linux-kernel - Re: [PATCH 1/9] io-throttle documentation

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Date:	Fri, 17 Apr 2009 20:35:17 +0900
From:	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>
To:	Li Zefan <lizf@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>, agk@...rceware.org,
	akpm@...ux-foundation.org, axboe@...nel.dk, baramsori72@...il.com,
	Carl Henrik Lunde <chlunde@...g.uio.no>,
	dave@...ux.vnet.ibm.com, Divyesh Shah <dpshah@...gle.com>,
	eric.rannaud@...il.com, fernando@....ntt.co.jp,
	Hirokazu Takahashi <taka@...inux.co.jp>, matt@...ehost.com,
	dradford@...ehost.com, ngupta@...gle.com, randy.dunlap@...cle.com,
	roberto@...it.it, Ryo Tsuruta <ryov@...inux.co.jp>,
	Satoshi UCHIDA <s-uchida@...jp.nec.com>,
	subrata@...ux.vnet.ibm.com, yoshikawa.takuya@....ntt.co.jp,
	containers@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/9] io-throttle documentation

Andrea Righi wrote:
> On Fri, Apr 17, 2009 at 09:56:31AM +0800, Li Zefan wrote:
>> KAMEZAWA Hiroyuki wrote:
>>> On Tue, 14 Apr 2009 22:21:12 +0200
>>> Andrea Righi <righi.andrea@...il.com> wrote:
>>>
>>>> +Example:
>>>> +* Create an association between an io-throttle group and a bio-cgroup group
>>>> +  with "bio" and "blockio" subsystems mounted in different mount points:
>>>> +  # mount -t cgroup -o bio bio-cgroup /mnt/bio-cgroup/
>>>> +  # cd /mnt/bio-cgroup/
>>>> +  # mkdir bio-grp
>>>> +  # cat bio-grp/bio.id
>>>> +  1
>>>> +  # mount -t cgroup -o blockio blockio /mnt/io-throttle
>>>> +  # cd /mnt/io-throttle
>>>> +  # mkdir foo
>>>> +  # echo 1 > foo/blockio.bio_id
>>> Why do we need multiple cgroups at once to track I/O ?
>>> Seems complicated to me.
>>>
>> IIUC, it also disallows other subsystems to be binded with blockio subsys:
>>   # mount -t cgroup -o blockio cpuset xxx /mnt
>>   (failed)
>>
>> and if a task is moved from cg1(id=1) to cg2(id=2) in bio subsys, this task
>> will be moved from CG1(id=1) to CG2(id=2) automatically in blockio subsys.
>>
>> All these are odd, unexpected, complex and bug-prone I think..
> 
> Implementing bio-cgroup functionality as pure infrastructure framework
> instead of a cgroup subsystem would remove all this oddity and
> complexity.

Andrea, I agree with you completely. In fact, we have been working on that for
a while and have just proposed doing exactly that on a different mail thread
(you are CC'ed). It would be great if you could comment on that proposal.

Thanks,

Fernando

> For example, the actual functionality that I need for the io-throttle
> controller is just an interface to set and get the cgroup owner of a
> page. I think it should be the same also for other potential users of
> bio-cgroup.
> 
> So, what about implementing the bio-cgroup functionality as cgroup "page
> tracking" infrastructure and provide the following interfaces:
> 
> /*
>  * Encode the cgrp->css.id in page_group->flags
>  */
> void set_cgroup_page_owner(struct page *page, struct cgroup *cgrp);
> 
> /*
>  * Returns the cgroup owner of a page, decoding the cgroup id from
>  * page_cgroup->flags.
>  */
> struct cgroup *get_cgroup_page_owner(struct page *page);
> 
> This also wouldn't increase the size of page_cgroup because we can
> encode the cgroup id in the unused bits of page_cgroup->flags, as
> originally suggested by Kame.
> 
> And I think it could be used also by dm-ioband, even if it's not a
> cgroup-based subsystem... but I may be wrong. Ryo what's your opinion?
> 
> -Andrea

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