Re: [PATCH-next v3 3/3] cgroup/cpuset: Check remote partition underneath before making an invalid partition valid

[Guopeng Zhang <guopeng.zhang@linux.dev>]

在 2026/11/11 10:02, Waiman Long 写道:
> When re-evaluating an invalid local partition via partcmd_update,
> update_parent_effective_cpumask() does not check whether a descendant
> remote partition is using its exclusive CPUs, promoting the cpuset to a
> valid partition root over the remote partition. This condition should only
> be possible if parent is the top_cpuset and exclusive_cpus isn't empty.
> 
> This leaves an active remote partition underneath a valid
> local partition root with overlapping exclusive CPUs, breaking
> scheduler domain partitioning and triggering warnings in
> compute_partition_effective_cpumask() and generate_sched_domains().
> 
> Fix that by checking the effective_xcpus against subpartitions_cpus
> when an underneath remote partition is possible to make sure that all
> the exclusive CPUs are available before allowing the invalid partition
> to be switched back to valid.
> 
> Also make sure that effective_xcpus is non-empty before swtiching an
> invalid partition back to valid.
> 
> Add a test case in test_cpuset_prs.sh to verify that an invalid partition
> with a remote partition underneath cannot be switched back to valid
> with a hotplug event.
> 
> Fixes: 181c8e091aae ("cgroup/cpuset: Introduce remote partition")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Link: https://sashiko.dev/#/bug/linux-47715b89-d5fb-4731-b7c9-070773c6ff75
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/cgroup/cpuset.c                        | 42 ++++++++++++-------
>  .../selftests/cgroup/test_cpuset_prs.sh       |  4 ++
>  2 files changed, 30 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 8b684361c94e..af5fd543fe88 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2034,11 +2034,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
>  		 * tasks_nocpu_error() with this very mask, and a failing
>  		 * write is routed to partcmd_invalidate instead.
>  		 */
> -		if (is_partition_invalid(cs)) {
> -			adding = false;
> -			deleting = cpumask_and(tmp->delmask,
> -					newmask, parent->effective_xcpus);
> -		} else {
> +		if (is_partition_valid(cs)) {
>  			cpumask_andnot(tmp->addmask, cs->effective_xcpus, newmask);
>  			adding = cpumask_and(tmp->addmask, tmp->addmask,
>  					     parent->effective_xcpus);
> @@ -2046,6 +2042,17 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
>  			cpumask_andnot(tmp->delmask, newmask, cs->effective_xcpus);
>  			deleting = cpumask_and(tmp->delmask, tmp->delmask,
>  					       parent->effective_xcpus);
> +		} else {
> +			deleting = cpumask_and(tmp->delmask,
> +					newmask, parent->effective_xcpus);
> +
> +			/* Check if a remote partition may be underneath */
> +			if ((parent == &top_cpuset) &&
> +			    !cpumask_empty(cs->exclusive_cpus) &&
> +			    cpumask_intersects(tmp->delmask, subpartitions_cpus)) {
> +				part_error = PERR_REMOTE;
> +				deleting = false;
> +			}
>  		}
>  
>  		/*
> @@ -2126,34 +2133,37 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
>  		} else if (is_partition_invalid(cs)) {
>  			struct cgroup_subsys_state *css;
>  			struct cpuset *child;
> -			bool exclusive = true;
>  
>  			/*
>  			 * Convert invalid partition to valid has to pass the
> -			 * cpu exclusivity test when cpuset.cpus.exclusive is
> -			 * empty as well as the housekeeping check.
> +			 * following tests:
> +			 * 1) housekeeping CPU conflict check
> +			 * 2) cpu exclusivity test when cpuset.cpus.exclusive is
> +			 *    empty.
> +			 * 3) if parent is top_cpuset, none of the effective_xcpus
> +			 *    (tmp->delmask) are in subpartitions_cpus, if not a
> +			 *    remote partition may be underneath.
>  			 */
>  			if (prstate_housekeeping_conflict(cs->partition_root_state,
>  							  parent_prs, tmp->delmask, NULL)) {
>  				part_error = PERR_HKEEPING;
> -				goto write_error;
> -			}
> -			if (cpumask_empty(cs->exclusive_cpus)) {
> +			} else if (cpumask_empty(cs->exclusive_cpus)) {
>  				rcu_read_lock();
>  				cpuset_for_each_child(child, css, parent) {
>  					if (child == cs)
>  						continue;
>  					if (!cpusets_are_exclusive(cs, child)) {
> -						exclusive = false;
> +						part_error = PERR_NOTEXCL;
>  						break;
>  					}
>  				}
>  				rcu_read_unlock();
> +			} else if ((parent == &top_cpuset) &&
> +				   cpumask_intersects(tmp->delmask, subpartitions_cpus)) {
> +				part_error = PERR_REMOTE;
>  			}

Hi,Longman

I think this check can still miss a remote partition during hotplug.
When CPU hotplug leaves no active CPU available to the top partition,
cpuset_handle_hotplug() clears subpartitions_cpus before walking the
descendants. The remote partitions are still valid at that point.

An invalid parent is visited before its remote descendant. Since the
mask is empty, this check allows the parent to become valid and add
its exclusive CPUs back to subpartitions_cpus. When the remote
descendant is visited later, those CPUs are already back in the mask,
so it passes the subset check and stays remote. We are then left with
a valid remote partition below a valid local partition, and
compute_partition_effective_cpumask() warns about the remote child.

Could we check whether there is still a valid remote partition below cs
before allowing cs to become valid? subpartitions_cpus may have been
cleared by hotplug, so the lack of overlap does not tell us that the
remote partition has actually been disabled.

Thanks,
Guopeng

> -			if (exclusive)
> +			if (!part_error)
>  				deleting = !cpumask_empty(tmp->delmask);
> -			else
> -				part_error = PERR_NOTEXCL;
>  		}
>  	}
>  
> @@ -2174,7 +2184,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
>  			break;
>  		case PRS_INVALID_ROOT:
>  		case PRS_INVALID_ISOLATED:
> -			if (!part_error)
> +			if (!part_error && deleting)
>  				new_prs = -old_prs;
>  			break;
>  		}
> diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> index 1e9e6b4c6f7b..3d873804abc2 100755
> --- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> +++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> @@ -563,6 +563,10 @@ TEST_MATRIX=(
>  	" C1-6:P1 C3-6:P2 C5-6:P2  .       .    P1      .      .     0 A1:1-2|A2:3-4|A3:5-6 \
>  								       A1:P1|A2:P1|A3:P2 5-6"
>  
> +	# A hotplug event will not incorrectly activate an invalid partition with
> +	# a remote partition underneath.
> +	"  CX1-4  C3-4:P1   .      .      P1   O1=0     .      .     0 A1:2|A2:3-4 A1:P-1|A2:P1"
> +
>  	#
>  	#  old-A1 old-A2 old-A3 old-B1 new-A1 new-A2 new-A3 new-B1 fail ECPUs Pstate ISOLCPUS
>  	#  ------ ------ ------ ------ ------ ------ ------ ------ ---- ----- ------ --------