numa: add generic percpu var numa_node_id() implementation

From: Lee Schermerhorn <[email protected]>

Rework the generic version of the numa_node_id() function to use the new generic percpu variable infrastructure.

Guard the new implementation with a new config option:

CONFIG_USE_PERCPU_NUMA_NODE_ID.

Archs which support this new implemention will default this option to 'y' when NUMA is configured. This config option could be removed if/when all archs switch over to the generic percpu implementation of numa_node_id(). Arch support involves:

1) converting any existing per cpu variable implementations to use this implementation. x86_64 is an instance of such an arch.
2) archs that don't use a per cpu variable for numa_node_id() will need to initialize the new per cpu variable "numa_node" as cpus are brought on-line. ia64 is an example.
3) Defining USE_PERCPU_NUMA_NODE_ID in arch dependent Kconfig--e.g., when NUMA is configured. This is required because I have retained the old implementation by default to allow archs to be modified incrementally, as desired.

Subsequent patches will convert x86_64 and ia64 to use this implemenation.

---

 arch/x86/include/asm/topology.h |    4 ++
 include/linux/topology.h        |   51 +++++++++++++++++++++++++++---
 mm/page_alloc.c                 |    5 ++
 3 files changed, 55 insertions(+), 5 deletions(-)

diff -puN arch/x86/include/asm/topology.h~numa-add-generic-percpu-var-numa_node_id-implementation arch/x86/include/asm/topology.h
@@ -170,6 +170,10 @@ static inline int numa_node_id(void)
 {
 	return 0;
 }
+/*
+ * indicate override:
+ */
+#define numa_node_id numa_node_id
 
 static inline int early_cpu_to_node(int cpu)
 {

diff -puN mm/page_alloc.c~numa-add-generic-percpu-var-numa_node_id-implementation mm/page_alloc.c
@@ -57,6 +57,11 @@
 #include <asm/div64.h>
 #include "internal.h"
 
+#ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
+DEFINE_PER_CPU(int, numa_node);
+EXPORT_PER_CPU_SYMBOL(numa_node);
+#endif
+
 /*
  * Array of node states.
  */

_