Re: [PATCH] sched: Relax a restriction in sched_rt_can_attach()

[Zefan Li <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

>>> 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...