sched: Add support for lazy preemption

It has become an obsession to mitigate the determinism vs. throughput
loss of RT. Looking at the mainline semantics of preemption points
gives a hint why RT sucks throughput wise for ordinary SCHED_OTHER
tasks. One major issue is the wakeup of tasks which are right away
preempting the waking task while the waking task holds a lock on which
the woken task will block right after having preempted the wakee. In
mainline this is prevented due to the implicit preemption disable of
spin/rw_lock held regions. On RT this is not possible due to the fully
preemptible nature of sleeping spinlocks.

Though for a SCHED_OTHER task preempting another SCHED_OTHER task this
is really not a correctness issue. RT folks are concerned about
SCHED_FIFO/RR tasks preemption and not about the purely fairness
driven SCHED_OTHER preemption latencies.

So I introduced a lazy preemption mechanism which only applies to
SCHED_OTHER tasks preempting another SCHED_OTHER task. Aside of the
existing preempt_count each tasks sports now a preempt_lazy_count
which is manipulated on lock acquiry and release. This is slightly
incorrect as for lazyness reasons I coupled this on
migrate_disable/enable so some other mechanisms get the same treatment
(e.g. get_cpu_light).

Now on the scheduler side instead of setting NEED_RESCHED this sets
NEED_RESCHED_LAZY in case of a SCHED_OTHER/SCHED_OTHER preemption and
therefor allows to exit the waking task the lock held region before
the woken task preempts. That also works better for cross CPU wakeups
as the other side can stay in the adaptive spinning loop.

For RT class preemption there is no change. This simply sets
NEED_RESCHED and forgoes the lazy preemption counter.

 Initial test do not expose any observable latency increasement, but
history shows that I've been proven wrong before :)

The lazy preemption mode is per default on, but with
CONFIG_SCHED_DEBUG enabled it can be disabled via:

 # echo NO_PREEMPT_LAZY >/sys/kernel/debug/sched_features

and reenabled via

 # echo PREEMPT_LAZY >/sys/kernel/debug/sched_features

The test results so far are very machine and workload dependent, but
there is a clear trend that it enhances the non RT workload
performance.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

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