linux-kernel - Re: [PATCH 3/5] page_cgroup: make page tracking available for blkio

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Date:	Wed, 23 Feb 2011 00:48:33 +0100
From:	Andrea Righi <arighi@...eler.com>
To:	Jonathan Corbet <corbet@....net>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Greg Thelen <gthelen@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>,
	Ryo Tsuruta <ryov@...inux.co.jp>,
	Hirokazu Takahashi <taka@...inux.co.jp>,
	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	containers@...ts.linux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] page_cgroup: make page tracking available for blkio

On Tue, Feb 22, 2011 at 04:27:29PM -0700, Jonathan Corbet wrote:
> On Wed, 23 Feb 2011 00:01:47 +0100
> Andrea Righi <arighi@...eler.com> wrote:
> 
> > > My immediate observation is that you're not really tracking the "owner"
> > > here - you're tracking an opaque 16-bit token known only to the block
> > > controller in a field which - if changed by anybody other than the block
> > > controller - will lead to mayhem in the block controller.  I think it
> > > might be clearer - and safer - to say "blkcg" or some such instead of
> > > "owner" here.
> > 
> > Basically the idea here was to be as generic as possible and make this
> > feature potentially available also to other subsystems, so that cgroup
> > subsystems may represent whatever they want with the 16-bit token.
> > However, no more than a single subsystem may be able to use this feature
> > at the same time.
> 
> That makes me nervous; it can't really be used that way unless we want to
> say that certain controllers are fundamentally incompatible and can't be
> allowed to play together.  For whatever my $0.02 are worth (given the
> state of the US dollar, that's not a whole lot), I'd suggest keeping the
> current mechanism, but make it clear that it belongs to your controller.
> If and when another controller comes along with a need for similar
> functionality, somebody can worry about making it more general.

OK, I understand. I'll use "blkio" instead of "owner". Also because I
wouldn't like to introduce additional logic and overhead to check if two
controllers are using this feature at the same time. Better to hard-code
this information in the name of the functions.

Probably the most generic solution is the one that you suggested:
replace the mem_cgroup with a pointer to css_set. I'll also try to
investigate this way.

Thanks,
-Andrea
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