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

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Message-ID: <38800495-464f-4bbf-b605-9a6b8d2b4c11@huaweicloud.com>
Date: Thu, 7 Aug 2025 10:44:26 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Waiman Long <longman@...hat.com>, 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 2025/8/7 1:24, 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>
> ---
>  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;

This path looks good to me.

However, I found the update_parent_effective_cpumask function a bit difficult to follow due to its
complexity.

To improve readability, could we refactor the partcmd_enable, partcmd_disable, partcmd_update and
partcmd_invalidate logic into separate, well-defined function blocks?  I'd be happy to take
ownership of this refactoring work if you agree with the approach.

Best regards,
Ridong

> @@ -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) {