linux-kernel - Re: [PATCH 3/9] cgroup: implement generic child / descendant walk macros

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Date:	Wed, 7 Nov 2012 18:49:58 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	lizefan@...wei.com, rjw@...k.pl,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	fweisbec@...il.com
Subject: Re: [PATCH 3/9] cgroup: implement generic child / descendant walk
 macros

On Wed 07-11-12 09:01:18, Tejun Heo wrote:
> Hello, Michal.
> 
> On Wed, Nov 07, 2012 at 05:54:57PM +0100, Michal Hocko wrote:
> > > +struct cgroup *cgroup_next_descendant_pre(struct cgroup *pos,
> > > +					  struct cgroup *cgroup)
> > > +{
> > > +	struct cgroup *next;
> > > +
> > > +	WARN_ON_ONCE(!rcu_read_lock_held());
> > > +
> > > +	/* if first iteration, pretend we just visited @cgroup */
> > > +	if (!pos) {
> > > +		if (list_empty(&cgroup->children))
> > > +			return NULL;
> > > +		pos = cgroup;
> > > +	}
> > 
> > Is there any specific reason why the root of the tree is excluded?
> > This is bit impractical because you have to special case the root
> > in the code.
> 
> Yeah, thought about including it but decided against it for two
> reasons.
> 
> * To be consistent with cgroup_for_each_children() - it's a bit weird
>   for descendants to include self when children don't.
> 
> * Iteration root is likely to require different treatment anyway.
>   e.g. for cgroup_freezer, the root is updated to the specified config
>   while all the descendants inherit config from its immediate parent.
>   They are different.

OK if there is a code which handles root differently then let's not
overcomplicate this. The naming should be clear that root needs a
special treatment.

I will continue with the review tomorrow.
-- 
Michal Hocko
SUSE Labs
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