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

[Waiman Long]

On 6/11/26 9:47 AM, Peter Zijlstra wrote:
> 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.
I thought so.
> > 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!
Thanks,
Longman