RCU init

Linux 6.18.37 LTS: kernel/rcu/tree.c
Linux 6.18.37 LTS: kernel/rcu/update.c
Linux 6.18.37 LTS: include/linux/rcupdate.h

void __init rcu_init(void)
{
	int cpu = smp_processor_id();

	rcu_early_boot_tests();

	rcu_bootup_announce();
	sanitize_kthread_prio();
	rcu_init_geometry();
	rcu_init_one();
	if (dump_tree)
		rcu_dump_rcu_node_tree();
	if (use_softirq)
		open_softirq(RCU_SOFTIRQ, rcu_core_si);

	/*
	 * We don't need protection against CPU-hotplug here because
	 * this is called early in boot, before either interrupts
	 * or the scheduler are operational.
	 */
	pm_notifier(rcu_pm_notify, 0);
	WARN_ON(num_online_cpus() > 1); // Only one CPU this early in boot.
	rcutree_prepare_cpu(cpu);
	rcutree_report_cpu_starting(cpu);
	rcutree_online_cpu(cpu);

	/* Create workqueue for Tree SRCU and for expedited GPs. */
	rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM | WQ_PERCPU, 0);
	WARN_ON(!rcu_gp_wq);

	sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM | WQ_UNBOUND, 0);
	WARN_ON(!sync_wq);

	/* Respect if explicitly disabled via a boot parameter. */
	if (rcu_normal_wake_from_gp < 0) {
		if (num_possible_cpus() <= WAKE_FROM_GP_CPU_THRESHOLD)
			rcu_normal_wake_from_gp = 1;
	}

	/* Fill in default value for rcutree.qovld boot parameter. */
	/* -After- the rcu_node ->lock fields are initialized! */
	if (qovld < 0)
		qovld_calc = DEFAULT_RCU_QOVLD_MULT * qhimark;
	else
		qovld_calc = qovld;

	// Kick-start in case any polled grace periods started early.
	(void)start_poll_synchronize_rcu_expedited();

	rcu_test_sync_prims();

	tasks_cblist_init_generic();
}

void call_rcu(struct rcu_head *head, rcu_callback_t func)
{
	__call_rcu_common(head, func, enable_rcu_lazy);
}