linux-kernel - Re: [PATCH] sched: Tweak default dynamic preempt mode selection

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Date:   Tue, 09 Nov 2021 06:30:47 +0100
From:   Mike Galbraith <efault@....de>
To:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH] sched: Tweak default dynamic preempt mode selection

On Mon, 2021-11-08 at 15:21 +0000, Valentin Schneider wrote:
> On 08/11/21 13:27, Mike Galbraith wrote:
> >
> > As long as RT depends on EXPERT it'll be a bit annoying regardless.  I
> > just thought it worth mention that what you want now and what RT will
> > presumably want upon merge completion appear to be mutually exclusive.
> >
>
> Hmm actually I think your approach should work, i.e. have
>
>   config PREEMPT_DYNAMIC
>     depends on [...] && !PREEMPT_RT
>
> rather than
>
>   config PREEMPT_RT
>     depends on [...] && !PREEMPT_DYNAMIC
>
> This essentially gives priority to the preemption model type over the
> preemption model dynamicness, which I think makes sense. I can fold that in
> v2.

Not seeing your v2 land yet, I grabbed my mallet and had a go at goal
reconciliation over morning java.  Non-lovely result seems to work.

sched, Kconfig: Fix preemption model selection

Switch PREEMPT_DYNAMIC/PREEMPT_RT dependency around so PREEMPT_RT
can be selected during the initial preemption model selection.
Further, since PREEMPT_DYNAMIC requires PREEMPT, make it depend
upon it instead of selecting it, and add a menu to allow selection
of the boot time behavior, this to allow arches that do not support
PREEMPT_DYNAMIC to retain their various configs untouched.

Signed-off-by: Mike Galbraith <efault@....de>
---
 kernel/Kconfig.preempt |   46 +++++++++++++++++++++++++---------------
------
 1 file changed, 25 insertions(+), 21 deletions(-)

--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -2,11 +2,10 @@

 choice
 	prompt "Preemption Model"
-	default PREEMPT_NONE_BEHAVIOUR
+	default PREEMPT_NONE

-config PREEMPT_NONE_BEHAVIOUR
+config PREEMPT_NONE
 	bool "No Forced Preemption (Server)"
-	select PREEMPT_NONE if !PREEMPT_DYNAMIC
 	help
 	  This is the traditional Linux preemption model, geared
towards
 	  throughput. It will still provide good latencies most of the
@@ -18,10 +17,9 @@ config PREEMPT_NONE_BEHAVIOUR
 	  raw processing power of the kernel, irrespective of
scheduling
 	  latencies.

-config PREEMPT_VOLUNTARY_BEHAVIOUR
+config PREEMPT_VOLUNTARY
 	bool "Voluntary Kernel Preemption (Desktop)"
 	depends on !ARCH_NO_PREEMPT
-	select PREEMPT_VOLUNTARY if !PREEMPT_DYNAMIC
 	help
 	  This option reduces the latency of the kernel by adding more
 	  "explicit preemption points" to the kernel code. These new
@@ -37,10 +35,11 @@ config PREEMPT_VOLUNTARY_BEHAVIOUR

 	  Select this if you are building a kernel for a desktop
system.

-config PREEMPT_BEHAVIOUR
+config PREEMPT
 	bool "Preemptible Kernel (Low-Latency Desktop)"
 	depends on !ARCH_NO_PREEMPT
-	select PREEMPT
+	select PREEMPTION
+	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
 	help
 	  This option reduces the latency of the kernel by making
 	  all kernel code (that is not executing in a critical
section)
@@ -58,7 +57,7 @@ config PREEMPT_BEHAVIOUR

 config PREEMPT_RT
 	bool "Fully Preemptible Kernel (Real-Time)"
-	depends on EXPERT && ARCH_SUPPORTS_RT && !PREEMPT_DYNAMIC
+	depends on EXPERT && ARCH_SUPPORTS_RT
 	select PREEMPTION
 	help
 	  This option turns the kernel into a real-time kernel by
replacing
@@ -75,17 +74,6 @@ config PREEMPT_RT

 endchoice

-config PREEMPT_NONE
-	bool
-
-config PREEMPT_VOLUNTARY
-	bool
-
-config PREEMPT
-	bool
-	select PREEMPTION
-	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
-
 config PREEMPT_COUNT
        bool

@@ -95,8 +83,7 @@ config PREEMPTION

 config PREEMPT_DYNAMIC
 	bool "Preemption behaviour defined on boot"
-	depends on HAVE_PREEMPT_DYNAMIC
-	select PREEMPT
+	depends on HAVE_PREEMPT_DYNAMIC && PREEMPT
 	default y
 	help
 	  This option allows to define the preemption model on the
kernel
@@ -114,6 +101,23 @@ config PREEMPT_DYNAMIC
 	  Interesting if you want the same pre-built kernel should be
used for
 	  both Server and Desktop workloads.

+if PREEMPT_DYNAMIC
+choice
+	prompt "Boot Time Preemption Model"
+	default PREEMPT_NONE_BEHAVIOR
+
+config PREEMPT_NONE_BEHAVIOR
+	bool "No Forced Preemption (Server)"
+
+config PREEMPT_VOLUNTARY_BEHAVIOR
+	bool "Voluntary Kernel Preemption (Desktop)"
+
+config PREEMPT_BEHAVIOR
+	bool "Preemptible Kernel (Low-Latency Desktop)"
+
+endchoice
+endif # PREEMPT_DYNAMIC
+
 config SCHED_CORE
 	bool "Core Scheduling for SMT"
 	depends on SCHED_SMT