[PATCH 3/9] cgroup: implement generic child / descendant walk macros

[Michal Hocko]

On Tue 06-11-12 12:31:54, Tejun Heo wrote:
> On Sat, Nov 03, 2012 at 01:38:29AM -0700, Tejun Heo wrote:
> > Currently, cgroup doesn't provide any generic helper for walking a
> > given cgroup's children or descendants.  This patch adds the following
> > three macros.
> > 
> > * cgroup_for_each_child() - walk immediate children of a cgroup.
> > 
> > * cgroup_for_each_descendant_pre() - visit all descendants of a cgroup
> >   in pre-order tree traversal.
> > 
> > * cgroup_for_each_descendant_post() - visit all descendants of a
> >   cgroup in post-order tree traversal.
> > 
> > All three only require the user to hold RCU read lock during
> > traversal.  Verifying that each iterated cgroup is online is the
> > responsibility of the user.  When used with proper synchronization,
> > cgroup_for_each_descendant_pre() can be used to propagate config
> > updates to descendants in reliable way.  See comments for details.
> 
> Michal, Li, how does this look to you?  Would this be okay for memcg
> too?

Yes, definitely. We are currently iterating by css->id which is, ehm,
impractical. Having a deterministic tree walk is definitely a plus.

-- 
Michal Hocko
SUSE Labs