Re: [PATCH] cgroup/cpuset: Invalidate remote partition on housekeeping conflict

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

On 9/23/2026 6:36 PM, Guopeng Zhang wrote:
> From: Guopeng Zhang <zhangguopeng@kylinos.cn>
> 
> Widening an ancestor's exclusive CPU mask can add a boot-isolated CPU
> to a valid remote partition root without touching the partition's own
> control files. The partition then load balances that CPU, silently
> defeating isolcpus=domain for it.
> 
> This can be reproduced on a 32-CPU system booted with
> isolcpus=domain,4:
> 
>      cd /sys/fs/cgroup
>      echo +cpuset > cgroup.subtree_control
>      mkdir -p A/B
>      echo +cpuset > A/cgroup.subtree_control
>      echo 2-4 > A/cpuset.cpus
>      echo 2-3 > A/cpuset.cpus.exclusive
>      echo 2-4 > A/B/cpuset.cpus
>      echo 2-4 > A/B/cpuset.cpus.exclusive
>      echo root > A/B/cpuset.cpus.partition
>      cat A/B/cpuset.cpus.effective               # 2-3
>      echo 2-4 > A/cpuset.cpus.exclusive
>      cat A/B/cpuset.cpus.partition               # root
>      cat A/B/cpuset.cpus.effective               # 2-4
> 
> The last write returns 0 and leaves the hierarchy in this state:
> 
>      root (cpuset.cpus.effective=0-1,5-31)
>      |
>      \-- A (member):           cpuset.cpus=2-4
>          |                     cpuset.cpus.exclusive=2-4
>          \-- B (root, remote): cpuset.cpus=2-4
>                                cpuset.cpus.effective=2-4
> 
> B is a remote partition: it takes its CPUs directly from the root
> cpuset, and A only passes its exclusive list down. Before the last
> write, that list is 2-3, so B holds 2-3 and CPU 4 stays in the
> root cpuset as a boot-isolated CPU. The write widens A's exclusive
> list to 2-4, which additionally grants CPU 4 to B. Nothing rejects
> the grant: B remains a valid root partition, and CPU 4 is still
> listed in cpuset.cpus.isolated while sitting in a load-balanced
> partition.
> 
> remote_partition_enable() and validate_partition() already call
> prstate_housekeeping_conflict() before granting CPUs. The ancestor
> update path in remote_cpus_update() does not. Add the missing check so
> that the existing prs_err path invalidates the remote partition instead
> of adding the boot-isolated CPU.
> 
> Fixes: f62a5d39368e ("cgroup/cpuset: Remove remote_partition_check() & make update_cpumasks_hier() handle remote partition")
> Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
> ---
>   kernel/cgroup/cpuset.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 753aa65afcd7..362e5b5dccaa 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1699,6 +1699,8 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
>   		else if ((prs == PRS_ISOLATED) &&
>   			 !isolated_cpus_can_update(tmp->addmask, tmp->delmask))
>   			WRITE_ONCE(cs->prs_err, PERR_HKEEPING);
> +		else if (prstate_housekeeping_conflict(prs, tmp->addmask))
> +			WRITE_ONCE(cs->prs_err, PERR_HKEEPING);
>   		if (cs->prs_err)
>   			goto invalidate;
>   	}

Hi Guopeng,

Thank you for your test and patch.

I noticed that this statement existed in remote_partition_enable but was missing 
from remote_cpus_update. There may be other state leaks here as well.

In case this happens again, I would like to suggest adding 
'validate_remote_partition' as I tried to do before [1]. Could you put some 
effort into this?

[1] 
https://lore.kernel.org/cgroups/20251225123058.231765-19-chenridong@huaweicloud.com/

-- 
Best regards
Ridong