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    SubjectRe: [RFC][-mm] Memory controller hierarchy support (v1)
    Paul Menage wrote:
    > On Fri, Apr 18, 2008 at 10:35 PM, Balbir Singh
    > <> wrote:
    >> 1. We need to hold cgroup_mutex while walking through the children
    >> in reclaim. We need to figure out the best way to do so. Should
    >> cgroups provide a helper function/macro for it?
    > There's already a function, cgroup_lock(). But it would be nice to
    > avoid such a heavy locking here, particularly since memory allocations
    > can occur with cgroup_mutex held, which could lead to a nasty deadlock
    > if the allocation triggered reclaim.

    Hmm.. probably..

    > One of the things that I've been considering was to put the
    > parent/child/sibling hierarchy explicitly in cgroup_subsys_state. This
    > would give subsystems their own copy to refer to, and could use their
    > own internal locking to synchronize with callbacks from cgroups that
    > might change the hierarchy. Cpusets could make use of this too, since
    > it has to traverse hierarchies sometimes.

    Very cool! I look forward to that infrastructure. I'll also look at the cpuset
    code and see how to traverse the hierarchy.

    >> 2. Do not allow children to have a limit greater than their parents.
    >> 3. Allow the user to select if hierarchial support is required
    > My thoughts on this would be:
    > 1) Never attach a first-level child's counter to its parent. As
    > Yamamoto points out, otherwise we end up with extra global operations
    > whenever any cgroup allocates or frees memory. Limiting the total
    > system memory used by all user processes doesn't seem to be something
    > that people are going to generally want to do, and if they really do
    > want to they can just create a non-root child and move the whole
    > system into that.
    > The one big advantage that you currently get from having all
    > first-level children be attached to the root is that the reclaim logic
    > automatically scans other groups when it reaches the top-level - but I
    > think that can be provided as a special-case in the reclaim traversal,
    > avoiding the overhead of hitting the root cgroup that we have in this
    > patch.

    I've been doing some thinking along these lines, I'll think more about this.

    > 2) Always attach other children's counters to their parents - if the
    > user didn't want a hierarchy, they could create a flat grouping rather
    > than nested groupings.

    Yes, that's a TODO

    > Paul

    Warm Regards,
    Balbir Singh
    Linux Technology Center

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