From: Valentin Schneider <valentin.schneider@arm.com>
To: Mike Galbraith <efault@gmx.de>, linux-kernel@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>,
Frederic Weisbecker <frederic@kernel.org>
Subject: Re: [PATCH] sched: Tweak default dynamic preempt mode selection
Date: Tue, 09 Nov 2021 12:19:30 +0000 [thread overview]
Message-ID: <87pmr9ser1.mognet@arm.com> (raw)
In-Reply-To: <1905cf613576d04f585d752d85ce21a3504a40d6.camel@gmx.de>
On 09/11/21 12:00, Mike Galbraith wrote:
> 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.
>
Yep...
>> > 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.
>
:)
> -config PREEMPT_NONE
> +if PREEMPT_DYNAMIC
> +config PREEMPT
> bool
>
On my end this doesn't let PREEMPT_DYNAMIC select PREEMPT, which I came to
realize we need (places like vermagic.h and ftrace's print_trace_header();
also it avoids a lot of headaches).
AIUI this is because the 'if <expr>' is equivalent to appending
depends on <expr>
to every choice item.
The below lets me have PREEMPT w/ PREEMPT_DYNAMIC. The one annoying thing
is the Preemption Model prompt remains visible in menuconfig when
PREEMPT_DYNAMIC is selected, but eh...
---
diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index 12ac42a3415f..e01588f9de1f 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -1,18 +1,9 @@
# SPDX-License-Identifier: GPL-2.0-only
-choice
- prompt "Preemption Model"
- default PREEMPT_STATIC
-
-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
@@ -28,15 +19,14 @@ config PREEMPT_DYNAMIC
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"
+ prompt "Preemption Model"
default PREEMPT_NONE
config PREEMPT_NONE
bool "No Forced Preemption (Server)"
+ depends on !PREEMPT_DYNAMIC
help
This is the traditional Linux preemption model, geared towards
throughput. It will still provide good latencies most of the
@@ -50,7 +40,7 @@ config PREEMPT_NONE
config PREEMPT_VOLUNTARY
bool "Voluntary Kernel Preemption (Desktop)"
- depends on !ARCH_NO_PREEMPT
+ depends on !ARCH_NO_PREEMPT && !PREEMPT_DYNAMIC
help
This option reduces the latency of the kernel by adding more
"explicit preemption points" to the kernel code. These new
@@ -88,7 +78,7 @@ config PREEMPT
config PREEMPT_RT
bool "Fully Preemptible Kernel (Real-Time)"
- depends on EXPERT && ARCH_SUPPORTS_RT
+ depends on EXPERT && ARCH_SUPPORTS_RT && !PREEMPT_DYNAMIC
select PREEMPTION
help
This option turns the kernel into a real-time kernel by replacing
@@ -104,14 +94,10 @@ config PREEMPT_RT
require real-time guarantees.
endchoice
-endif # PREEMPT_STATIC
-
-if PREEMPT_DYNAMIC
-config PREEMPT
- bool
choice
- prompt "Boot Time Preemption Flavor"
+ prompt "Boot Time Preemption Model"
+ depends on PREEMPT_DYNAMIC
default PREEMPT_NONE_BEHAVIOR
config PREEMPT_NONE_BEHAVIOR
@@ -123,7 +109,6 @@ config PREEMPT_VOLUNTARY_BEHAVIOR
config PREEMPT_BEHAVIOR
bool "Preemptible Kernel (Low-Latency Desktop)"
endchoice
-endif # PREEMPT_DYNAMIC
config PREEMPT_COUNT
bool
@@ -149,5 +134,3 @@ config SCHED_CORE
SCHED_CORE is default disabled. When it is enabled and unused,
which is the likely usage by Linux distributions, there should
be no measurable impact on performance.
-
-
next prev parent reply other threads:[~2021-11-09 12:19 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-05 10:40 [PATCH] sched: Tweak default dynamic preempt mode selection Valentin Schneider
2021-11-06 4:40 ` Mike Galbraith
2021-11-08 11:17 ` Valentin Schneider
2021-11-08 12:27 ` Mike Galbraith
2021-11-08 15:21 ` Valentin Schneider
2021-11-09 5:30 ` Mike Galbraith
2021-11-09 9:52 ` Valentin Schneider
2021-11-09 11:00 ` Mike Galbraith
2021-11-09 12:19 ` Valentin Schneider [this message]
2021-11-09 13:31 ` Mike Galbraith
2021-11-06 5:05 ` kernel test robot
2021-11-06 5:05 ` kernel test robot
2021-11-08 12:00 ` Valentin Schneider
2021-11-08 12:00 ` Valentin Schneider
2021-11-06 5:33 ` kernel test robot
2021-11-06 5:33 ` kernel test robot
2021-11-06 7:58 ` kernel test robot
2021-11-06 7:58 ` kernel test robot
2021-11-09 10:25 ` Frederic Weisbecker
2021-11-10 0:03 ` Valentin Schneider
2021-11-10 5:44 ` Mike Galbraith
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