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    SubjectRe: [RFC][PATCH 2/7][memcg] cgroup arbitarary ID allocation
    On Tue, 27 Jul 2010 22:30:27 -0400
    Vivek Goyal <vgoyal@redhat.com> wrote:

    > > Index: mmotm-2.6.35-0719/Documentation/cgroups/cgroups.txt
    > > ===================================================================
    > > --- mmotm-2.6.35-0719.orig/Documentation/cgroups/cgroups.txt
    > > +++ mmotm-2.6.35-0719/Documentation/cgroups/cgroups.txt
    > > @@ -621,6 +621,15 @@ and root cgroup. Currently this will onl
    > > the default hierarchy (which never has sub-cgroups) and a hierarchy
    > > that is being created/destroyed (and hence has no sub-cgroups).
    > >
    > > +void custom_id(struct cgroup_subsys *ss, struct cgroup *cgrp)
    > > +
    > > +Called at assigning a new ID to cgroup subsystem state struct. This
    > > +is called when ss->use_id == true. If this function is not provided,
    > > +a new ID is automatically assigned. If you enable ss->use_id,
    > > +you can use css_lookup() and css_get_next() to access "css" objects
    > > +via IDs.
    > > +
    >
    > Couple of lines to explain why a subsystem would like to assign its
    > own ids and not be happy with generic cgroup assigned id be helpful.
    > In this case, I think you are using this id as index into array
    > and want to control the index, hence you seem to be doing it.
    >
    > But I am not sure again why do you want to control index?
    >

    Now, the subsystem allocation/id-allocation order is

    ->create()
    alloc_id.

    Otherwise "id" of memory cgroup is just determined by the place in virtual-indexed
    array.
    As
    memcg = mem_cgroup_base + id

    This "id" is determined at create().

    If "id" is determined regardless of memory cgroup's placement, it's of no use.
    My original design of css_id() allocates id in create() but it was moved to
    generic part. So, this is expected change in my plan.

    We have 2 choices.
    id = alloc_id()
    create(id)
    or
    this patch.

    Both are okay for me. But alloc id before create() may add some ugly rollback.

    Thanks,
    -Kame



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