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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, May 05, 2015 at 11:54:31AM +0800, Zefan Li wrote:

> But I was wondering if we can change the default value of cpu.rt_runtime_us
> from 0 to -1? So by default the RT tasks can be attached to a newly-created
> cgroup without users having to make any configuration, and those tasks are
> confined by the parent cgroup, which is what we have with cfs bw control.
> This require some changes to the code, but I guess it's do-able?

Its tricky.

Imagine:

	  root
	 /    \
	A      B
       / \    / \
      a1 a2  b1 b2

Now if they all have -1, I cannot set a bw on any except the leaf nodes
([ab][12]). Because the sum of child bw must strictly be smaller or
equal to the parent bandwidth, and -1 if effective inf.

Similarly, if A has bw enabled I cannot create a new child with -1.
Because above.

Now you can kludge around some of this, for example you can make the
default depend on the parent setting etc.. But that's horribly
inconsistent.

So I really prefer not to go that way; if people use RR/FIFO they had
better bloody know what they're doing; which includes setting up the
system.

The whole RR/FIFO thing is so enormously broken (by definition; this
truly is unfixable) that you simply _cannot_ automate it.