[PATCH] init: refactor the generic cpu_to_node for NUMA

[Huang Shijie]

(0) We list the ARCHs which support the NUMA:
       arm64, loongarch, powerpc, riscv,
       sparc, mips, s390, x86,

(1) Some ARCHs in (0) override the generic cpu_to_node(), such as:
       sparc, mips, s390, x86.

    Since these ARCHs have their own cpu_to_node(), we do not care
    about them.

(2) The ARCHs enable NUMA and use the generic cpu_to_node.
    From (0) and (1), we can know that four ARCHs support NUMA and
    use the generic cpu_to_node:
        arm64, loongarch, powerpc, riscv,

    The generic cpu_to_node depends on percpu "numa_node".

    (2.1) The loongarch sets "numa_node" in:
          start_kernel --> smp_prepare_boot_cpu()

    (2.2) The arm64, powerpc, riscv set "numa_node" in:
       	  start_kernel --> arch_call_rest_init() --> rest_init()
       	               --> kernel_init() --> kernel_init_freeable()
                       --> smp_prepare_cpus()

    (2.3) The first place calling the cpu_to_node() is early_trace_init():
          start_kernel --> early_trace_init()--> __ring_buffer_alloc()
	               --> rb_allocate_cpu_buffer()

    (2.4) So it safe for loongarch. But for arm64, powerpc and riscv,
          there are at least four places in the common code where
	  the cpu_to_node() is called before it is initialized:
	   a.) early_trace_init()         in kernel/trace/trace.c
	   b.) sched_init()               in kernel/sched/core.c
	   c.) init_sched_fair_class()    in kernel/sched/fair.c
	   d.) workqueue_init_early()     in kernel/workqueue.c

(3) In order to fix the issue, the patch refactors the generic cpu_to_node:
    (3.1) change cpu_to_node to function pointer,
          and export it for kernel modules.

    (3.2) introduce _cpu_to_node() which is the original cpu_to_node().

    (3.3) introduce smp_prepare_boot_cpu_start() to wrap the original
          smp_prepare_boot_cpu(), and set cpu_to_node with
	  early_cpu_to_node which works fine for arm64, powerpc,
	  riscv and loongarch.

    (3.4) introduce smp_prepare_cpus_done() to wrap the original
          smp_prepare_cpus().
	  The "numa_node" is ready after smp_prepare_cpus(),
	  then set cpu_to_node with _cpu_to_node().

Signed-off-by: Huang Shijie <shijie@xxxxxxxxxxxxxxxxxxxxxx>
---
 drivers/base/arch_numa.c | 11 +++++++++++
 include/linux/topology.h |  6 ++----
 init/main.c              | 29 +++++++++++++++++++++++++++--
 3 files changed, 40 insertions(+), 6 deletions(-)

diff --git a/drivers/base/arch_numa.c b/drivers/base/arch_numa.c
index 5b59d133b6af..867a477fa975 100644
--- a/drivers/base/arch_numa.c
+++ b/drivers/base/arch_numa.c
@@ -61,6 +61,17 @@ EXPORT_SYMBOL(cpumask_of_node);
 
 #endif
 
+#ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
+#ifndef cpu_to_node
+int _cpu_to_node(int cpu)
+{
+	return per_cpu(numa_node, cpu);
+}
+int (*cpu_to_node)(int cpu);
+EXPORT_SYMBOL(cpu_to_node);
+#endif
+#endif
+
 static void numa_update_cpu(unsigned int cpu, bool remove)
 {
 	int nid = cpu_to_node(cpu);
diff --git a/include/linux/topology.h b/include/linux/topology.h
index 52f5850730b3..e7ce2bae11dd 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -91,10 +91,8 @@ static inline int numa_node_id(void)
 #endif
 
 #ifndef cpu_to_node
-static inline int cpu_to_node(int cpu)
-{
-	return per_cpu(numa_node, cpu);
-}
+extern int (*cpu_to_node)(int cpu);
+extern int _cpu_to_node(int cpu);
 #endif
 
 #ifndef set_numa_node
diff --git a/init/main.c b/init/main.c
index e24b0780fdff..b142e9c51161 100644
--- a/init/main.c
+++ b/init/main.c
@@ -870,6 +870,18 @@ static void __init print_unknown_bootoptions(void)
 	memblock_free(unknown_options, len);
 }
 
+static void __init smp_prepare_boot_cpu_start(void)
+{
+	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */
+
+#ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
+#ifndef cpu_to_node
+	/* The early_cpu_to_node should be ready now. */
+	cpu_to_node = early_cpu_to_node;
+#endif
+#endif
+}
+
 asmlinkage __visible __init __no_sanitize_address __noreturn __no_stack_protector
 void start_kernel(void)
 {
@@ -899,7 +911,7 @@ void start_kernel(void)
 	setup_command_line(command_line);
 	setup_nr_cpu_ids();
 	setup_per_cpu_areas();
-	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */
+	smp_prepare_boot_cpu_start();
 	boot_cpu_hotplug_init();
 
 	pr_notice("Kernel command line: %s\n", saved_command_line);
@@ -1519,6 +1531,19 @@ void __init console_on_rootfs(void)
 	fput(file);
 }
 
+static void __init smp_prepare_cpus_done(unsigned int setup_max_cpus)
+{
+	/* Different ARCHs may override smp_prepare_cpus() */
+	smp_prepare_cpus(setup_max_cpus);
+
+#ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
+#ifndef cpu_to_node
+	/* Change to the formal function. */
+	cpu_to_node = _cpu_to_node;
+#endif
+#endif
+}
+
 static noinline void __init kernel_init_freeable(void)
 {
 	/* Now the scheduler is fully set up and can do blocking allocations */
@@ -1531,7 +1556,7 @@ static noinline void __init kernel_init_freeable(void)
 
 	cad_pid = get_pid(task_pid(current));
 
-	smp_prepare_cpus(setup_max_cpus);
+	smp_prepare_cpus_done(setup_max_cpus);
 
 	workqueue_init();
 
-- 
2.40.1