linux-kernel - Re: [PATCH 1/8] cgroup: implement cgroup_is

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Message-ID: <20130408163344.GP17178@dhcp22.suse.cz>
Date:	Mon, 8 Apr 2013 18:33:44 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Li Zefan <lizefan@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Glauber Costa <glommer@...allels.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 1/8] cgroup: implement cgroup_is_ancestor()

On Mon 08-04-13 08:57:26, Tejun Heo wrote:
> Hello,
> 
> On Mon, Apr 08, 2013 at 04:47:50PM +0200, Michal Hocko wrote:
> > On Mon 08-04-13 16:20:11, Li Zefan wrote:
> > [...]
> > > @@ -5299,6 +5300,26 @@ struct cgroup_subsys_state *cgroup_css_from_dir(struct file *f, int id)
> > >  	return css ? css : ERR_PTR(-ENOENT);
> > >  }
> > >  
> > > +/**
> > > + * cgroup_is_ancestor - test "root" cgroup is an ancestor of "child"
> > > + * @child: the cgroup to be tested.
> > > + * @root: the cgroup supposed to be an ancestor of the child.
> > > + *
> > > + * Returns true if "root" is an ancestor of "child" in its hierarchy.
> > > + */
> > > +bool cgroup_is_ancestor(struct cgroup *child, struct cgroup *root)
> > > +{
> > > +	int depth = child->depth;
> > 
> > Is this functionality helpful for other controllers but memcg?
> > css_is_ancestor is currently used only by memcg code AFAICS and we can
> > get the same functionality easily by using something like:
> 
> It's a basic hierarchy operation. 

Which nobody seem to need ATM so it could be added later when an user
emerges.

> I'd prefer it to be in cgroup and in general let's try to avoid
> memcg-specific infrastructure. 

Now that I am thinking about that some more css_is_ancestor is even not
correct. Consider this for example
root
 \
  A (use_hierarchy = 0)
   \
    B (use_hierarchy = 1)
     \
      C

__mem_cgroup_same_or_subtree(A, C) would happily return true even though
this is not correct because A is not a part of the same hierarchy.
So it seems that memcg will need a special treatment here anyway because
cgroup core doesn't know about use_hierarchy thingy which is still with
us...
I will post a patch.

> It doesn't seem to end well.
> 
> Thanks.
> 
> -- 
> tejun
> --
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-- 
Michal Hocko
SUSE Labs
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