linux-kernel - [tip: core/rseq] cpumask: Cache num_possible

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Message-ID: <176363760025.498.8053574440000863451.tip-bot2@tip-bot2>
Date: Thu, 20 Nov 2025 11:20:00 -0000
From: "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Yury Norov <yury.norov@...il.com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Shrikanth Hegde <sshegde@...ux.ibm.com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: core/rseq] cpumask: Cache num_possible_cpus()

The following commit has been merged into the core/rseq branch of tip:

Commit-ID:     d0f23ccf6ba9e2cc202e9ad25a427b8e5ea3ca1e
Gitweb:        https://git.kernel.org/tip/d0f23ccf6ba9e2cc202e9ad25a427b8e5ea3ca1e
Author:        Thomas Gleixner <tglx@...utronix.de>
AuthorDate:    Wed, 19 Nov 2025 18:27:01 +01:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 20 Nov 2025 12:14:54 +01:00

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Yury Norov <yury.norov@...il.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Reviewed-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
Link: https://patch.msgid.link/20251119172549.578653738@linutronix.de
---
 include/linux/cpumask.h | 10 ++++++++--
 kernel/cpu.c            | 19 +++++++++++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index feba06e..66694ee 100644
--- 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 cpumask *src);
 #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_online_cpus(void)
 {
 	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)
diff --git a/kernel/cpu.c b/kernel/cpu.c
index db9f6c5..18e530d 100644
--- 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 cpumask *src)
 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)
@@ -3140,6 +3144,21 @@ void set_cpu_online(unsigned int cpu, bool online)
 }
 
 /*
+ * This should be marked __init, but there is a boatload of call sites
+ * which need to be fixed up to do so. Sigh...
+ */
+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.
  */
 void __init boot_cpu_init(void)