linux-kernel - Re: [PATCH -mm 1/5] quick lookup memcg by ID

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Date:	Tue, 3 Aug 2010 13:31:09 +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>,
	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>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH -mm 1/5] quick lookup memcg by ID

Hi.

Thank you for all of your works.

Several comments are inlined.

On Mon, 2 Aug 2010 19:13:04 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> 
> Now, memory cgroup has an ID per cgroup and make use of it at
>  - hierarchy walk,
>  - swap recording.
> 
> This patch is for making more use of it. The final purpose is
> to replace page_cgroup->mem_cgroup's pointer to an unsigned short.
> 
> This patch caches a pointer of memcg in an array. By this, we
> don't have to call css_lookup() which requires radix-hash walk.
> This saves some amount of memory footprint at lookup memcg via id.
> 
> Changelog: 20100730
>  - fixed rcu_read_unlock() placement.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
>  init/Kconfig    |   11 +++++++++++
>  mm/memcontrol.c |   48 ++++++++++++++++++++++++++++++++++--------------
>  2 files changed, 45 insertions(+), 14 deletions(-)
> 
> Index: mmotm-0727/mm/memcontrol.c
> ===================================================================
> --- mmotm-0727.orig/mm/memcontrol.c
> +++ mmotm-0727/mm/memcontrol.c
> @@ -292,6 +292,30 @@ static bool move_file(void)
>  					&mc.to->move_charge_at_immigrate);
>  }
>  
> +/* 0 is unused */
> +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;
> +
> +static struct mem_cgroup *id_to_memcg(unsigned short id)
> +{
> +	/*
> +	 * This array is set to NULL when mem_cgroup is freed.
> +	 * IOW, there are no more references && rcu_synchronized().
> +	 * This lookup-caching is safe.
> +	 */
> +	if (unlikely(!mem_cgroups[id])) {
> +		struct cgroup_subsys_state *css;
> +
> +		rcu_read_lock();
> +		css = css_lookup(&mem_cgroup_subsys, id);
> +		rcu_read_unlock();
> +		if (!css)
> +			return NULL;
> +		mem_cgroups[id] = container_of(css, struct mem_cgroup, css);
> +	}
> +	return mem_cgroups[id];
> +}
id_to_memcg() seems to be called under rcu_read_lock() already, so I think
rcu_read_lock()/unlock() would be unnecessary.

> Index: mmotm-0727/init/Kconfig
> ===================================================================
> --- mmotm-0727.orig/init/Kconfig
> +++ mmotm-0727/init/Kconfig
> @@ -594,6 +594,17 @@ config CGROUP_MEM_RES_CTLR_SWAP
>  	  Now, memory usage of swap_cgroup is 2 bytes per entry. If swap page
>  	  size is 4096bytes, 512k per 1Gbytes of swap.
>  
> +config MEM_CGROUP_MAX_GROUPS
> +	int "Maximum number of memory cgroups on a system"
> +	range 1 65535
> +	default 8192 if 64BIT
> +	default 2048 if 32BIT
> +	help
> +	  Memory cgroup has limitation of the number of groups created.
> +	  Please select your favorite value. The more you allow, the more
> +	  memory will be consumed. This consumes vmalloc() area, so,
> +	  this should be small on 32bit arch.
> +
We don't use vmalloc() area in this version :)


Thanks,
Daisuke Nishimura.
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