[RFC] cgroup TODOs

[Tejun Heo]

Hello, Vivek.

On Fri, Sep 14, 2012 at 10:25:39AM -0400, Vivek Goyal wrote:
> On Thu, Sep 13, 2012 at 01:58:27PM -0700, Tejun Heo wrote:
> 
> [..]
> >   * blkio is the most problematic.  It has two sub-controllers - cfq
> >     and blk-throttle.  Both are utterly broken in terms of hierarchy
> >     support and the former is known to have pretty hairy code base.  I
> >     don't see any other way than just biting the bullet and fixing it.
> 
> I am still little concerned about changing the blkio behavior
> unexpectedly. Can we have some kind of mount time flag which retains
> the old flat behavior and we warn user that this mode is deprecated
> and will soon be removed. Move over to hierarchical mode. Then after
> few release we can drop the flag and cleanup any extra code which
> supports flat mode in CFQ. This will atleast make transition smooth.

I don't know.  That essentially is what we're doing with memcg now and
it doesn't seem any less messy.  Given the already scary complexity,
do we really want to support both flat and hierarchy models at the
same time?

> >   memcg can be handled by memcg people and I can handle cgroup_freezer
> >   and others with help from the authors.  The problematic one is
> >   blkio.  If anyone is interested in working on blkio, please be my
> >   guest.  Vivek?  Glauber?
> 
> I will try to spend some time on this. Doing changes in blk-throttle
> should be relatively easy. Painful part if CFQ. It does so much that
> it is not clear whether a particular change will bite us badly or
> not. So doing changes becomes hard. There are heuristics, preemptions,
> queue selection logic, service tree and bringing it all together
> for full hierarchy becomes interesting.
> 
> I think first thing which needs to be done is merge group scheduling
> and cfqq scheduling. Because of flat hierarchy currently we use two
> scheduling algorithm. Old logic for queue selection and new logic
> for group scheduling. If we treat task and group at same level then
> we have to merge two and come up with single logic.

I think this depends on how we decide to handle tasks vs. groups,
right?

> [..]
> >   * Vivek brought up the issue of distributing resources to tasks and
> >     groups in the same cgroup.  I don't know.  Need to think more
> >     about it.
> 
> This one will require some thought. I have heard arguments for both the
> models. Treating tasks and groups at same level seem to have one
> disadvantange and that is that people can't think of system resources
> in terms of %. People often say, give 20% of disk resources to a
> particular cgroup. But it is not possible as there are all kernel
> threads running in root cgroup and tasks come and go and that means
> % share of a group is variable and not fixed.

Another problem is that configuration isn't contained in cgroup
proper.  We need a way to assign weights to individual tasks which can
be somehow directly compared against group weights.  cpu cooks
priority for this and blkcg may be able to cook ioprio but it's nasty
and unobvious.  Also, let's say we grow network bandwidth controller
for whatever reason.  What value are we gonna use?

> To make it fixed, we will need to make sure that number of entities
> fighting for resources are not variable. That means only group fight
> for resources at a level and tasks with-in groups. 
> 
> Now the question is should kernel enforce it or should it be left to 
> user space. I think doing it in user space is also messy as different
> agents control different part of hiearchy. For example, if somebody says
> that give a particular virtual machine a x% of system resource, libvirt
> has no way to do that. At max it can ensure x% of parent group but above
> that hierarchy is controlled by systemd and libvirtd has no control
> over that.
>
> Only possible way to do this will seem to be that systemd creates libvirt
> group at top level with a minimum fixed % of quota and then libvirt can
> figure out % share of each virtual machine. But it is hard to do.
> 
> So while % model is more intutive to users, it is hard to implement. So
> an easier way is to stick to the model of relative weights/share and
> let user specify relative importance of a virtual machine and actual
> quota or % will vary dynamically depending on other tasks/components
> in the system.

Why is it hard to implement?  You just need to treat tasks in the
current node as another group competing with other cgroups on equal
terms.  If anything, isn't that simpler than treating scheduling
"entities"?

Thanks.

-- 
tejun