FIXUP: sched/tune: fix accounting for runnable tasks

Contains:

sched/tune: fix accounting for runnable tasks (1/5)

The accounting for tasks into boost groups of different CPUs is currently
broken mainly because:
a) we do not properly track the change of boost group of a RUNNABLE task
b) there are race conditions between migration code and accounting code

This patch provides a fixes to ensure enqueue/dequeue
accounting also for throttled tasks.

Without this patch is can happen that a task is enqueued into a throttled
RQ thus not being accounted for the boosting of the corresponding RQ.
We could argue that a throttled task should not boost a CPU, however:
a) properly implementing CPU boosting considering throttled tasks will
   increase a lot the complexity of the solution
b) it's not easy to quantify the benefits introduced by such a more
   complex solution

Since task throttling requires the usage of the CFS bandwidth controller,
which is not widely used on mobile systems (at least not by Android kernels
so far), for the time being we go for the simple solution and boost also
for throttled RQs.

sched/tune: fix accounting for runnable tasks (2/5)

This patch provides the code required to enforce proper locking.
A per boost group spinlock has been added to grant atomic
accounting of tasks as well as to serialise enqueue/dequeue operations,
triggered by tasks migrations, with cgroups's attach/detach operations.

sched/tune: fix accounting for runnable tasks (3/5)

This patch adds cgroups {allow,can,cancel}_attach callbacks.

Since a task can be migrated between boost groups while it's running,
the CGroups's attach callbacks have been added to properly migrate
boost contributions of RUNNABLE tasks.

The RQ's lock is used to serialise enqueue/dequeue operations, triggered
by tasks migrations, with cgroups's attach/detach operations. While the
SchedTune's CPU lock is used to grant atrocity of the accounting within
the CPU.

NOTE: the current implementation does not allows a concurrent CPU migration
      and CGroups change.

sched/tune: fix accounting for runnable tasks (4/5)

This fixes accounting for exiting tasks by adding a dedicated call early
in the do_exit() syscall, which disables SchedTune accounting as soon as a
task is flagged PF_EXITING.

This flag is set before the multiple dequeue/enqueue dance triggered
by cgroup_exit() which is useful only to inject useless tasks movements
thus increasing possibilities for race conditions with the migration code.
The schedtune_exit_task() call does the last dequeue of a task from its
current boost group. This is a solution more aligned with what happens in
mainline kernels (>v4.4) where the exit_cgroup does not move anymore a dying
task to the root control group.

sched/tune: fix accounting for runnable tasks (5/5)

To avoid accounting issues at startup, this patch disable the SchedTune
accounting until the required data structures have been properly
initialized.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
[jstultz: fwdported to 4.4]
Signed-off-by: John Stultz <john.stultz@linaro.org>

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