sched: Add on_rq states and remove several double rq locks

This series aims to get rid of some places where locks of two RQs are held at the same time.

Patch [1/5] is a preparation/cleanup. It replaces old (.on_rq == 1) with new (.on_rq == ONRQ_QUEUED) everywhere. No functional changes.

Patch [2/5] is main in the series. It introduces new state: ONRQ_MIGRATING and teaches scheduler to understand it (we need a little changes predominantly in try_to_wake_up()). This will be used in the following way:

	(we are changing task's rq)

        raw_spin_lock(&src_rq->lock);
        dequeue_task(src_rq, p, 0);
        p->on_rq = ONRQ_MIGRATING;
        set_task_cpu(p, dst_cpu);
        raw_spin_unlock(&src_rq->lock);
    
        raw_spin_lock(&dst_rq->lock);
        p->on_rq = ONRQ_QUEUED;
        enqueue_task(dst_rq, p, 0);
        raw_spin_unlock(&dst_rq->lock);

Patches [3-5/5] remove double locks and use new ONRQ_MIGRATING state. They allow unlocked using of 3-4 function, which looks safe for me.

The series doesn't add any overhead, and it shouldn't worsen performance, I think. It improves granularity, and it's possible to imagine situations, which will be happier without double rq lock.

I tested the reliability for 2-3 week (in sum, with breaks) on my work laptop, and bugs did not happen. Looks like, it's ready for people's review.

Commentaries are welcome. Thanks!

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Kirill Tkhai (5):
sched: Wrapper for checking task_struct's .on_rq
sched: Teach scheduler to understand ONRQ_MIGRATING state
sched: Remove double_rq_lock() from __migrate_task() sched/fair: Remove double_lock_balance() from active_load_balance_cpu_stop() sched/fair: Remove double_lock_balance() from load_balance()

 kernel/sched/core.c      |  109 +++++++++++++++++++-------------
 kernel/sched/deadline.c  |   14 ++--
 kernel/sched/fair.c      |  156 +++++++++++++++++++++++++++++-----------------
 kernel/sched/rt.c        |   16 ++---
 kernel/sched/sched.h     |    8 ++
 kernel/sched/stop_task.c |    2 -
 6 files changed, 187 insertions(+), 118 deletions(-)

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