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 (task_struct::on_rq == 1) with new (task_struct::on_rq == ONRQ_QUEUED) everywhere. No functional changes.

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

        (we are changing task's rq)

        raw_spin_lock(&src_rq->lock);

	p = ...; /* Some src_rq task */

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

	/*
	 * Now p os dequeued, and both
         * RQ locks are unlocked, but
	 * its on_rq is not zero.
	 * Nobody can manipulate p
	 * while it's migrating,
	 * even when spinlocks are
	 * unlocked.
	 */

        raw_spin_lock(&dst_rq->lock);
        p->on_rq = ONRQ_QUEUED;
        enqueue_task(dst_rq, p, 0);
        raw_spin_unlock(&dst_rq->lock);

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

The profit is double_rq_lock() is not need now, so we reduce the total time when RQs are locked.

---

Kirill Tkhai (5):
sched: Wrapper for checking task_struct::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      |  115 +++++++++++++++++--------------
 kernel/sched/deadline.c  |   14 ++--
 kernel/sched/fair.c      |  172 +++++++++++++++++++++++++++++++---------------
 kernel/sched/rt.c        |   16 ++--
 kernel/sched/sched.h     |   13 +++
 kernel/sched/stop_task.c |    2 -
 6 files changed, 211 insertions(+), 121 deletions(-)

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