) = id;
> > +				return id;
> > +			}
> > +		}
> > +	}
> > +
> > +	/*
> > +	 * When 1. there is no id assigned to this LLC domain,
> > +	 * or 2. the sd is NULL, we reach here.
> > +	 * Consider the following scenario,
> > +	 * CPU0~CPU95 are in the node0, CPU96~CPU191 are
> > +	 * in the node1. During bootup, maxcpus=96 is
> > +	 * appended.
> > +	 * case 1: When running cpu_attach_domain(CPU24)
> > +	 * during boot up, CPU24 is the first CPU in its
> > +	 * non-NULL LLC domain. However,
> > +	 * its corresponding llc id has not been assigned yet.
> > +	 *
> > +	 * case 2: After boot up, the CPU100 is brought up
> > +	 * via sysfs manually. As a result, CPU100 has only a
> > +	 * Numa domain attached, because CPU100 is the only CPU
> > +	 * of a sched domain, all its bottom domains are degenerated.
> > +	 * The LLC domain pointer sd is NULL for CPU100.
> > +	 *
> > +	 * For both cases, we want to increase the number of LLCs.
> > +	 */
> > +	per_cpu(sd_llc_id, cpu) = max_llcs++;
> > +
> > +	return per_cpu(sd_llc_id, cpu);
> > +}
> 
> I'm not sure I follow. So partition_sched_domains() first calls
> detach_destroy_domains() on the old set, and then build_sched_domains()
> on the new set.
> 
> Do detach_destroy_domain() will do:
> 
>   cpu_attach_domain(NULL,..);
> 
> That is, it will explicitly attach the NULL sched_domain to a CPU. At
> which point I feel update_llc_id() should be returning -1, no?
> 
> Then later, build_sched_domains() will set a !NULL sched_domain, at
> which point update_llc_id() can set a real value.
> 
> This should then also get rid of that weird max_llcs check in llc_id(),
> right?

Thanks for pointing this out.  Yes, we should take care of the
attachment of NULL sd. Will update the code accordingly.

Tim