Re: [PATCH v2 3/3] cgroup/cpuset: Release isolated CPUs when the parent is invalid

[Ridong Chen <ridong.chen@linux.dev>]

On 11/5/2026 9:03 PM, Guopeng Zhang wrote:
> From: Guopeng Zhang <zhangguopeng@kylinos.cn>
> 
> An isolated partition can be invalidated after its parent has already
> become invalid. update_parent_effective_cpumask() then skips returning
> CPUs to the parent, but also skips removing them from isolated_cpus.
> The invalid child continues to isolate CPUs that no longer belong to
> any valid isolated partition.
> 
> For example, on a 32-CPU system with all CPUs online and without
> isolcpus= or nohz_full= boot parameters:
> 
>    cd /sys/fs/cgroup
>    echo +cpuset > cgroup.subtree_control
>    mkdir A
>    echo 0,2-31 > A/cpuset.cpus
>    echo root > A/cpuset.cpus.partition
>    echo +cpuset > A/cgroup.subtree_control
>    mkdir A/B
>    echo 2-3 > A/B/cpuset.cpus
>    echo isolated > A/B/cpuset.cpus.partition
>    cat cpuset.cpus.isolated
>    2-3
>    echo 0 > /sys/devices/system/cpu/cpu1/online
>    cat A/B/cpuset.cpus.partition
>    isolated invalid
>    cat cpuset.cpus.isolated
>    2-3
> 
> Release the invalidated child's isolated CPUs when its parent is no
> longer a valid partition. Use isolated_cpus_release() to preserve CPUs
> still covered by a valid isolated ancestor and by boot isolation.
> 
> After this fix, cpuset.cpus.isolated is empty for this reproducer.
> 
> Fixes: 11e5f407b64a ("cgroup/cpuset: Keep track of CPUs in isolated partitions")
> Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
> ---
>   kernel/cgroup/cpuset.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index fc5454c2740f..f3b808cea51c 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1976,10 +1976,15 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
>   		/*
>   		 * Make the current partition invalid.
>   		 */
> -		if (is_partition_valid(parent))
> +		if (is_partition_valid(parent)) {
>   			adding = cpumask_and(tmp->addmask,
>   					     cs->effective_xcpus,
>   					     parent->effective_xcpus);
> +		} else if (old_prs == PRS_ISOLATED) {
> +			/* No valid parent will release these isolated CPUs. */
> +			cpumask_copy(tmp->addmask, cs->effective_xcpus);
> +			isolated_cpus_release(cs, tmp->addmask);
> +		}
>   		if (old_prs > 0)
>   			new_prs = -old_prs;
>   

Hi Guopeng,

Thanks for the fix. Could you try this patch to see if it fixes the
issue?

https://lore.kernel.org/cgroups/20261105125215.2205892-4-ridong.chen@linux.dev/T/#u

I'm concerned about whether the related issue has been fixed in all
the places that need fixing. In your reproducer, CPUs 2-3 are already
wrong when A becomes invalid: partition_xcpus_del() only updates
isolated_cpus when the old state differs from the parent's state, so
the CPUs that A gave back but had distributed to the isolated child B
are left in isolated_cpus. This patch only cleans them up later, when
B is invalidated with an invalid parent. Other paths that give back
CPUs of an isolated descendant may have the same problem.

My patch makes partition_xcpus_del() always set the isolation state of
the given back CPUs, including those of the descendants, to the state
of the parent that receives them. With it, isolated_cpus_release()
should no longer be needed.

It would also be good to add your reproducer to test_cpuset_prs.sh.
It can be expressed with O1=0 in the test matrix.

-- 
Best regards
Ridong