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    SubjectRe: [RFC] cgroup TODOs
    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.

    >
    > * cgroup_freezer and others shouldn't be too difficult to fix.
    >
    > Who:
    >
    > 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.

    Glauber feel free to jump into it if you like to. We can sort it out
    together.

    [..]
    > * 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.

    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.

    Thoughts?

    Thanks
    Vivek


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