linux-kernel - Re: [PATCH 2/3] cgroup/cpuset.c: Fix a partition error with CPU hotplug

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Message-ID: <4ebbbe0b-0f4c-4002-b13b-263b7efc9f69@redhat.com>
Date: Wed, 6 Aug 2025 13:30:35 -0400
From: Waiman Long <llong@...hat.com>
To: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
 Michal Koutný <mkoutny@...e.com>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 Ingo Molnar <mingo@...nel.org>, Juri Lelli <juri.lelli@...hat.com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [PATCH 2/3] cgroup/cpuset.c: Fix a partition error with CPU
 hotplug

On 8/6/25 1:24 PM, Waiman Long wrote:
> It was found during testing that an invalid leaf partition with an
> empty effective exclusive CPU list can become a valid empty partition
> with no CPU afer an offline/online operation of an unrelated CPU. An
> empty partition root is allowed in the special case that it has no
> task in its cgroup and has distributed out all its CPUs to its child
> partitions. That is certainly not the case here.
>
> The problem is in the cpumask_subsets() test in the hotplug case
> (update with no new mask) of update_parent_effective_cpumask() as it
> also returns true if the effective exclusive CPU list is empty. Fix that
> by addding the cpumask_empty() test to root out this exception case.
> Also add the cpumask_empty() test in cpuset_hotplug_update_tasks()
> to avoid calling update_parent_effective_cpumask() for this special case.
>
> Fixes: 0c7f293efc87 ("cgroup/cpuset: Add cpuset.cpus.exclusive.effective for v2")
> Signed-off-by: Waiman Long <longman@...hat.com>

Oh, forget to remove the unneeded ".c" from the patch title.

-Longman

> ---
>   kernel/cgroup/cpuset.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index bf149246e001..d993e058a663 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1843,7 +1843,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
>   			if (is_partition_valid(cs))
>   				adding = cpumask_and(tmp->addmask,
>   						xcpus, parent->effective_xcpus);
> -		} else if (is_partition_invalid(cs) &&
> +		} else if (is_partition_invalid(cs) && !cpumask_empty(xcpus) &&
>   			   cpumask_subset(xcpus, parent->effective_xcpus)) {
>   			struct cgroup_subsys_state *css;
>   			struct cpuset *child;
> @@ -3870,9 +3870,10 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
>   		partcmd = partcmd_invalidate;
>   	/*
>   	 * On the other hand, an invalid partition root may be transitioned
> -	 * back to a regular one.
> +	 * back to a regular one with a non-empty effective xcpus.
>   	 */
> -	else if (is_partition_valid(parent) && is_partition_invalid(cs))
> +	else if (is_partition_valid(parent) && is_partition_invalid(cs) &&
> +		 !cpumask_empty(cs->effective_xcpus))
>   		partcmd = partcmd_update;
>   
>   	if (partcmd >= 0) {