Re: [PATCH 04/17] cgroup/cpuset: Limit type-change accounting to owned CPUs

[Waiman Long <longman@redhat.com>]

On 8/20/26 8:41 AM, Guopeng Zhang wrote:
> From: Guopeng Zhang <zhangguopeng@kylinos.cn>
>
> effective_xcpus includes CPUs granted to valid child partitions. Passing
> the whole mask to isolated_cpus_update() during a root-to-isolated or
> isolated-to-root change applies the parent's new state to child-owned
> CPUs as well.
>
> Build a mask of CPUs owned by the partition by subtracting the
> effective_xcpus of valid children. Use this mask when updating
> isolated_cpus for a partition type change.
>
> Fixes: 11e5f407b64a ("cgroup/cpuset: Keep track of CPUs in isolated partitions")
> Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
> ---
>   kernel/cgroup/cpuset.c | 30 ++++++++++++++++++++++++++++--
>   1 file changed, 28 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 2538faac9aba..468272baadb2 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2155,6 +2155,30 @@ static void compute_partition_effective_cpumask(struct cpuset *cs,
>   	rcu_read_unlock();
>   }
>   
> +/*
> + * Compute CPUs owned directly by a partition.
> + *
> + * effective_xcpus includes CPUs granted to valid child partitions. Exclude
> + * those CPUs when changing only this partition's type.
> + */
> +static void compute_partition_owned_cpumask(struct cpuset *cs,
> +					    struct cpumask *owned_cpus)
> +{
> +	struct cgroup_subsys_state *css;
> +	struct cpuset *child;
> +
> +	lockdep_assert_held(&cpuset_mutex);
> +	cpumask_copy(owned_cpus, cs->effective_xcpus);
> +
> +	rcu_read_lock();
> +	cpuset_for_each_child(child, css, cs) {
> +		if (is_partition_valid(child))
> +			cpumask_andnot(owned_cpus, owned_cpus,
> +				       child->effective_xcpus);
> +	}
> +	rcu_read_unlock();
> +}
> +
>   /*
>    * update_cpumasks_hier - Update effective cpumasks and tasks in the subtree
>    * @cs:  the cpuset to consider
> @@ -2990,8 +3014,10 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>   	} else if (old_prs && new_prs) {
>   		/*
>   		 * A change in load balance state only, no change in cpumasks.
> -		 * Need to update isolated_cpus.
> +		 * Need to update isolated_cpus for CPUs owned by this partition,
> +		 * excluding CPUs distributed to valid child partitions.
>   		 */
> +		compute_partition_owned_cpumask(cs, tmpmask.new_cpus);
>   		if (((new_prs == PRS_ISOLATED) &&
>   		     !isolated_cpus_can_update(cs->effective_xcpus, NULL)) ||
>   		    prstate_housekeeping_conflict(new_prs, cs->effective_xcpus))
> @@ -3030,7 +3056,7 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>   	if (!is_partition_valid(cs))
>   		reset_partition_data(cs);
>   	else if (isolcpus_updated)
> -		isolated_cpus_update(old_prs, new_prs, cs->effective_xcpus);
> +		isolated_cpus_update(old_prs, new_prs, tmpmask.new_cpus);
>   	spin_unlock_irq(&callback_lock);
>   
>   	/* Force update if switching back to member & update effective_xcpus */

Your use of tmpmask.new_cpus in isolated_cpus_update() can be 
problematic. isolcpus_updated can be set when an isolated partition is 
enabled or disabled. In both cases, tmpmask.new_cpus can be used 
temporarily. So the content of this temporary cpumask may not be what we 
want to pass into isolated_cpus_update(). I will suggest you only use 
tmpmask.new_cpus if it is determined to be coming from partition state 
transition instead of enabling/disabling of partition.

Cheers,
Longman