Re: [PATCH 09/10] cpuset: allow to keep tasks in empty cpusets

[Li Zefan <lizefan@huawei.com>]

On 2013/4/20 4:58, Tejun Heo wrote:
> Hello,
> 
> On Fri, Apr 19, 2013 at 08:29:24PM +0800, Li Zefan wrote:
>> +static void update_tasks_cpumask_hier(struct cpuset *root_cs,
>> +				      bool update_root, struct ptr_heap *heap)
>> +{
>> +	struct cpuset *cp;
>> +	struct cgroup *pos_cgrp;
>> +
>> +	if (update_root)
>> +		update_tasks_cpumask(root_cs, heap);
>> +
>> +	rcu_read_lock();
>> +	cpuset_for_each_descendant_pre(cp, pos_cgrp, root_cs) {
>> +		/* skip the whole subtree if @cp have some CPU */
>> +		if (!cpumask_empty(cp->cpus_allowed)) {
>> +			pos_cgrp = cgroup_rightmost_descendant(pos_cgrp);
>> +			continue;
>> +		}
>> +
>> +		update_tasks_cpumask(cp, heap);
>> +	}
>> +	rcu_read_unlock();
> 
> I don't think we can call update_tasks_cpumask() under
> rcu_read_lock().  It calls into set_cpus_allowed_ptr() which may
> block, so you'll probably have to punt it to a work item like how

Oh, will fix.

> migration is being done.  Another approach would be converting cgroup
> to use SRCU instead, which would lessen pain on other places too.  The
> only problem there would be that srcu_read_lock() is a bit more
> expensive than rcu_read_lock().  I'm not sure whether that'd show up
> in some hot path or not.  Ideas?
> 

I guess we can live with rcu_read_lock() for now, and see if we can
change it to srcu when other significant changes are done in all
cgroup controllers. (hierarchy support in blkcg, etc..)