sched/core: uclamp: Extend CPU's cgroup controller

The cgroup CPU bandwidth controller allows to assign a specified
(maximum) bandwidth to the tasks of a group. However this bandwidth is
defined and enforced only on a temporal base, without considering the
actual frequency a CPU is running on. Thus, the amount of computation
completed by a task within an allocated bandwidth can be very different
depending on the actual frequency the CPU is running that task.
The amount of computation can be affected also by the specific CPU a
task is running on, especially when running on asymmetric capacity
systems like Arm's big.LITTLE.

With the availability of schedutil, the scheduler is now able
to drive frequency selections based on actual task utilization.
Moreover, the utilization clamping support provides a mechanism to
bias the frequency selection operated by schedutil depending on
constraints assigned to the tasks currently RUNNABLE on a CPU.

Giving the mechanisms described above, it is now possible to extend the
cpu controller to specify the minimum (or maximum) utilization which
should be considered for tasks RUNNABLE on a cpu.
This makes it possible to better defined the actual computational
power assigned to task groups, thus improving the cgroup CPU bandwidth
controller which is currently based just on time constraints.

Extend the CPU controller with a couple of new attributes util.{min,max}
which allows to enforce utilization boosting and capping for all the
tasks in a group. Specifically:

- util.min: defines the minimum utilization which should be considered
	    i.e. the RUNNABLE tasks of this group will run at least at a
		 minimum frequency which corresponds to the util.min
		 utilization

- util.max: defines the maximum utilization which should be considered
	    i.e. the RUNNABLE tasks of this group will run up to a
		 maximum frequency which corresponds to the util.max
		 utilization

These attributes:

a) are available only for non-root nodes, both on default and legacy
   hierarchies, while system wide clamps are defined by a generic
   interface which does not depends on cgroups. This system wide
   interface enforces constraints on tasks in the root node.

b) enforce effective constraints at each level of the hierarchy which
   are a restriction of the group requests considering its parent's
   effective constraints. Root group effective constraints are defined
   by the system wide interface.
   This mechanism allows each (non-root) level of the hierarchy to:
   - request whatever clamp values it would like to get
   - effectively get only up to the maximum amount allowed by its parent

c) have higher priority than task-specific clamps, defined via
   sched_setattr(), thus allowing to control and restrict task requests

Add two new attributes to the cpu controller to collect "requested"
clamp values. Allow that at each non-root level of the hierarchy.
Validate local consistency by enforcing util.min < util.max.
Keep it simple by do not caring now about "effective" values computation
and propagation along the hierarchy.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>

--
Changes in v8:
 Message-ID: <20190214154817.GN50184@devbig004.ftw2.facebook.com>
 - update changelog description for points b), c) and following paragraph