linux-kernel - Re: [PATCH v3 1/5] cpusets, hotplug: Implement cpuset tree traversal in a helper function

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Date:	Tue, 15 May 2012 17:45:23 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	a.p.zijlstra@...llo.nl, mingo@...nel.org, pjt@...gle.com,
	paul@...lmenage.org, akpm@...ux-foundation.org, rjw@...k.pl,
	nacc@...ibm.com, paulmck@...ux.vnet.ibm.com, tglx@...utronix.de,
	seto.hidetoshi@...fujitsu.com, tj@...nel.org, mschmidt@...hat.com,
	berrange@...hat.com, nikunj@...ux.vnet.ibm.com,
	vatsa@...ux.vnet.ibm.com, liuj97@...il.com,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 1/5] cpusets, hotplug: Implement cpuset tree traversal
 in a helper function

On 05/15/2012 05:33 AM, David Rientjes wrote:

> On Mon, 14 May 2012, Srivatsa S. Bhat wrote:
> 
>> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
>> index 14f7070..23e5da6 100644
>> --- a/kernel/cpuset.c
>> +++ b/kernel/cpuset.c
>> @@ -2000,6 +2000,23 @@ static void remove_tasks_in_empty_cpuset(struct cpuset *cs)
>>  	move_member_tasks_to_cpuset(cs, parent);
>>  }
>>  
>> +static struct cpuset *traverse_cpusets(struct list_head *queue)
> 
> I suggest a different name for this, traverse_cpusets() doesn't imply that 
> it will be returning struct cpuset *.


OK, I guess something like cpuset_next() or next_cpuset() should be fine?

> 
>> +{
>> +	struct cpuset *cp;
>> +	struct cpuset *child;	/* scans child cpusets of cp */
>> +	struct cgroup *cont;
>> +
>> +	cp = list_first_entry(queue, struct cpuset, stack_list);
>> +	list_del(queue->next);
>> +	list_for_each_entry(cont, &cp->css.cgroup->children, sibling) {
>> +		child = cgroup_cs(cont);
>> +		list_add_tail(&child->stack_list, queue);
>> +	}
>> +
>> +	return cp;
> 
> Eek, what happens if queue is empty?  It can't happen with only this 
> patch applied, but since you're doing this to be used in other places then 
> you'd need to check for list_empty().
> 


Actually, I didn't intend this to be used in other places because their
traversals are a little bit different anyway.. But yes, having a check for
list_empty() would be good. I'll add it and change the loop to something like:

while (cpuset_next(&queue) {
	...
}

Thanks for pointing this out!

>> +}
>> +
>> +
>>  /*
>>   * Walk the specified cpuset subtree and look for empty cpusets.
>>   * The tasks of such cpuset must be moved to a parent cpuset.
>> @@ -2019,19 +2036,12 @@ static void scan_for_empty_cpusets(struct cpuset *root)
>>  {
>>  	LIST_HEAD(queue);
>>  	struct cpuset *cp;	/* scans cpusets being updated */
>> -	struct cpuset *child;	/* scans child cpusets of cp */
>> -	struct cgroup *cont;
>>  	static nodemask_t oldmems;	/* protected by cgroup_mutex */
>>  
>>  	list_add_tail((struct list_head *)&root->stack_list, &queue);
>>  
>>  	while (!list_empty(&queue)) {
>> -		cp = list_first_entry(&queue, struct cpuset, stack_list);
>> -		list_del(queue.next);
>> -		list_for_each_entry(cont, &cp->css.cgroup->children, sibling) {
>> -			child = cgroup_cs(cont);
>> -			list_add_tail(&child->stack_list, &queue);
>> -		}
>> +		cp = traverse_cpusets(&queue);
>>  
>>  		/* Continue past cpusets with all cpus, mems online */
>>  		if (cpumask_subset(cp->cpus_allowed, cpu_active_mask) &&
> 
> Otherwise, looks good.
> 


Thanks a lot!

Regards,
Srivatsa S. Bhat

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