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@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Yury Norov <yury.norov@gmail.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://patch.msgid.link/20251119172549.578653738@linutronix.de

kernel/cpu.c

@@ -3085,10 +3085,13 @@ EXPORT_SYMBOL(cpu_all_bits);
 #ifdef CONFIG_INIT_ALL_POSSIBLE
 struct cpumask __cpu_possible_mask __ro_after_init
 	= {CPU_BITS_ALL};
+unsigned int __num_possible_cpus __ro_after_init = NR_CPUS;
 #else
 struct cpumask __cpu_possible_mask __ro_after_init;
+unsigned int __num_possible_cpus __ro_after_init;
 #endif
 EXPORT_SYMBOL(__cpu_possible_mask);
+EXPORT_SYMBOL(__num_possible_cpus);
 
 struct cpumask __cpu_online_mask __read_mostly;
 EXPORT_SYMBOL(__cpu_online_mask);
@@ -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)
@@ -3139,6 +3143,21 @@ 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.
  */