Re: [PATCH 4/4] memcg: always enable kmemcg on the default hierarchy

[Tejun Heo <tj@kernel.org>]

Hello,

On Fri, Aug 28, 2015 at 07:49:18PM +0300, Vladimir Davydov wrote:
> On Fri, Aug 28, 2015 at 11:25:30AM -0400, Tejun Heo wrote:
> > On the default hierarchy, all memory consumption will be accounted
> > together and controlled by the same set of limits.  Always enable
> > kmemcg on the default hierarchy.
> 
> IMO we should introduce a boot time knob for disabling it, because kmem
> accounting is still not perfect, besides some users might prefer to go
> w/o it for performance reasons.

Yeah, fair enough.

> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index c94b686..8a5dd01 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -4362,6 +4362,13 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css)
> >  	if (ret)
> >  		return ret;
> >  
> > +	/* kmem is always accounted together on the default hierarchy */
> > +	if (cgroup_on_dfl(css->cgroup)) {
> > +		ret = memcg_activate_kmem(memcg, PAGE_COUNTER_MAX);
> > +		if (ret)
> > +			return ret;
> > +	}
> > +
> 
> This is a wrong place for this. The kernel will panic on an attempt to
> create a sub memcg, because memcg_init_kmem already enables kmem
> accounting in this case. I guess we should add this hunk to
> memcg_propagate_kmem instead.

Yeap, bypassing "parent is active" test in memcg_propagate_kmem()
seems like the right thing to do.

Thanks.

-- 
tejun

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