linux-kernel - Re: [PATCH] memcg: use ID in page

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Date:	Tue, 24 Aug 2010 08:52:43 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc:	linux-mm@...ck.org,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	gthelen@...gle.com, m-ikeda@...jp.nec.com,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kamezawa.hiroyuki@...il.com
Subject: Re: [PATCH] memcg: use ID in page_cgroup

On Mon, 23 Aug 2010 14:32:37 +0900
Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:

> On Fri, 20 Aug 2010 19:01:32 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> 
> > 
> > I have an idea to remove page_cgroup->page pointer, 8bytes reduction per page.
> > But it will be after this work.
> Another off topic. I think we can reduce the size of mem_cgroup by packing
> some boolean members into one "unsinged long flags".
> 
I'll use "flags" to remove struct page pointer.


> > @@ -300,12 +300,13 @@ static atomic_t mem_cgroup_num;
> >  #define NR_MEMCG_GROUPS (CONFIG_MEM_CGROUP_MAX_GROUPS + 1)
> >  static struct mem_cgroup *mem_cgroups[NR_MEMCG_GROUPS] __read_mostly;
> >  
> > -/* Must be called under rcu_read_lock */
> > -static struct mem_cgroup *id_to_memcg(unsigned short id)
> > +/* Must be called under rcu_read_lock, set safe==true if under lock */
> Do you mean, "Set safe==true if we can ensure by some locks that the id can be
> safely dereferenced without rcu_read_lock", right ?
> 

yes. that's just for rcu_deerference_check().


> > +static struct mem_cgroup *id_to_memcg(unsigned short id, bool safe)
> >  {
> >  	struct mem_cgroup *ret;
> >  	/* see mem_cgroup_free() */
> > -	ret = rcu_dereference_check(mem_cgroups[id], rch_read_lock_held());
> > +	ret = rcu_dereference_check(mem_cgroups[id],
> > +				rch_read_lock_held() || safe);
> >  	if (likely(ret && ret->valid))
> >  		return ret;
> >  	return NULL;
> 
> (snip)
> > @@ -723,6 +729,11 @@ static inline bool mem_cgroup_is_root(st
> >  	return (mem == root_mem_cgroup);
> >  }
> >  
> > +static inline bool mem_cgroup_is_rootid(unsigned short id)
> > +{
> > +	return (id == 1);
> > +}
> > +
> It might be better to add
> 
> 	BUG_ON(newid->id != 1)
> 
> in cgroup.c::cgroup_init_idr().
> 

Why ??

Thanks,
-Kame



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