linux-kernel - [patch 1/1] Kconfig: Introduce CONFIG_PREEMPT

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Date:   Mon, 15 Jul 2019 17:04:03 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...uxfoundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...uxfoundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Christoph Hellwig <hch@....de>, Tejun Heo <tj@...nel.org>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Daniel Wagner <wagi@...om.org>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Clark Williams <clark.williams@...il.com>,
        Julia Cartwright <julia@...com>,
        Marc Zyngier <marc.zyngier@....com>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: [patch 1/1] 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@...utronix.de>
---
 arch/Kconfig           |    3 +++
 kernel/Kconfig.preempt |   25 +++++++++++++++++++++++--
 2 files changed, 26 insertions(+), 2 deletions(-)

--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -809,6 +809,9 @@ config ARCH_NO_COHERENT_DMA_MMAP
 config ARCH_NO_PREEMPT
 	bool
 
+config ARCH_SUPPORTS_RT
+	bool
+
 config CPU_NO_EFFICIENT_FFS
 	def_bool n
 
--- 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-preemtible sections. This makes the kernel, except for very
+	  low level and critical code pathes (entry code, scheduler, low
+	  level interrupt handling) fully preemtible 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