Re: [PATCH] cgroup/cpuset: Use cpuset_v2() in cpuset_num_cpus()

[Andrea Righi <arighi@nvidia.com>]

Hi Waiman,

On Wed, Sep 30, 2026 at 11:48:00AM -0400, Waiman Long wrote:
> The cpuset code has 2 different helpers for determining if it is in a
> v1 or v2 working environment - cpuset_v2() and is_in_v2_mode().
> 
> cpuset_v2() checks the binding of the cpuset subsystem is on a v1
> or v2 hierarchy. It can also help to optimize out v1 specific code
> if CONFIG_CPUSETS_V1 isn't defined.
> 
> is_in_v2_mode() layers on top of cpuset_v2() and returns true if it is in
> a v2 hierarchy or in a v1 hierarchy with "cpuset_v2_mode" mount option
> enabled to make "cpuset.cpus" and "cpuset.mems" behave in a way similar
> to their v2 counterparts. However the cpuset hierarchy may still be a
> standalone one separated from the other cgroup hierarchies.
> 
> Commit 90ac22ffef48 ("sched/fair: Add cgroup_mode: max") adds a new
> cpuset_num_cpus() helper to return the number of effective CPUs in a given
> cgroup. It is part of the "sched: Flatten the pick" patch series to help
> flatten the cpu cgroup runqueue hierachies. cpuset_num_cpus() currently
> uses is_in_v2_mode() to determine if it is in a v2 environment. However
> is_in_v2_mode() can be true in a v1 environment where the cpuset and cpu
> cgroup subsystems can be in two separate hierachies. The current use
> case [1] for cpuset_num_cpus() assumes that it can return the number
> of effective CPUs present in the current cpu cgroup which may not be
> the case.
> 
> Fix that by switching to use cpuset_v2() which signifies that it is in
> a unified hierarchy where both cpuset and cpu cgroup subsystems share
> the same cgroup hierarchy if it returns true.
> 
> [1] https://lore.kernel.org/lkml/20260930081125.GF88198@noisy.programming.kicks-ass.net
> 
> Fixes: 90ac22ffef48 ("sched/fair: Add cgroup_mode: max")
> Signed-off-by: Waiman Long <longman@redhat.com>

Works for my particular sched_ext use case.

Reviewed-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> ---
>  kernel/cgroup/cpuset.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 4a2b6f154609..cbf67ad13874 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -4251,7 +4251,7 @@ int cpuset_num_cpus(struct cgroup *cgrp)
>  	int nr = num_online_cpus();
>  	struct cpuset *cs;
>  
> -	if (is_in_v2_mode()) {
> +	if (cpuset_v2()) {
>  		guard(rcu)();
>  		cs = css_cs(cgroup_e_css(cgrp, &cpuset_cgrp_subsys));
>  		if (cs)
> -- 
> 2.55.0
>