Hello.

On Tue, Feb 09, 2021 at 11:33:00AM -0500, Johannes Weiner <hannes@cmpxchg.org> wrote:
> @@ -1971,10 +1978,14 @@ int cgroup_setup_root(struct cgroup_root *root, u16 ss_mask)
>  	if (ret)
>  		goto destroy_root;
>  
> -	ret = rebind_subsystems(root, ss_mask);
> +	ret = cgroup_rstat_init(root_cgrp);
Would it make sense to do cgroup_rstat_init() only if there's a subsys
in ss_mask that makes use of rstat?
(On legacy systems there could be individual hierarchy for each
controller so the rstat space can be saved.)

> @@ -5159,11 +5170,9 @@ static struct cgroup *cgroup_create(struct cgroup *parent, const char *name,
>  	if (ret)
>  		goto out_free_cgrp;
>  
> -	if (cgroup_on_dfl(parent)) {
> -		ret = cgroup_rstat_init(cgrp);
> -		if (ret)
> -			goto out_cancel_ref;
> -	}
> +	ret = cgroup_rstat_init(cgrp);
And here do cgroup_rstat_init() only when parent has it.

> @@ -285,8 +285,6 @@ void __init cgroup_rstat_boot(void)
>  
>  	for_each_possible_cpu(cpu)
>  		raw_spin_lock_init(per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu));
> -
> -	BUG_ON(cgroup_rstat_init(&cgrp_dfl_root.cgrp));
>  }
Regardless of the suggestion above, this removal obsoletes the comment
cgroup_rstat_init:

         int cpu;
 
-        /* the root cgrp has rstat_cpu preallocated */
         if (!cgrp->rstat_cpu) {
                 cgrp->rstat_cpu = alloc_percpu(struct cgroup_rstat_cpu);

Regards,
Michal