modules: Use a better scheme for refcounting

Current refcounting for modules (done if CONFIG_MODULE_UNLOAD=y) is
using a lot of memory.

Each 'struct module' contains an [NR_CPUS] array of full cache lines.

This patch uses existing infrastructure (percpu_modalloc() &
percpu_modfree()) to allocate percpu space for the refcount storage.

Instead of wasting NR_CPUS*128 bytes (on i386), we now use
nr_cpu_ids*sizeof(local_t) bytes.

On a typical distro, where NR_CPUS=8, shiping 2000 modules, we reduce
size of module files by about 2 Mbytes. (1Kb per module)

Instead of having all refcounters in the same memory node - with TLB misses
because of vmalloc() - this new implementation permits to have better
NUMA properties, since each  CPU will use storage on its preferred node,
thanks to percpu storage.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 9 deletions

diff --git a/include/linux/module.h b/include/linux/module.h
index 4f7ea12463d3..f3b8329eb5b8 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -219,11 +219,6 @@ void *__symbol_get_gpl(const char *symbol);
 
 #endif
 
-struct module_ref
-{
-        local_t count;
-} ____cacheline_aligned;
-
 enum module_state
 {
         MODULE_STATE_LIVE,
@@ -344,8 +339,11 @@ struct module
         /* Destruction function. */
         void (*exit)(void);
 
-        /* Reference counts */
-        struct module_ref ref[NR_CPUS];
+#ifdef CONFIG_SMP
+        char *refptr;
+#else
+        local_t ref;
+#endif
 #endif
 };
 #ifndef MODULE_ARCH_INIT
@@ -395,13 +393,22 @@ void __symbol_put(const char *symbol);
 #define symbol_put(x) __symbol_put(MODULE_SYMBOL_PREFIX #x)
 void symbol_put_addr(void *addr);
 
+static inline local_t *__module_ref_addr(struct module *mod, int cpu)
+{
+#ifdef CONFIG_SMP
+        return (local_t *) (mod->refptr + per_cpu_offset(cpu));
+#else
+        return &mod->ref;
+#endif
+}
+
 /* Sometimes we know we already have a refcount, and it's easier not
    to handle the error case (which only happens with rmmod --wait). */
 static inline void __module_get(struct module *module)
 {
         if (module) {
                 BUG_ON(module_refcount(module) == 0);
-                local_inc(&module->ref[get_cpu()].count);
+                local_inc(__module_ref_addr(module, get_cpu()));
                 put_cpu();
         }
 }
@@ -413,7 +420,7 @@ static inline int try_module_get(struct module *module)
         if (module) {
                 unsigned int cpu = get_cpu();
                 if (likely(module_is_live(module)))
-                        local_inc(&module->ref[cpu].count);
+                        local_inc(__module_ref_addr(module, cpu));
                 else
                         ret = 0;
                 put_cpu();