linux-kernel - [PATCH] cpu: memoise number of possible cpus

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Date: Thu, 18 Apr 2024 07:19:27 +0300
From: Alexey Dobriyan <adobriyan@...il.com>
To: tglx@...utronix.de,
	peterz@...radead.org
Cc: adobriyan@...il.com,
	yury.norov@...il.com,
	linux@...musvillemoes.dk,
	akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] cpu: memoise number of possible cpus

cpu_possible_mask is fixed after boot, so it makes sense
to calculate number of possible cpus to
a) make num_possible_cpus() faster (distros ship with _large_ NR_CPUS),
b) unscrew codegen elsewhere replacing function call
   with simple memory load.

Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
---
 include/linux/cpumask.h | 3 ++-
 init/main.c             | 3 +++
 kernel/cpu.c            | 3 +++
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 04536a29f10f..a98843ca6131 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -1106,7 +1106,8 @@ static __always_inline unsigned int num_online_cpus(void)
 {
 	return raw_atomic_read(&__num_online_cpus);
 }
-#define num_possible_cpus()	cpumask_weight(cpu_possible_mask)
+extern unsigned int num_possible_cpus;
+#define num_possible_cpus()	num_possible_cpus
 #define num_present_cpus()	cpumask_weight(cpu_present_mask)
 #define num_active_cpus()	cpumask_weight(cpu_active_mask)
 
diff --git a/init/main.c b/init/main.c
index 881f6230ee59..fe0291b44d78 100644
--- a/init/main.c
+++ b/init/main.c
@@ -904,6 +904,9 @@ void start_kernel(void)
 	setup_boot_config();
 	setup_command_line(command_line);
 	setup_nr_cpu_ids();
+#if NR_CPUS > 1
+	num_possible_cpus = cpumask_weight(cpu_possible_mask);
+#endif
 	setup_per_cpu_areas();
 	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */
 	boot_cpu_hotplug_init();
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 07ad53b7f119..4a75f95fec82 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3106,6 +3106,9 @@ EXPORT_SYMBOL_GPL(cpu_bit_bitmap);
 const DECLARE_BITMAP(cpu_all_bits, NR_CPUS) = CPU_BITS_ALL;
 EXPORT_SYMBOL(cpu_all_bits);
 
+unsigned int num_possible_cpus __ro_after_init = 1;
+EXPORT_SYMBOL(num_possible_cpus);
+
 #ifdef CONFIG_INIT_ALL_POSSIBLE
 struct cpumask __cpu_possible_mask __ro_after_init
 	= {CPU_BITS_ALL};
-- 
2.43.2