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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 08, 2021 at 02:04:41PM +0200, Frederic Weisbecker wrote:
> Currently the default behaviour for CONFIG_PREEMPT_DYNAMIC is
> preempt=full. So distros always have to override that with the boot
> option if it's not their default choice.
> 
> Make things more convenient for them with providing that choice at
> Kconfig time.
> 
> This should also encourage automatic testing robots relying on randconfig
> to run through all the various preempt dynamic flavours.
> 
> (Unfortunately this involved copy-pasting help text for static PREEMPT
> Kconfig entries. Perhaps referring to them would be enough?)
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
>  kernel/Kconfig.preempt | 53 ++++++++++++++++++++++++++++++++++++++++++
>  kernel/sched/core.c    | 23 +++++++++++++++++-
>  2 files changed, 75 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> index bd7c4147b9a8..384110d1a215 100644
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -100,6 +100,59 @@ config PREEMPT_DYNAMIC
>  	  Interesting if you want the same pre-built kernel should be used for
>  	  both Server and Desktop workloads.
>  
> +choice
> +	prompt "Preemption dynamic default boot mode"
> +	default PREEMPT_DYNAMIC_FULL
> +	depends on PREEMPT_DYNAMIC
> +
> +config PREEMPT_DYNAMIC_NONE
> +	bool "Default boot with no Forced Preemption (Server)"
> +	help
> +	  This is the traditional Linux preemption model, geared towards
> +	  throughput. It will still provide good latencies most of the
> +	  time, but there are no guarantees and occasional longer delays
> +	  are possible.
> +
> +	  Select this option if you are building a kernel for a server or
> +	  scientific/computation system, or if you want to maximize the
> +	  raw processing power of the kernel, irrespective of scheduling
> +	  latencies.
> +
> +config PREEMPT_DYNAMIC_VOLUNTARY
> +	bool "Default boot with Voluntary Kernel Preemption (Desktop)"
> +	help
> +	  This option reduces the latency of the kernel by adding more
> +	  "explicit preemption points" to the kernel code. These new
> +	  preemption points have been selected to reduce the maximum
> +	  latency of rescheduling, providing faster application reactions,
> +	  at the cost of slightly lower throughput.
> +
> +	  This allows reaction to interactive events by allowing a
> +	  low priority process to voluntarily preempt itself even if it
> +	  is in kernel mode executing a system call. This allows
> +	  applications to run more 'smoothly' even when the system is
> +	  under load.
> +
> +	  Select this if you are building a kernel for a desktop system.
> +
> +config PREEMPT_DYNAMIC_FULL
> +	bool "Default boot with Preemptible Kernel (Low-Latency Desktop)"
> +	help
> +	  This option reduces the latency of the kernel by making
> +	  all kernel code (that is not executing in a critical section)
> +	  preemptible.  This allows reaction to interactive events by
> +	  permitting a low priority process to be preempted involuntarily
> +	  even if it is in kernel mode executing a system call and would
> +	  otherwise not be about to reach a natural preemption point.
> +	  This allows applications to run more 'smoothly' even when the
> +	  system is under load, at the cost of slightly lower throughput
> +	  and a slight runtime overhead to kernel code.
> +
> +	  Select this if you are building a kernel for a desktop or
> +	  embedded system with latency requirements in the milliseconds
> +	  range.
> +endchoice
> +
>  config SCHED_CORE
>  	bool "Core Scheduling for SMT"
>  	default y

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_NONE
+	default PREEMPT_TYPE_NONE
 
-config PREEMPT_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_VOLUNTARY
+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 PREEMPTION
-	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
-	select PREEMPT_DYNAMIC if HAVE_PREEMPT_DYNAMIC
+	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
 
-config PREEMPT_COUNT
-       bool
+# default model for PREEMPT_DYNAMIC
 
-config PREEMPTION
-       bool
-       select PREEMPT_COUNT
+config PREEMPT_DYNAMIC_NONE
+	bool
 
-config PREEMPT_DYNAMIC
+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
 	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