linux-kernel - Re: [RFC] writeback and cgroup

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Date:	Thu, 12 Apr 2012 16:37:19 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Tejun Heo <tj@...nel.org>, Fengguang Wu <fengguang.wu@...el.com>,
	Jens Axboe <axboe@...nel.dk>, linux-mm@...ck.org,
	sjayaraman@...e.com, andrea@...terlinux.com, jmoyer@...hat.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	kamezawa.hiroyu@...fujitsu.com, lizefan@...wei.com,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	ctalbott@...gle.com, rni@...gle.com, lsf@...ts.linux-foundation.org
Subject: Re: [RFC] writeback and cgroup

On Wed, Apr 11, 2012 at 09:22:31PM +0200, Jan Kara wrote:

[..]
> > >   Well, btrfs plays tricks with bdi's but there is a special bdi called
> > > "btrfs" which backs the whole filesystem and that is what's put in
> > > sb->s_bdi or in each inode's i_mapping->backing_dev_info. So we have a
> > > global bdi to work with.
> > 
> > Ok, that's good to know. How would we configure this special bdi? I am
> > assuming there is no backing device visible in /sys/block/<device>/queue/?
> > Same is true for network file systems.
>   Where should be the backing device visible? Now it's me who is lost :)

I mean how are we supposed to put cgroup throttling rules using cgroup
interface for network filesystems and for btrfs global bdi. Using "dev_t"
associated with bdi? I see that all the bdi's are showing up in
/sys/class/bdi, but how do I know which one I am intereste in or which
one belongs to filesystem I am interestd in putting throttling rule on.

For block devices, we simply use "major:min limit" format to write to
a cgroup file and this configuration will sit in one of the per queue
per cgroup data structure.

I am assuming that when you say throttling should happen at bdi, you
are thinking of maintaining per cgroup per bdi data structures and user
is somehow supposed to pass "bdi_maj:bdi_min  limit" through cgroup files?
If yes, how does one map a filesystem's bdi we want to put rules on?

Also, at request queue level we have bios and we throttle bios. At bdi
level, I think there are no bios yet. So somehow we got to deal with
pages. Not sure how exactly will throttling happen.

Thanks
Vivek
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