linux-kernel - Re: [PATCH REPOST RFC cgroup/for-3.7] cgroup: mark subsystems with broken hierarchy support and whine if cgroups are nested for them

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Date:	Wed, 12 Sep 2012 17:49:07 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Glauber Costa <glommer@...allels.com>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	Li Zefan <lizefan@...wei.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Turner <pjt@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Graf <tgraf@...g.ch>,
	"Serge E. Hallyn" <serue@...ibm.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Neil Horman <nhorman@...driver.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH REPOST RFC cgroup/for-3.7] cgroup: mark subsystems with
 broken hierarchy support and whine if cgroups are nested for them

On Wed 12-09-12 13:31:55, Glauber Costa wrote:
> On 09/11/2012 02:04 PM, Michal Hocko wrote:
> > I like the approach in general but see the comments bellow:
> > 
> > On Mon 10-09-12 15:33:55, Tejun Heo wrote:
> > [...]
> >> --- a/mm/memcontrol.c
> >> +++ b/mm/memcontrol.c
> >> @@ -3855,12 +3855,17 @@ static int mem_cgroup_hierarchy_write(st
> >>  	 */
> >>  	if ((!parent_memcg || !parent_memcg->use_hierarchy) &&
> >>  				(val == 1 || val == 0)) {
> >> -		if (list_empty(&cont->children))
> >> +		if (list_empty(&cont->children)) {
> >>  			memcg->use_hierarchy = val;
> >> -		else
> >> +			/* we're fully hierarchical iff root uses hierarchy */
> >> +			if (mem_cgroup_is_root(memcg))
> >> +				mem_cgroup_subsys.broken_hierarchy = !val;
> >> +		} else {
> >>  			retval = -EBUSY;
> >> -	} else
> >> +		}
> >> +	} else {
> >>  		retval = -EINVAL;
> >> +	}
> >>  
> >>  out:
> >>  	cgroup_unlock();
> >> @@ -4953,6 +4958,7 @@ mem_cgroup_create(struct cgroup *cont)
> >>  						&per_cpu(memcg_stock, cpu);
> >>  			INIT_WORK(&stock->work, drain_local_stock);
> >>  		}
> >> +		mem_cgroup_subsys.broken_hierarchy = !memcg->use_hierarchy;
> > 
> > Hmmm, this will warn even if we have
> > root (default use_hierarchy=0)
> >  \
> >   A (use_hierarchy=1)
> >    \
> >     B <- here
> > 
> > which is unfortunate because it will add a noise to a reasonable
> > configuration.
> > I think this is fixable if you move the warning after 
> > cgroup_subsys_state::create and broken_hierarchy would be set only if
> > parent is not root and use_hierarchy==0 in mem_cgroup_create. Something
> > like:
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 795e525..d5c93ab 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -4973,6 +4973,13 @@ mem_cgroup_create(struct cgroup *cont)
> >  	} else {
> >  		res_counter_init(&memcg->res, NULL);
> >  		res_counter_init(&memcg->memsw, NULL);
> > +		/*
> > +		 * Deeper hierachy with use_hierarchy == false doesn't make
> > +		 * much sense so let cgroup subsystem know about this unfortunate
> > +		 * state in our controller.
> > +		 */
> > +		if (parent && parent != root_mem_cgroup)
> > +			mem_cgroup_subsys.broken_hierarchy = true;
> >  	}
> >  	memcg->last_scanned_node = MAX_NUMNODES;
> >  	INIT_LIST_HEAD(&memcg->oom_notify);
> > 
> > What do you think?
> > 
> I side with Tejun's original intentions.
> 
> While I respect your goal of not warning about any configuration
> with max_level = 1, I believe the only sane configuration as soon
> as we get any 2nd-level child is use_hierarchy = 1 for everybody.
> 
> Everything aside from it should be warned.

Defintely. And that what the above guarantess, doesn't it?

-- 
Michal Hocko
SUSE Labs
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