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    SubjectRe: [PATCH] sched: Relax a restriction in sched_rt_can_attach()
    >>> Some degree of flexibility is provided so that you may disable some controllers
    >>> in a subtree. For example:
    >>>
    >>> root ---> child1
    >>> (cpuset,memory,cpu) (cpuset,memory)
    >>> \
    >>> \-> child2
    >>> (cpu)
    >>
    >> Whew, that's a relief. Thanks.
    >
    > But somehow I'm not feeling a whole lot better.
    >
    > "May" means if you don't explicitly take some action to disable group
    > scheduling, you get it (I don't care if I have an off button), but that
    > would also seemingly mean that we would then have rt tasks in taskgroups
    > with no bandwidth allocated, ie you have to make group scheduling for rt
    > tasks meaningless until a bandwidth appeared, and to make bandwidth
    > appear, you'd have to stop the world, distribute, continue, no?
    >
    > The current "just say no" seems a lot more sensible.
    >

    I just realized we allow removing/adding controllers from/to cgroups
    while there are tasks in them, which isn't safe unless we eliminate all
    can_attach callbacks. We've done so for some cgroup subsystems, but
    there are still a few of them...



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