sched/fair: Ensure that the CFS parent is added after unthrottling

sched/fair: Ensure that the CFS parent is added after unthrottling

Ensure that a CFS parent will be in the list whenever one of its children is also
in the list.

A warning on rq->tmp_alone_branch != &rq->leaf_cfs_rq_list has been
reported while running LTP test cfs_bandwidth01.

Odin Ugedal found the root cause:

	$ tree /sys/fs/cgroup/ltp/ -d --charset=ascii
	/sys/fs/cgroup/ltp/
	|-- drain
	`-- test-6851
	    `-- level2
		|-- level3a
		|   |-- worker1
		|   `-- worker2
		`-- level3b
		    `-- worker3

Timeline (ish):
- worker3 gets throttled
- level3b is decayed, since it has no more load
- level2 get throttled
- worker3 get unthrottled
- level2 get unthrottled
  - worker3 is added to list
  - level3b is not added to list, since nr_running==0 and is decayed

 [ Vincent Guittot: Rebased and updated to fix for the reported warning. ]

Fixes: a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
Reported-by: Sachin Sant <[email protected]>
Suggested-by: Vincent Guittot <[email protected]>
Signed-off-by: Rik van Riel <[email protected]>
Signed-off-by: Vincent Guittot <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Tested-by: Sachin Sant <[email protected]>
Acked-by: Odin Ugedal <[email protected]>
Link: https://lore.kernel.org/r/[email protected]