sched/core: uclamp: Add CPU's clamp buckets refcounting

Utilization clamping allows to clamp the CPU's utilization within a
[util_min, util_max] range, depending on the set of RUNNABLE tasks on
that CPU. Each task references two "clamp buckets" defining its minimum
and maximum (util_{min,max}) utilization "clamp values". A CPU's clamp
bucket is active if there is at least one RUNNABLE tasks enqueued on
that CPU and refcounting that bucket.

When a task is {en,de}queued {on,from} a rq, the set of active clamp
buckets on that CPU can change. Since each clamp bucket enforces a
different utilization clamp value, when the set of active clamp buckets
changes, a new "aggregated" clamp value is computed for that CPU.

Clamp values are always MAX aggregated for both util_min and util_max.
This ensures that no tasks can affect the performance of other
co-scheduled tasks which are more boosted (i.e. with higher util_min
clamp) or less capped (i.e. with higher util_max clamp).

Each task has a:
   task_struct::uclamp[clamp_id]::bucket_id
to track the "bucket index" of the CPU's clamp bucket it refcounts while
enqueued, for each clamp index (clamp_id).

Each CPU's rq has a:
   rq::uclamp[clamp_id]::bucket[bucket_id].tasks
to track how many tasks, currently RUNNABLE on that CPU, refcount each
clamp bucket (bucket_id) of a clamp index (clamp_id).

Each CPU's rq has also a:
   rq::uclamp[clamp_id]::bucket[bucket_id].value
to track the clamp value of each clamp bucket (bucket_id) of a clamp
index (clamp_id).

The rq::uclamp::bucket[clamp_id][] array is scanned every time we need
to find a new MAX aggregated clamp value for a clamp_id. This operation
is required only when we dequeue the last task of a clamp bucket
tracking the current MAX aggregated clamp value. In these cases, the CPU
is either entering IDLE or going to schedule a less boosted or more
clamped task.
The expected number of different clamp values, configured at build time,
is small enough to fit the full unordered array into a single cache
line.

Add the basic data structures required to refcount, in each CPU's rq,
the number of RUNNABLE tasks for each clamp bucket. Add also the max
aggregation required to update the rq's clamp value at each
enqueue/dequeue event.

Use a simple linear mapping of clamp values into clamp buckets.
Pre-compute and cache bucket_id to avoid integer divisions at
enqueue/dequeue time.

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

---
Changes in v7:
 Message-ID: <20190123191007.GG17749@hirez.programming.kicks-ass.net>
 - removed buckets mapping code
 - use a simpler linear mapping of clamp values into buckets
 Message-ID: <20190124161443.lv2pw5fsspyelckq@e110439-lin>
 - move this patch at the beginning of the series,
   in the attempt to make the overall series easier to digest by moving
   at the very beginning the core bits and main data structures
 Others:
 - update the mapping logic to use exactly and only
   UCLAMP_BUCKETS_COUNT buckets, i.e. no more "special" bucket
 - update uclamp_rq_update() to do top-bottom max search