Re: [PATCH v1 7/8] sched/topology: Avoid redundant percpu allocations and bitmap walks in build_sched_domains()

[Pasha Tatashin]

On 10-15 16:17, Tim Chen wrote:
> On Thu, 2026-10-15 at 11:33 -0400, Pasha Tatashin wrote:
> > During CPU hotplug, build_sched_domains() is invoked on every CPU
> > activation and deactivation. Profiling build_sched_domains() on a
> > 480-CPU Intel Xeon 6985P-C and a 512-CPU AMD EPYC 9B45 shows several
> > sources of avoidable O(N^2) and O(N) work per step:
> > 
> > 1. build_sched_groups() and build_overlap_sched_groups() iterate over
> >    every bit of sched_domain_span(sd) via for_each_cpu_wrap(i, span, cpu)
> >    and check cpumask_test_cpu(i, covered) one CPU at a time, even after
> >    entire child groups (e.g., 120-CPU NUMA nodes) have been added to
> >    covered. Instead, track the remaining uncovered CPUs
> >    (cpumask_copy(uncovered, span)), clear each built group's span with
> >    cpumask_andnot(), and advance directly to the next uncovered CPU.
> > 
> > 2. __visit_domain_allocation_hell() and __free_domain_allocs() allocate
> >    and free 3 * nr_levels + 2 temporary per-CPU pointer arrays via
> >    alloc_percpu() / free_percpu() on every domain rebuild. Reuse the
> >    per-CPU pointer arrays across domain rebuilds under
> >    sched_domains_mutex, clearing entries in __free_domain_allocs() and
> >    only freeing the topology-level arrays when sched_domain_topology is
> >    replaced in sched_init_numa() / sched_reset_numa().
> 
> sched_domains_mutex does not protect the free:
> 
> [snip]
> 
> >  #ifdef CONFIG_NUMA
> > +static void __sdt_free_percpu(void)
> > +{
> > +	struct sched_domain_topology_level *tl;
> > +
> > +	for_each_sd_topology(tl) {
> > +		struct sd_data *sdd = &tl->data;
> > +
> 
> sched_init_numa() and sched_reset_numa() are called from
> sched_update_numa(), which runs from sched_cpu_activate() and
> sched_cpu_deactivate() without sched_domains_mutex. The free is safe
> because every domain builder holds at least cpus_read_lock(), and
> sched_update_numa() runs under the hotplug write lock. 
> 
> Please fix the changelog and add lockdep_assert_cpus_held()
> in __sdt_free_percpu(), so a future caller cannot break this without a warning.

I will update the changelog to note that __sdt_free_percpu() is serialized 
against domain rebuilds by the CPU hotplug lock, and add
lockdep_assert_cpus_held() to __sdt_free_percpu().

