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

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Date:	Mon, 23 Aug 2010 14:32:37 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
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,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH] memcg: use ID in page_cgroup

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".

> @@ -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 ?

> +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().


Thanks,
Daisuke Nishimura.
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