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

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Message-ID: <20121107153835.GC4131@dhcp22.suse.cz>
Date:	Wed, 7 Nov 2012 16:38:35 +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 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
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