linux-kernel - Re: [PATCH -mm 2/5] use ID in page cgroup

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Date:	Tue, 3 Aug 2010 09:15:13 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	linux-mm@...ck.org,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	vgoyal@...hat.com, m-ikeda@...jp.nec.com, gthelen@...gle.com,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm 2/5] use ID in page cgroup

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-08-02 19:14:10]:

> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> 
> Now, addresses of memory cgroup can be calculated by their ID without complex.
> This patch relplaces pc->mem_cgroup from a pointer to a unsigned short.
> On 64bit architecture, this offers us more 6bytes room per page_cgroup.
> Use 2bytes for blkio-cgroup's page tracking. More 4bytes will be used for
> some light-weight concurrent access.
> 
> We may able to move this id onto flags field but ...go step by step.
> 
> Changelog: 20100730
>  - fixed some garbage added by debug code in early stage
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
>  include/linux/page_cgroup.h |    3 ++-
>  mm/memcontrol.c             |   32 +++++++++++++++++++-------------
>  mm/page_cgroup.c            |    2 +-
>  3 files changed, 22 insertions(+), 15 deletions(-)
> 
> Index: mmotm-0727/include/linux/page_cgroup.h
> ===================================================================
> --- mmotm-0727.orig/include/linux/page_cgroup.h
> +++ mmotm-0727/include/linux/page_cgroup.h
> @@ -12,7 +12,8 @@
>   */
>  struct page_cgroup {
>  	unsigned long flags;
> -	struct mem_cgroup *mem_cgroup;
> +	unsigned short mem_cgroup;	/* ID of assigned memory cgroup */
> +	unsigned short blk_cgroup;	/* Not Used..but will be. */
>  	struct page *page;
>  	struct list_head lru;		/* per cgroup LRU list */
>  };

Can I recommend that on 64 bit systems, we merge the flag, mem_cgroup
and blk_cgroup into one 8 byte value. We could use
__attribute("packed") and do something like this

struct page_cgroup {
        unsigned int flags;
        unsigned short mem_cgroup;
        unsigned short blk_cgroup;
        ...
} __attribute(("packed"));

Then we need to make sure we don't use more that 32 bits for flags,
which is very much under control at the moment.

This will save us 8 bytes in total on 64 bit systems and nothing on 32
bit systems, but will enable blkio cgroup to co-exist.


-- 
	Three Cheers,
	Balbir
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