linux-kernel - Re: Low overhead patches for the memory cgroup controller (v5)

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Date:	Tue, 23 Jun 2009 10:23:16 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	kamezawa.hiroyuki@...fujitsu.com, nishimura@....nes.nec.co.jp,
	lizf@...fujitsu.com, menage@...gle.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: Low overhead patches for the memory cgroup controller (v5)

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-06-23 09:01:16]:

> On Mon, 22 Jun 2009 15:43:43 -0700
> Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > On Mon, 15 Jun 2009 10:09:00 +0530
> > Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> > 
> > >
> > > ...
> > > 
> > > This patch changes the memory cgroup and removes the overhead associated
> > > with accounting all pages in the root cgroup. As a side-effect, we can
> > > no longer set a memory hard limit in the root cgroup.
> > > 
> > > A new flag to track whether the page has been accounted or not
> > > has been added as well. Flags are now set atomically for page_cgroup,
> > > pcg_default_flags is now obsolete and removed.
> > > 
> > > ...
> > >
> > > @@ -1114,9 +1121,22 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *mem,
> > >  		css_put(&mem->css);
> > >  		return;
> > >  	}
> > > +
> > >  	pc->mem_cgroup = mem;
> > >  	smp_wmb();
> > > -	pc->flags = pcg_default_flags[ctype];
> > > +	switch (ctype) {
> > > +	case MEM_CGROUP_CHARGE_TYPE_CACHE:
> > > +	case MEM_CGROUP_CHARGE_TYPE_SHMEM:
> > > +		SetPageCgroupCache(pc);
> > > +		SetPageCgroupUsed(pc);
> > > +		break;
> > > +	case MEM_CGROUP_CHARGE_TYPE_MAPPED:
> > > +		ClearPageCgroupCache(pc);
> > > +		SetPageCgroupUsed(pc);
> > > +		break;
> > > +	default:
> > > +		break;
> > > +	}
> > 
> > Do we still need the smp_wmb()?
> > 
> > It's hard to say, because we forgot to document it :(
> > 
> Sorry for lack of documentation.
> 
> pc->mem_cgroup should be visible before SetPageCgroupUsed(). Othrewise,
> A routine believes USED bit will see bad pc->mem_cgroup.
> 
> I'd like to  add a comment later (againt new mmotm.)
>

Thanks Kamezawa! We do use the barrier Andrew, an easy way to find
affected code is to look at the smp_rmb()'s we have. But it is better
documented.

-- 
	Balbir
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