Re: [PATCH/for-next v3 2/3] cgroup/cpuset: Defer housekeeping_update() calls from CPU hotplug to workqueue

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Feb 02, 2026 at 03:11:43PM -0500, Waiman Long wrote:

> @@ -1310,14 +1321,34 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus)
>   */
>  static void update_isolation_cpumasks(void)
>  {
> -	int ret;
> +	static DECLARE_WORK(isolcpus_work, isolcpus_workfn);
>  
>  	if (!isolated_cpus_updating)
>  		return;
>  
> -	ret = housekeeping_update(isolated_cpus);
> -	WARN_ON_ONCE(ret < 0);
> +	/*
> +	 * This function can be reached either directly from regular cpuset
> +	 * control file write or via CPU hotplug. In the latter case, it is
> +	 * the per-cpu kthread that calls cpuset_handle_hotplug() on behalf
> +	 * of the task that initiates CPU shutdown or bringup.
> +	 *
> +	 * To have better flexibility and prevent the possibility of deadlock
> +	 * when calling from CPU hotplug, we defer the housekeeping_update()
> +	 * call to after the current cpuset critical section has finished.
> +	 * This is done via workqueue.
> +	 */
> +	if (current->flags & PF_KTHREAD) {

		/* Serializes the static isolcpus_workfn. */
		lockdep_assert_held(&cpuset_mutex);

> +		/*
> +		 * We rely on WORK_STRUCT_PENDING_BIT to not requeue a work
> +		 * item that is still pending.
> +		 */
> +		queue_work(system_unbound_wq, &isolcpus_work);
> +		/* Also defer sched domains regeneration to the work function */
> +		force_sd_rebuild = false;
> +		return;
> +	}
>  
> +	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
>  	isolated_cpus_updating = false;
>  }