linux-kernel - [PATCH v2] cpumask: limit visibility of FORCE_NR

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Date:   Wed, 19 Oct 2022 15:59:39 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     linux-kernel@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Sander Vanheule <sander@...nheule.net>,
        Alexey Klimov <klimov.linux@...il.com>,
        Eric Biggers <ebiggers@...gle.com>
Cc:     Yury Norov <yury.norov@...il.com>
Subject: [PATCH v2] cpumask: limit visibility of FORCE_NR_CPUS

In current form, FORCE_NR_CPUS is visible to all users building their
kernels, even not experts. It is also set in allmodconfig or allyesconfig,
which is not a correct behavior.

The 'choice' and unused config UNFORCE_NR_CPUS are used to ensure that
auto-generated configs that try to enable as much options as possible,
like allmodconfig, don't enable FORCE_NR_CPUS.

Suggested-by: Geert Uytterhoeven <geert@...ux-m68k.org>
Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Yury Norov <yury.norov@...il.com>
---
v2: extend commit message with an explanation for what we need 'choice'.

 lib/Kconfig | 31 ++++++++++++++++++++++++-------
 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/lib/Kconfig b/lib/Kconfig
index 9bbf8a4b2108..1ada12f5dda6 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -528,14 +528,31 @@ config CPUMASK_OFFSTACK
 	  them on the stack.  This is a bit more expensive, but avoids
 	  stack overflow.
 
+choice
+	prompt "Number of CPUs detection method"
+	default UNFORCE_NR_CPUS
+	depends on SMP && EXPERT
+	help
+	  Select between boot-time and compile-time detection of number
+	  of CPUs. If it's possible to provide exact number of CPUs at
+	  compile-time, kernel code may be optimized better.
+	  For general-purpose kernel, choose "boot time" option.
+
+config UNFORCE_NR_CPUS
+	bool "Set number of CPUs at boot time"
+	help
+	  Choose it if you build general-purpose kernel and want to rely
+	  on kernel to detect actual number of CPUs.
+
 config FORCE_NR_CPUS
-       bool "NR_CPUS is set to an actual number of CPUs"
-       depends on SMP
-       help
-         Say Yes if you have NR_CPUS set to an actual number of possible
-         CPUs in your system, not to a default value. This forces the core
-         code to rely on compile-time value and optimize kernel routines
-         better.
+	bool "Set number of CPUs at compile time"
+	help
+	  Choose it if NR_CPUS corresponds to an actual number of
+	  possible CPUs in your system. This forces the core code
+	  to rely on compile-time value and optimize kernel routines
+	  better.
+
+endchoice
 
 config CPU_RMAP
 	bool
-- 
2.34.1