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

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Message-Id: <20100803132559.9d0fcb69.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Tue, 3 Aug 2010 13:25:59 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.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

On Tue, 3 Aug 2010 09:36:45 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> * 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.
> 

Hmm. As I already wrote, packing id to flags is not easy. 
leave 4 bytes space _pad for a while and drop this patch ?

I don't like to add CONFIG_DEBUG in this core.

Thanks,
-Kame


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