Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull CONFIG_PREEMPT_RT stub config from Thomas Gleixner:
"The real-time preemption patch set exists for almost 15 years now and
while the vast majority of infrastructure and enhancements have found
their way into the mainline kernel, the final integration of RT is
still missing.

Over the course of the last few years, we have worked on reducing the
intrusivenness of the RT patches by refactoring kernel infrastructure
to be more real-time friendly. Almost all of these changes were
benefitial to the mainline kernel on their own, so there was no
objection to integrate them.

Though except for the still ongoing printk refactoring, the remaining
changes which are required to make RT a first class mainline citizen
are not longer arguable as immediately beneficial for the mainline
kernel. Most of them are either reordering code flows or adding RT
specific functionality.

But this now has hit a wall and turned into a classic hen and egg
problem:

Maintainers are rightfully wary vs. these changes as they make only
sense if the final integration of RT into the mainline kernel takes
place.

Adding CONFIG_PREEMPT_RT aims to solve this as a clear sign that RT
will be fully integrated into the mainline kernel. The final
integration of the missing bits and pieces will be of course done with
the same careful approach as we have used in the past.

While I'm aware that you are not entirely enthusiastic about that, I
think that RT should receive the same treatment as any other widely
used out of tree functionality, which we have accepted into mainline
over the years.

RT has become the de-facto standard real-time enhancement and is
shipped by enterprise, embedded and community distros. It's in use
throughout a wide range of industries: telecommunications, industrial
automation, professional audio, medical devices, data acquisition,
automotive - just to name a few major use cases.

RT development is backed by a Linuxfoundation project which is
supported by major stakeholders of this technology. The funding will
continue over the actual inclusion into mainline to make sure that the
functionality is neither introducing regressions, regressing itself,
nor becomes subject to bitrot. There is also a lifely user community
around RT as well, so contrary to the grim situation 5 years ago, it's
a healthy project.

As RT is still a good vehicle to exercise rarely used code paths and
to detect hard to trigger issues, you could at least view it as a QA
tool if nothing else"

* 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
sched/rt, Kconfig: Introduce CONFIG_PREEMPT_RT