sched/fair: Prevent unlimited runtime on throttled group

sched/fair: Prevent unlimited runtime on throttled group

When a running task is moved on a throttled task group and there is no
other task enqueued on the CPU, the task can keep running using 100% CPU
whatever the allocated bandwidth for the group and although its cfs rq is
throttled. Furthermore, the group entity of the cfs_rq and its parents are
not enqueued but only set as curr on their respective cfs_rqs.

We have the following sequence:

sched_move_task
  -dequeue_task: dequeue task and group_entities.
  -put_prev_task: put task and group entities.
  -sched_change_group: move task to new group.
  -enqueue_task: enqueue only task but not group entities because cfs_rq is
    throttled.
  -set_next_task : set task and group_entities as current sched_entity of
    their cfs_rq.

Another impact is that the root cfs_rq runnable_load_avg at root rq stays
null because the group_entities are not enqueued. This situation will stay
the same until an "external" event triggers a reschedule. Let trigger it
immediately instead.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Ben Segall <bsegall@google.com>
Link: https://lkml.kernel.org/r/1579011236-31256-1-git-send-email-vincent.guittot@linaro.org