Date:   Tue, 09 Nov 2021 12:00:39 +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 Tue, 2021-11-09 at 09:52 +0000, Valentin Schneider wrote:
> On 09/11/21 06:30, Mike Galbraith wrote:
> > 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.
> >
>
> Yeah so I went down a debatable path, gave up on that and started something
> different, and gave up on that because it was late :-)
>
> Now interestingly my second attempt is pretty close to what you have
> below.

Well that's a shame, because while it seems to function, it also puts
PREEMPT_DYNAMIC in a pretty darn similar spot to the one PREEMPT_RT was
in.  Drat.

> > 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.
> >
>
> Have some nits below, but otherwise where I stand right now I think it's
> the least ugly way of tackling this :)

We're supposed to be going for _least_ ugly?  Oh ;)  This is really a
job for someone who knows their way around Kconfig-land, but since I'm
not hearing "Yawn, here ya go", it gets whacked with a mallet until one
of us gives up.

I'm looking at two straight up choice sets, one for those who have and
want PREEMPT_DYNAMIC, and another for the rest of us.  Unfortunately,
there's no "else", so I end up with this mess, which would likely be
better served by "source foo/bar".

---
 kernel/Kconfig.preempt |   90 +++++++++++++++++++++++++++++--------------------
 1 file changed, 54 insertions(+), 36 deletions(-)

--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -2,11 +2,41 @@

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

-config PREEMPT_NONE_BEHAVIOUR
+config PREEMPT_STATIC
+	bool "Preemption behaviour defined at build"
+
+config PREEMPT_DYNAMIC
+	bool "Preemption behaviour defined on boot"
+	depends on HAVE_PREEMPT_DYNAMIC && !ARCH_NO_PREEMPT
+	select PREEMPT
+	select PREEMPTION
+	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
+	help
+	  This option allows to define the preemption model on the kernel
+	  command line parameter and thus override the default preemption
+	  model defined during compile time.
+
+	  The feature is primarily interesting for Linux distributions which
+	  provide a pre-built kernel binary to reduce the number of kernel
+	  flavors they offer while still offering different usecases.
+
+	  The runtime overhead is negligible with HAVE_STATIC_CALL_INLINE enabled
+	  but if runtime patching is not available for the specific architecture
+	  then the potential overhead should be considered.
+
+	  Interesting if you want the same pre-built kernel should be used for
+	  both Server and Desktop workloads.
+endchoice
+
+if PREEMPT_STATIC
+choice
+	prompt "Preemption Flavor"
+	default PREEMPT_NONE
+
+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 +48,9 @@ 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 +66,11 @@ 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 +88,7 @@ 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
@@ -74,17 +104,26 @@ config PREEMPT_RT
 	  require real-time guarantees.

 endchoice
+endif # PREEMPT_STATIC

-config PREEMPT_NONE
+if PREEMPT_DYNAMIC
+config PREEMPT
 	bool

-config PREEMPT_VOLUNTARY
-	bool
+choice
+	prompt "Boot Time Preemption Flavor"
+	default PREEMPT_NONE_BEHAVIOR

-config PREEMPT
-	bool
-	select PREEMPTION
-	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
+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 PREEMPT_COUNT
        bool
@@ -93,27 +132,6 @@ config PREEMPTION
        bool
        select PREEMPT_COUNT

-config PREEMPT_DYNAMIC
-	bool "Preemption behaviour defined on boot"
-	depends on HAVE_PREEMPT_DYNAMIC
-	select PREEMPT
-	default y
-	help
-	  This option allows to define the preemption model on the kernel
-	  command line parameter and thus override the default preemption
-	  model defined during compile time.
-
-	  The feature is primarily interesting for Linux distributions which
-	  provide a pre-built kernel binary to reduce the number of kernel
-	  flavors they offer while still offering different usecases.
-
-	  The runtime overhead is negligible with HAVE_STATIC_CALL_INLINE enabled
-	  but if runtime patching is not available for the specific architecture
-	  then the potential overhead should be considered.
-
-	  Interesting if you want the same pre-built kernel should be used for
-	  both Server and Desktop workloads.
-
 config SCHED_CORE
 	bool "Core Scheduling for SMT"
 	depends on SCHED_SMT

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