sched/fair: Search a task from the tail of the queue

As a first step this patch makes cfs_tasks list as MRU one.
It means, that when a next task is picked to run on physical
CPU it is moved to the front of the list.

Therefore, the cfs_tasks list is more or less sorted (except
woken tasks) starting from recently given CPU time tasks toward
tasks with max wait time in a run-queue, i.e. MRU list.

Second, as part of the load balance operation, this approach
starts detach_tasks()/detach_one_task() from the tail of the
queue instead of the head, giving some advantages:

 - tends to pick a task with highest wait time;

 - tasks located in the tail are less likely cache-hot,
   therefore the can_migrate_task() decision is higher.

hackbench illustrates slightly better performance. For example
doing 1000 samples and 40 groups on i5-3320M CPU, it shows below
figures:

 default: 0.657 avg
 patched: 0.646 avg

Signed-off-by: Uladzislau Rezki (Sony) <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Kirill Tkhai <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Nicolas Pitre <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Oleksiy Avramchenko <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Tim Chen <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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