Re: [PATCH v2 08/10] cpuset: allow to keep tasks in empty cpusets

[Tejun Heo <tj@kernel.org>]

Hello, Li.

On Wed, Jun 05, 2013 at 05:16:59PM +0800, Li Zefan wrote:
> @@ -2092,11 +2183,13 @@ static void cpuset_propagate_hotplug_workfn(struct work_struct *work)
>  	mutex_unlock(&cpuset_mutex);
>  
>  	/*
> -	 * If @cs became empty, move tasks to the nearest ancestor with
> -	 * execution resources.  This is full cgroup operation which will
> +	 * If sane_behavior flag is set, we'll keep tasks in empty cpusets.
> +	 *
> +	 * Otherwise move tasks to the nearest ancestor with execution
> +	 *  resources.  This is full cgroup operation which will
>  	 * also call back into cpuset.  Should be done outside any lock.
>  	 */
> -	if (is_empty)
> +	if (!sane && is_empty)
>  		remove_tasks_in_empty_cpuset(cs);
>  
>  	/* the following may free @cs, should be the last operation */
> @@ -2171,6 +2264,7 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
>  		cpumask_copy(top_cpuset.cpus_allowed, &new_cpus);
>  		mutex_unlock(&callback_mutex);
>  		/* we don't mess with cpumasks of tasks in top_cpuset */
> +		update_tasks_cpumask_hier(&top_cpuset, false, NULL);
>  	}

I'm a little confused by the order of operation.  We now have two
different hierarchical walks for hotplug propagation, right?  I
suppose the above one is added because we now also need to update the
mask when cpus are being brought online?

I wonder whether it'd be possible to merge the two paths.  My
suspicion is that we probably don't need propagate_hotplug_work
anymore now that we can drop RCU read lock while doing the pre-order
walk.  What do you think?

Thanks.

-- 
tejun