Re: [PATCH v3 3/7] sched/fair: Add cgroup_mode: max

[Peter Zijlstra]

On Wed, Jun 10, 2026 at 11:09:59AM -0400, Waiman Long wrote:

> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -4116,6 +4116,21 @@ bool cpuset_cpus_allowed_fallback(struct
> >   	return changed;
> >   }
> > +int cpuset_num_cpus(struct cgroup *cgrp)
> > +{
> > +	int nr = num_online_cpus();
> > +	struct cpuset *cs;
> > +
> > +	if (is_in_v2_mode()) {
> > +		guard(rcu)();
> > +		cs = css_cs(cgroup_e_css(cgrp, &cpuset_cgrp_subsys));
> > +		if (cs)
> > +			nr = cpumask_weight(cs->effective_cpus);
> > +	}
> > +
> > +	return nr;
> > +}
> 
> I just have a question about cgroup v1 support. I am assuming that cgroup v1
> without the cpuset_v2_mode mount option is not supported. 

Correct.

> To fully support
> cgroup v1, you may have to use guarantee_active_cpus() to return the actual
> set of CPUs that the task can run on.

Except this is group based, we'd need an iteration of all tasks in the
group and compute a union of guarantee_active_cpus(). Which all seems
far too expensive and not worth the effort.

> Also there is a caveat about the arm64 specific
> task_cpu_possible_mask() for certain arm64 CPUs. That is for 32-bit
> binary running on 64-bit core which are allowed only on a selected
> subset of cores within the CPU.
> 
> This is probably not what you want to focus on right now, but it will be
> good to have a comment to list items that are not fully supported here.

Will add a comment!