sched: Remove double_rq_lock() from __migrate_task()

Avoid double_rq_lock() and use TASK_ON_RQ_MIGRATING for
__migrate_task(). The advantage is (obviously) not holding two
rq->lock's at the same time and thereby increasing parallelism.

The important point to note is that because we acquire dst->lock
immediately after releasing src->lock the potential wait time of
task_rq_lock() callers on TASK_ON_RQ_MIGRATING is not longer
than it would have been in the double rq lock scenario.

Signed-off-by: Kirill Tkhai <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Kirill Tkhai <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Nicolas Pitre <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: http://lkml.kernel.org/r/1408528070.23412.89.camel@tkhai
Signed-off-by: Ingo Molnar <[email protected]>

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