Re: [PATCH-next v2 2/5] cgroup/cpuset: Correctly set cpus_updated in cpuset_hotplug_update_tasks()

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

On 11/10/2026 3:09 AM, Waiman Long wrote:
> When cpuset_hotplug_update_tasks() is called with the conditions that
> either remote_partition_disable() or update_parent_effective_cpumask()
> has to be called, the cs->effective_cpus can be updated such that it
> matches what is computed in new_cpus. In such case, cpus_updated can
> be incorrectly set to false leading to stale CPU afinity remained in the
> tasks not matching the new cpuset value. Fix that by forcing cpus_updated
> to true whenever one of these partition helpers is being called.
> 
> Fixes: 4b842da276a8a ("cpuset: Make CPU hotplug work with partition")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Link: https://sashiko.dev/#/bug/linux-ab90f7e8-0d60-40ca-83e9-e405075b24d2
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/cgroup/cpuset.c | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 6074327c3477..3955d2d574cf 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -4002,7 +4002,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
>   {
>   	static cpumask_t new_cpus;
>   	static nodemask_t new_mems;
> -	bool cpus_updated;
> +	bool cpus_updated = false;
>   	bool mems_updated;
>   	int partcmd = -1;
>   	struct cpuset *parent;
> @@ -4015,12 +4015,18 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
>   	if (!tmp || !cs->partition_root_state)
>   		goto update_tasks;
>   
> +	/*
> +	 * cpus_updated should be set to true when remote_partition_disable()
> +	 * or update_parent_effective_cpumask() is called as cs->effective_cpus
> +	 * may have been updated.
> +	 */
>   	if (is_remote_partition(cs) && (cpumask_empty(subpartitions_cpus) ||
>   				       (cpumask_empty(&new_cpus) &&
>   					partition_is_populated(cs, NULL)))) {
>   		WRITE_ONCE(cs->prs_err, PERR_HOTPLUG);
>   		remote_partition_disable(cs, tmp);
>   		compute_effective_cpumask(&new_cpus, cs, parent);
> +		cpus_updated = true;
>   	}
>   
>   	/*
> @@ -4045,6 +4051,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
>   		partcmd = partcmd_update;
>   
>   	if (partcmd >= 0) {
> +		cpus_updated = true;
>   		update_parent_effective_cpumask(cs, partcmd, NULL, tmp);
>   		if (is_partition_valid(cs) || (partcmd == partcmd_invalidate)) {
>   			compute_effective_cpumask(&new_cpus, cs, parent);
> @@ -4053,7 +4060,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
>   	}
>   
>   update_tasks:
> -	cpus_updated = !cpumask_equal(&new_cpus, cs->effective_cpus);
> +	cpus_updated |= !cpumask_equal(&new_cpus, cs->effective_cpus);
>   	mems_updated = !nodes_equal(new_mems, cs->effective_mems);
>   	if (!cpus_updated && !mems_updated)
>   		goto unlock;	/* Hotplug doesn't affect this cpuset */

I think the issue only happens in the remote partition case.
remote_partition_disable() calls reset_partition_data(), which sets
cs->effective_cpus to what compute_effective_cpumask() will return
afterwards, so the cpumask_equal() check misses the change while the
tasks still have the old affinity.

update_parent_effective_cpumask() only updates the effective_cpus of
the parent (through partition_xcpus_add/del()), and cs is skipped in
update_sibling_cpumasks(). So cs->effective_cpus isn't changed there and
the original check still works. If new_cpus is the same as
cs->effective_cpus in that case, forcing cpus_updated just walks all the
tasks in cs and calls set_cpus_allowed_ptr() with their current mask.
Should cpus_updated only be set in the remote partition case?

Also, as the problem comes from remote_partition_disable() in the
hotplug path, should the Fixes tag be the following instead?

Fixes: 181c8e091aae ("cgroup/cpuset: Introduce remote partition")

-- 
Best regards
Ridong