linux-kernel - Re: [RFC] Storing cgroup id in page->private (Was: Re: [RFC] [PATCH 0/6] Provide cgroup isolation for buffered writes.)

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Date:	Fri, 11 Mar 2011 12:13:43 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Andreas Dilger <adilger@...ger.ca>,
	Justin TerAvest <teravest@...gle.com>,
	m-ikeda <m-ikeda@...jp.nec.com>, jaxboe <jaxboe@...ionio.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	ryov <ryov@...inux.co.jp>, taka <taka@...inux.co.jp>,
	"righi.andrea" <righi.andrea@...il.com>,
	guijianfeng <guijianfeng@...fujitsu.com>,
	balbir <balbir@...ux.vnet.ibm.com>,
	ctalbott <ctalbott@...gle.com>, nauman <nauman@...gle.com>,
	mrubin <mrubin@...gle.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC] Storing cgroup id in page->private (Was: Re: [RFC] [PATCH
 0/6] Provide cgroup isolation for buffered writes.)

On Thu, 10 Mar 2011 22:15:04 -0500
Vivek Goyal <vgoyal@...hat.com> wrote:

> On Fri, Mar 11, 2011 at 11:52:35AM +0900, KAMEZAWA Hiroyuki wrote:
> > On Thu, 10 Mar 2011 21:15:31 -0500
> > Vivek Goyal <vgoyal@...hat.com> wrote:
> >  
> > > > IMO, if you really need some per-page information, then just put it
> > > > in the struct page - you can't hide the memory overhead just by
> > > > having the filesystem to store it for you. That just adds
> > > > unnecessary complexity...
> > > 
> > > Ok. I guess adding anything to struct page is going to be hard and 
> > > we might have to fall back to looking into using page_cgroup for
> > > tracking page state. I was trying to explore the ways so that we don't
> > > have to instantiate whole page_cgroup structure just for trying
> > > to figure out who dirtied the page.
> > > 
> > 
> > Is this bad ?
> > ==
> 
> Sounds like an interesting idea. I am primarily concered about the radix
> tree node size increase. Not sure how big a concern this is.
> 
> Also tracking is useful for two things.
> 
> - Proportinal IO
> - IO throttling
> 
> For proportional IO, anyway we have to use it with memory controller to
> control per cgroup dirty ratio so storing info in page_cgroup should
> not hurt. 
> 

dirty-ratio for memcg will be implemented. It's definitely necessary.

> The only other case where dependence on page_cgroup hurts is IO throttling
> where IO controller does not really need memory cgroup controller (I hope
> so). But we are still not sure if throttling IO at device level is a
> good idea and how to resolve issues related to priority inversion.
> 
Yes, that's priority inversion is my concern.

Thanks,
-Kame

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