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

[Li Zefan <lizefan@huawei.com>]

On 2013/6/6 4:51, Tejun Heo wrote:
> 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?
> 

The first one will only update tasks in empty cpusets (no matter online or
offline), and the second one will only update tasks in non-empty cpusets
(only when offline).

> 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?
> 

It indeed can be confusing. I'll see if we can make the code clearer.