linux-kernel - [patch V2 09/20] cpumask: Cache num_possible

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Message-ID: <20251022110555.967170782@linutronix.de>
Date: Wed, 22 Oct 2025 14:55:30 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
 Gabriele Monaco <gmonaco@...hat.com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Michael Jeanson <mjeanson@...icios.com>,
 Jens Axboe <axboe@...nel.dk>,
 "Paul E. McKenney" <paulmck@...nel.org>,
 "Gautham R. Shenoy" <gautham.shenoy@....com>,
 Florian Weimer <fweimer@...hat.com>,
 Tim Chen <tim.c.chen@...el.com>,
 Yury Norov <yury.norov@...il.com>
Subject: [patch V2 09/20] cpumask: Cache num_possible_cpus()

Reevaluating num_possible_cpus() over and over does not make sense. That
becomes a constant after init as cpu_possible_mask is marked ro_after_init.

Cache the value during initialization and provide that for consumption.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Yury Norov <yury.norov@...il.com>
---
V2: New patch
---
 include/linux/cpumask.h |   10 ++++++++--
 kernel/cpu.c            |   15 +++++++++++++++
 2 files changed, 23 insertions(+), 2 deletions(-)

--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -126,6 +126,7 @@ extern struct cpumask __cpu_dying_mask;
 #define cpu_dying_mask    ((const struct cpumask *)&__cpu_dying_mask)
 
 extern atomic_t __num_online_cpus;
+extern unsigned int __num_possible_cpus;
 
 extern cpumask_t cpus_booted_once_mask;
 
@@ -1152,13 +1153,13 @@ void init_cpu_possible(const struct cpum
 #define __assign_cpu(cpu, mask, val)	\
 	__assign_bit(cpumask_check(cpu), cpumask_bits(mask), (val))
 
-#define set_cpu_possible(cpu, possible)	assign_cpu((cpu), &__cpu_possible_mask, (possible))
 #define set_cpu_enabled(cpu, enabled)	assign_cpu((cpu), &__cpu_enabled_mask, (enabled))
 #define set_cpu_present(cpu, present)	assign_cpu((cpu), &__cpu_present_mask, (present))
 #define set_cpu_active(cpu, active)	assign_cpu((cpu), &__cpu_active_mask, (active))
 #define set_cpu_dying(cpu, dying)	assign_cpu((cpu), &__cpu_dying_mask, (dying))
 
 void set_cpu_online(unsigned int cpu, bool online);
+void set_cpu_possible(unsigned int cpu, bool possible);
 
 /**
  * to_cpumask - convert a NR_CPUS bitmap to a struct cpumask *
@@ -1211,7 +1212,12 @@ static __always_inline unsigned int num_
 {
 	return raw_atomic_read(&__num_online_cpus);
 }
-#define num_possible_cpus()	cpumask_weight(cpu_possible_mask)
+
+static __always_inline unsigned int num_possible_cpus(void)
+{
+	return __num_possible_cpus;
+}
+
 #define num_enabled_cpus()	cpumask_weight(cpu_enabled_mask)
 #define num_present_cpus()	cpumask_weight(cpu_present_mask)
 #define num_active_cpus()	cpumask_weight(cpu_active_mask)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3108,6 +3108,9 @@ EXPORT_SYMBOL(__cpu_dying_mask);
 atomic_t __num_online_cpus __read_mostly;
 EXPORT_SYMBOL(__num_online_cpus);
 
+unsigned int __num_possible_cpus __ro_after_init = NR_CPUS;
+EXPORT_SYMBOL(__num_possible_cpus);
+
 void init_cpu_present(const struct cpumask *src)
 {
 	cpumask_copy(&__cpu_present_mask, src);
@@ -3116,6 +3119,7 @@ void init_cpu_present(const struct cpuma
 void init_cpu_possible(const struct cpumask *src)
 {
 	cpumask_copy(&__cpu_possible_mask, src);
+	__num_possible_cpus = cpumask_weight(&__cpu_possible_mask);
 }
 
 void set_cpu_online(unsigned int cpu, bool online)
@@ -3139,6 +3143,17 @@ void set_cpu_online(unsigned int cpu, bo
 	}
 }
 
+void set_cpu_possible(unsigned int cpu, bool possible)
+{
+	if (possible) {
+		if (!cpumask_test_and_set_cpu(cpu, &__cpu_possible_mask))
+			__num_possible_cpus++;
+	} else {
+		if (cpumask_test_and_clear_cpu(cpu, &__cpu_possible_mask))
+			__num_possible_cpus--;
+	}
+}
+
 /*
  * Activate the first processor.
  */