> > +		free_percpu(sdd->sd);
> > +		sdd->sd = NULL;
> > +		free_percpu(sdd->sg);
> > +		sdd->sg = NULL;
> > +		free_percpu(sdd->sgc);
> > +		sdd->sgc = NULL;
> > +	}
> > +}
> > +
> >  static int cpu_numa_flags(void)
> >  {
> >  	return SD_NUMA;
> > @@ -2449,6 +2483,7 @@ void sched_init_numa(int offline_node)
> >  	/*
> >  	 * Copy the default topology bits..
> >  	 */
> > +	__sdt_free_percpu();
> >  	for (i = 0; sched_domain_topology[i].mask; i++)
> >  		tl[i] = sched_domain_topology[i];
> >  
> > @@ -2507,6 +2542,7 @@ static void sched_reset_numa(void)
> >  		kfree(masks);
> >  	}
> >  	if (sched_domain_topology_saved) {
> > +		__sdt_free_percpu();
> >  		kfree(sched_domain_topology);
> >  		sched_domain_topology = sched_domain_topology_saved;
> >  		sched_domain_topology_saved = NULL;
> > @@ -2708,17 +2744,23 @@ static int __sdt_alloc(const struct cpumask *cpu_map)
> >  	for_each_sd_topology(tl) {
> >  		struct sd_data *sdd = &tl->data;
> >  
> > -		sdd->sd = alloc_percpu(struct sched_domain *);
> > -		if (!sdd->sd)
> > -			return -ENOMEM;
> > +		if (!sdd->sd) {
> > +			sdd->sd = alloc_percpu(struct sched_domain *);
> > +			if (!sdd->sd)
> > +				return -ENOMEM;
> > +		}
> >  
> > -		sdd->sg = alloc_percpu(struct sched_group *);
> > -		if (!sdd->sg)
> > -			return -ENOMEM;
> > +		if (!sdd->sg) {
> > +			sdd->sg = alloc_percpu(struct sched_group *);
> > +			if (!sdd->sg)
> > +				return -ENOMEM;
> > +		}
> >  
> > -		sdd->sgc = alloc_percpu(struct sched_group_capacity *);
> > -		if (!sdd->sgc)
> > -			return -ENOMEM;
> > +		if (!sdd->sgc) {
> > +			sdd->sgc = alloc_percpu(struct sched_group_capacity *);
> > +			if (!sdd->sgc)
> > +				return -ENOMEM;
> > +		}
> >  
> 
> Keeping the arrays also means every slot must be NULL when a build 
> starts. Patch 5 depends on that: get_group(), 
> init_overlap_sched_group() and init_sched_domain_shared() treat a NULL 
> slot as "not allocated yet", and the overlap fast path treats 
> "sgc->ref > 0" as "set up in this build". An old pointer would pass
> these checks and be used without a warning.
> 
> The rule holds today, but nothing says so. It also depends on builds
> never filling a slot outside cpu_map, because the clearing loops only
> visit cpu_map. Please write this down and check it, for example in
> __sdt_alloc():
> 
> 
> >  		for_each_cpu(j, cpu_map) {
> >  			struct sched_domain *sd;
> 
> Maybe add
> 
>         /*
>          * The slot arrays are kept between builds. Every slot for a CPU
>          * in cpu_map must be NULL here; lazy allocation treats NULL as
>          * "not allocated in this build". claim_allocations() and
>          * __sdt_free()/__sds_free() clear the slots, but only for
>          * cpu_map, so a build must not fill slots outside cpu_map.
>          */
>                 for_each_cpu(j, cpu_map) {
>                         struct sched_domain *sd;
> 
>                         WARN_ON_ONCE(*per_cpu_ptr(sdd->sd, j) ||
>                                      *per_cpu_ptr(sdd->sg, j) ||
>                                      *per_cpu_ptr(sdd->sgc, j));
> 

Sure, I will add the comment, and WARN_ON_ONCE.

> > @@ -2748,30 +2790,36 @@ static void __sdt_free(const struct cpumask *cpu_map)
> >  
> >  			if (sdd->sd) {
> >  				sd = *per_cpu_ptr(sdd->sd, j);
> > -				if (sd && (sd->flags & SD_NUMA))
> > -					free_sched_groups(sd->groups, 0);
> > -				kfree(*per_cpu_ptr(sdd->sd, j));
> > +				if (sd) {
> > +					if (sd->flags & SD_NUMA)
> > +						free_sched_groups(sd->groups, 0);
> > +					kfree(sd);
> > +					*per_cpu_ptr(sdd->sd, j) = NULL;
> > +				}
> >  			}
> >  
> > -			if (sdd->sg)
> > +			if (sdd->sg) {
> >  				kfree(*per_cpu_ptr(sdd->sg, j));
> > -			if (sdd->sgc)
> > +				*per_cpu_ptr(sdd->sg, j) = NULL;
> > +			}
> > +			if (sdd->sgc) {
> >  				kfree(*per_cpu_ptr(sdd->sgc, j));
> > +				*per_cpu_ptr(sdd->sgc, j) = NULL;
> > +			}
> >  		}
> > -		free_percpu(sdd->sd);
> > -		sdd->sd = NULL;
> > -		free_percpu(sdd->sg);
> > -		sdd->sg = NULL;
> > -		free_percpu(sdd->sgc);
> > -		sdd->sgc = NULL;
> >  	}
> >  }
> >  
> > +static struct sched_domain_shared * __percpu *sched_domains_sds;
> > +
> >  static int __sds_alloc(struct s_data *d, const struct cpumask *cpu_map)
> >  {
> 
> and maybe similar check 
> 
>         for_each_cpu(j, cpu_map)
>                 WARN_ON_ONCE(*per_cpu_ptr(d->sds, j));

I will add the comment and the WARN_ON_ONCE() checks in
both __sdt_alloc() and __sds_alloc() for v2.

Thanks,
Pasha