[PATCH 00/14] sched: balance callbacks

[Peter Zijlstra <peterz@infradead.org>]

Mike stumbled over a cute bug where the RT/DL balancing ops caused a bug.

The exact scenario is __sched_setscheduler() changing a (runnable) task from
FIFO to OTHER. In swiched_from_rt(), where we do pull_rt_task() we temporarity
drop rq->lock. This gap allows regular cfs load-balancing to step in and
migrate our task.

However, check_class_changed() will happily continue with switched_to_fair()
which assumes our task is still on the old rq and makes the kernel go boom.

Instead of trying to patch this up and make things complicated; simply disallow
these methods to drop rq->lock and extend the current post_schedule stuff into
a balancing callback list, and use that.

This survives Mike's testcase.

Changes since -v2:
 - reworked the hrtimer patch. -- Kirill, tglx
 - added lock pinning

Changes since -v1:
 - make SMP=n build,
 - cured switched_from_dl()'s cancel_dl_timer().

no real tests on the new parts other than booting / building kernels.