sched/core: uclamp: Map TASK's clamp values into CPU's clamp buckets

Utilization clamping requires each CPU to know which clamp values are
assigned to tasks RUNNABLE on that CPU. A per-CPU array of reference
counters can be used where each entry tracks how many RUNNABLE tasks
require the same clamp value on each CPU. However, the range of clamp
values is too wide to track all the possible values in a per-CPU array.

Trade-off clamping precision for run-time and space efficiency using a
"bucketization and mapping" mechanism to translate "clamp values" into
"clamp buckets", each one representing a range of possible clamp values.

While the bucketization allows to use only a minimal set of clamp
buckets at run-time, the mapping ensures that the clamp buckets in use
are always at the beginning of the per-CPU array.

The minimum set of clamp buckets used at run-time depends on their
granularity and how many clamp values the target system expects to
use. Since on most systems we expect only a few different clamp
values, the bucketization and mapping mechanism increases our chances
to have all the required data fitting in one cache line.

For example, if we have only 20% and 25% clamped tasks, by setting:
   CONFIG_UCLAMP_BUCKETS_COUNT 20
we allocate 20 clamp buckets with 5% resolution each, however we will
use only 2 of them at run-time, since their 5% resolution is enough to
always distinguish the clamp values in use, and they will both fit
into a single cache line for each CPU.

Introduce the "bucketization and mapping" mechanisms which are required
for the implement of the per-CPU operations.

Add a new "uclamp_enabled" sched_class attribute to mark which class
will contribute to clamping the CPU utilization. Move few callbacks
around to ensure that the most used callbacks are all in the same cache
line along with the new attribute.

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

---
Changes in v6:
 Message-ID: <20181107144448.GH9761@hirez.programming.kicks-ass.net>
 - added bucketization support since the beginning to avoid
   semi-functional code in this patch
 Message-ID: <20181107141414.GF9761@hirez.programming.kicks-ass.net>
 - update cmpxchg loops to use "do { } while (cmpxchg(ptr, old, new) != old)"
 - switch to usage of try_cmpxchg()
 Message-ID: <20181107145527.GI9761@hirez.programming.kicks-ass.net>
 - use SCHED_WARN_ON() instead of CONFIG_SCHED_DEBUG guarded blocks
 - ensure se_count never underflow
 Message-ID: <20181112000910.GC3038@worktop>
 - wholesale s/group/bucket/
 Message-ID: <20181111164754.GA3038@worktop>
 - consistently use unary (++/--) operators
 Message-ID: <20181107142428.GG14309@e110439-lin>
 - added some better comments for invariant conditions
 Message-ID: <20181107145612.GJ14309@e110439-lin>
 - ensure UCLAMP_BUCKETS_COUNT >= 1
 Others:
 - added and make use of the bit_for() macro
 - wholesale s/_{get,put}/_{inc,dec}/ to match refcount APIs
 - documentation review and cleanup