linux-kernel - Re: [PATCH] cgroupfs: create /sys/fs/cgroup to mount cgroupfs on

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Message-ID: <20100722212634.GC2688@redhat.com>
Date:	Thu, 22 Jul 2010 17:26:34 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Greg KH <gregkh@...e.de>
Cc:	Paul Menage <menage@...gle.com>, Li Zefan <lizf@...fujitsu.com>,
	Lennart Poettering <lennart@...ttering.net>,
	Kay Sievers <kay.sievers@...y.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Ben Blum <bblum@...rew.cmu.edu>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	"Daniel P. Berrange" <berrange@...hat.com>,
	Jan Safranek <jsafrane@...hat.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>
Subject: Re: [PATCH] cgroupfs: create /sys/fs/cgroup to mount cgroupfs on

On Thu, Jul 22, 2010 at 02:18:56PM -0700, Greg KH wrote:
> On Thu, Jul 22, 2010 at 03:37:41PM -0400, Vivek Goyal wrote:
> > On Thu, Jul 22, 2010 at 11:36:15AM -0700, Greg KH wrote:
> > > On Thu, Jul 22, 2010 at 11:31:07AM -0700, Paul Menage wrote:
> > > > On Thu, Jul 22, 2010 at 11:26 AM, Greg KH <gregkh@...e.de> wrote:
> > > > > We really shouldn't be asking userspace to create new root filesystems.
> > > > > So follow along with all of the other in-kernel filesystems, and provide
> > > > > a mount point in sysfs.
> > > > >
> > > > > For cgroupfs, this should be in /sys/fs/cgroup/  This change provides
> > > > > that mount point when the cgroup filesystem is registered in the kernel.
> > > > 
> > > > But cgroups will typically have multiple mounts, with different
> > > > resource controllers/options on each mount. That doesn't really fit in
> > > > with this scheme.
> > > 
> > > Really?  I see systems mounting it at /cgroups/ in the filesystem today.
> > > Where are you expecting it to be mounted at?
> > > 
> > 
> > Greg,
> > 
> > [CCing few more folks who might be interested in this dicussion ]
> > 
> > We do want to retain facility to mount different controllers at different
> > mount points. We were discussing the other day that in libvirt we might
> > want to mount block IO controller and network controller separately as
> > by default we will not put a new virtual machine in a cgroup of its own
> > because of the penatly involved.
> 
> That's fine, I'm not changing that ability at all.  We just need a
> "default" mount point for "normal" users.
> 
> > For other controllers like cpu, memory etc, libvirt automatically puts
> > each new virtual machine in a cgroup of own. So this is one use case
> > where we might want to mount different controllers at different mount
> > points.
> > 
> > For my testing I now always use /cgroup/ and create directories under it
> > /cgroup/blkio /cgroup/cpu etc and mount controllers on respective
> > directories.
> 
> Lennart and Kay, is this what systemd is doing?  I really don't think we
> should be adding a root /cgroup/ mount point to the system for something
> like this.
> 
> Maybe /dev/cgroup/ is better to use, as that way users can create
> sub-mount points easier.  They can't do that in /sys/fs/cgroup/

The only problem with /dev/cgroup seems to be that it seems little
unintutive. To me, we have devices under /dev/ dir and cgroups are not
devices.

I think people have floated similar threads in the past on lkml with
various opinions and everybody had their own choices but nothing was
conclusive. 

Polluting / definitely sounds odd but it does not look that bad once
we can't find any other good choices.

Thanks
Vivek
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