sched: Remove double_rq_lock() from __migrate_task()

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 <ktkhai@parallels.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul Turner <pjt@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1408528070.23412.89.camel@tkhai
Signed-off-by: Ingo Molnar <mingo@kernel.org>