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Date:   Tue, 16 Jul 2019 11:20:51 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Linus Torvalds <torvalds@linuxfoundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linuxfoundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sebastian Siewior <bigeasy@linutronix.de>,
        Christoph Hellwig <hch@lst.de>, Tejun Heo <tj@kernel.org>,
        Lukas Bulwahn <lukas.bulwahn@gmail.com>,
        Daniel Wagner <wagi@monom.org>,
        Tom Zanussi <tom.zanussi@linux.intel.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Clark Williams <clark.williams@gmail.com>,
        Julia Cartwright <julia@ni.com>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Frederic Weisbecker <frederic@kernel.org>
Subject: Re: [patch 1/1] Kconfig: Introduce CONFIG_PREEMPT_RT
Reply-To: paulmck@linux.ibm.com
References: <20190715150402.798499167@linutronix.de>
 <20190715150601.205143057@linutronix.de>
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On Mon, Jul 15, 2019 at 05:04:03PM +0200, Thomas Gleixner wrote:
> 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>

This would make it easier to get some of the remaining RCU patches
from -rt to mainline, for example, the rcutorture changes proposed
recently.

Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>

> ---
>  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
> 
>