sched/rt, Kconfig: Introduce CONFIG_PREEMPT_RT

Add a new entry to the preemption menu which enables the real-time support
for the kernel. The choice is only enabled when an architecture supports
it.

It selects PREEMPT as the RT features depend on it. To achieve that the
existing PREEMPT choice is renamed to PREEMPT_LL which select PREEMPT as
well.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Clark Williams <williams@redhat.com>
Acked-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Daniel Wagner <wagi@monom.org>
Acked-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Acked-by: Julia Cartwright <julia@ni.com>
Acked-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Acked-by: Gratian Crisan <gratian.crisan@ni.com>
Acked-by: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Andrew Morton <akpm@linuxfoundation.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/alpine.DEB.2.21.1907172200190.1778@nanos.tec.linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 26 insertions, 2 deletions

diff --git a/arch/Kconfig b/arch/Kconfig
index c47b328eada0..ada51f36bd5d 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -801,6 +801,9 @@ config ARCH_NO_COHERENT_DMA_MMAP
 config ARCH_NO_PREEMPT
         bool
 
+config ARCH_SUPPORTS_RT
+        bool
+
 config CPU_NO_EFFICIENT_FFS
         def_bool n
 
diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index dc0b682ec2d9..fc020c09b7e8 100644
--- a/kernel/Kconfig.preempt
+++ b/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