Re: [PATCH 1/2] sched: Add default dynamic preempt mode Kconfig

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 08, 2021 at 03:55:09PM +0200, Peter Zijlstra wrote:
> Urgh, would something like this work?
> 
> ---
>  kernel/Kconfig.preempt | 64 ++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 51 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> index bd7c4147b9a8..43c68a806e4e 100644
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -1,11 +1,25 @@
>  # SPDX-License-Identifier: GPL-2.0-only
>  
> +config PREEMPT_COUNT
> +       bool
> +
> +config PREEMPTION
> +       bool
> +       select PREEMPT_COUNT
> +
> +config PREEMPT_DYNAMIC_OPTION
> +	bool
> +	help
> +	  Symbol that gates availablility of PREEMPT_DYNAMIC, selected
> +	  by preemption models that can be dynamically selected.
> +
>  choice
>  	prompt "Preemption Model"
> +	default PREEMPT_TYPE_NONE
>  
> +config PREEMPT_TYPE_NONE
>  	bool "No Forced Preemption (Server)"
> +	select PREEMPT_DYNAMIC_OPTION if HAVE_PREEMPT_DYNAMIC
>  	help
>  	  This is the traditional Linux preemption model, geared towards
>  	  throughput. It will still provide good latencies most of the
> @@ -17,9 +31,10 @@ config PREEMPT_NONE
>  	  raw processing power of the kernel, irrespective of scheduling
>  	  latencies.
>  
> +config PREEMPT_TYPE_VOLUNTARY
>  	bool "Voluntary Kernel Preemption (Desktop)"
>  	depends on !ARCH_NO_PREEMPT
> +	select PREEMPT_DYNAMIC_OPTION if HAVE_PREEMPT_DYNAMIC
>  	help
>  	  This option reduces the latency of the kernel by adding more
>  	  "explicit preemption points" to the kernel code. These new
> @@ -35,12 +50,10 @@ config PREEMPT_VOLUNTARY
>  
>  	  Select this if you are building a kernel for a desktop system.
>  
> -config PREEMPT
> +config PREEMPT_TYPE_FULL
>  	bool "Preemptible Kernel (Low-Latency Desktop)"
>  	depends on !ARCH_NO_PREEMPT
> +	select PREEMPT_DYNAMIC_OPTION if HAVE_PREEMPT_DYNAMIC
>  	help
>  	  This option reduces the latency of the kernel by making
>  	  all kernel code (that is not executing in a critical section)
> @@ -75,15 +88,24 @@ config PREEMPT_RT
>  
>  endchoice
>  
> +# default model for PREEMPT_DYNAMIC
>  
> +config PREEMPT_DYNAMIC_NONE
> +	bool
>  
> +config PREEMPT_DYNAMIC_VOLUNTARY
> +	bool
> +
> +config PREEMPT_DYNAMIC_FULL
>  	bool
> +
> +config PREEMPT_DYNAMIC
> +	bool "Dynamic Preemption Mode"
> +	depends on PREEMPT_DYNAMIC_OPTION
> +	select PREEMPT
> +	select PREEMPT_DYNAMIC_NONE if PREEMPT_TYPE_NONE
> +	select PREEMPT_DYNAMIC_VOLUNTARY if PREEMPT_TYPE_VOLUNTARY
> +	select PREEMPT_DYNAMIC_FULL if PREEMPT_TYPE_FULL

Arguably PREEMPT_DYNAMIC_{NONE,VOLUNTARY,FULL} are superfluous and the
default selection can use PREEMPT_TYPE_*.

>  	help
>  	  This option allows to define the preemption model on the kernel
>  	  command line parameter and thus override the default preemption
> @@ -100,6 +122,22 @@ config PREEMPT_DYNAMIC
>  	  Interesting if you want the same pre-built kernel should be used for
>  	  both Server and Desktop workloads.
>  
> +# actual preemption model
> +
> +config PREEMPT_NONE
> +	bool
> +	default y if !PREEMPT_DYNAMIC && PREEMPT_TYPE_NONE
> +
> +config PREEMPT_VOLUNTARY
> +	bool
> +	default y if !PREEMPT_DYNAMIC && PREEMPT_TYPE_VOLUNTARY
> +
> +config PREEMPT
> +	bool
> +	default y if !PREEMPT_DYNAMIC && PREEMPT_TYPE_FULL
> +	select PREEMPTION
> +	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
> +
>  config SCHED_CORE
>  	bool "Core Scheduling for SMT"
>  	default y