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

arch/Kconfig

@@ -796,6 +796,9 @@ config ARCH_NO_COHERENT_DMA_MMAP
 config ARCH_NO_PREEMPT
 	bool
 
+config ARCH_SUPPORTS_RT
+	bool
+
 config CPU_NO_EFFICIENT_FFS
 	def_bool n
 

kernel/Kconfig.preempt

@@ -35,10 +35,10 @@ config PREEMPT_VOLUNTARY
 
 	  Select this if you are building a kernel for a desktop system.
 
-config PREEMPT
+config PREEMPT_LL
 	bool "Preemptible Kernel (Low-Latency Desktop)"
 	depends on !ARCH_NO_PREEMPT
-	select PREEMPT_COUNT
+	select PREEMPT
 	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
 	help
 	  This option reduces the latency of the kernel by making
@@ -55,7 +55,28 @@ config PREEMPT
 	  embedded system with latency requirements in the milliseconds
 	  range.
 
+config PREEMPT_RT
+	bool "Fully Preemptible Kernel (Real-Time)"
+	depends on EXPERT && ARCH_SUPPORTS_RT
+	select PREEMPT
+	help
+	  This option turns the kernel into a real-time kernel by replacing
+	  various locking primitives (spinlocks, rwlocks, etc.) with
+	  preemptible priority-inheritance aware variants, enforcing
+	  interrupt threading and introducing mechanisms to break up long
+	  non-preemptible sections. This makes the kernel, except for very
+	  low level and critical code pathes (entry code, scheduler, low
+	  level interrupt handling) fully preemptible and brings most
+	  execution contexts under scheduler control.
+
+	  Select this if you are building a kernel for systems which
+	  require real-time guarantees.
+
 endchoice
 
 config PREEMPT_COUNT
        bool
+
+config PREEMPT
+       bool
+       select PREEMPT_COUNT