sched: Add on_rq states and remove several double rq locks

v5: New names: TASK_ON_RQ_QUEUED, TASK_ON_RQ_MIGRATING, task_on_rq_migrating() and task_on_rq_queued().

I've pulled the latest version from peterz/queue.git, and Peter's changes are included.

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 == TASK_ON_RQ_QUEUED) everywhere. No functional changes.

Patch [2/5] is main in the series. It introduces new TASK_ON_RQ_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 = TASK_ON_RQ_MIGRATING;
        set_task_cpu(p, dst_cpu);
        raw_spin_unlock(&src_rq->lock);

        /*
         * Now p is 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 = TASK_ON_RQ_QUEUED;
        enqueue_task(dst_rq, p, 0);
        raw_spin_unlock(&dst_rq->lock);

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

The profit is that double_rq_lock() is not need in several places. 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 TASK_ON_RQ_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      |  113 +++++++++++++++------------
 kernel/sched/deadline.c  |   15 ++--
 kernel/sched/fair.c      |  195 ++++++++++++++++++++++++++++++++--------------
 kernel/sched/rt.c        |   16 ++--
 kernel/sched/sched.h     |   13 +++
 kernel/sched/stop_task.c |    2 
 6 files changed, 228 insertions(+), 126 deletions(-)

--

Ok, looks good. I picked up this version, with a few minor tweaks and fixes to the changelogs.

Thanks,

Ingo