linux-kernel - Re: [PATCH -mm 5/5] memcg: use spinlock in page_cgroup instead of bit

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Date:	Tue, 3 Aug 2010 09:36:45 +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 5/5] memcg: use spinlock in page_cgroup instead of
 bit_spinlock

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-08-02 19:20:06]:

> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> 
> This patch replaces bit_spinlock with spinlock. In general,
> spinlock has good functinality than bit_spin_lock and we should use
> it if we have a room for it. In 64bit arch, we have extra 4bytes.
> Let's use it.
> expected effects:
>  - use better codes.
>  - ticket lock on x86-64
>  - para-vitualization aware lock
> etc..
> 
> Chagelog: 20090729
>  - fixed page_cgroup_is_locked().
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> --

The additional space usage is a big concern, I think saving space
would be of highest priority. I understand the expected benefits, but
a spinlock_t per page_cgroup is quite expensive at the moment. If
anything I think it should be a config option under CONFIG_DEBUG or
something else to play with and see the side effects.

-- 
	Three Cheers,
	Balbir
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