[PATCH] SLAB : Use num_possible_cpus() in enable_cpucache()

[Eric Dumazet]

The existing comment in mm/slab.c is *perfect*, so I reproduce it :

        /*
         * CPU bound tasks (e.g. network routing) can exhibit cpu bound
         * allocation behaviour: Most allocs on one cpu, most free operations
         * on another cpu. For these cases, an efficient object passing between
         * cpus is necessary. This is provided by a shared array. The array
         * replaces Bonwick's magazine layer.
         * On uniprocessor, it's functionally equivalent (but less efficient)
         * to a larger limit. Thus disabled by default.
         */

As most shiped linux kernels are now compiled with CONFIG_SMP, there is no way 
a preprocessor #if can detect if the machine is UP or SMP. Better to use 
num_possible_cpus().

This means on UP we allocate a 'size=0 shared array', to be more efficient.

Another patch can later avoid the allocations of 'empty shared arrays', to 
save some memory.

Signed-off-by: Eric Dumazet <dada1@xxxxxxxxxxxxx>
diff --git a/mm/slab.c b/mm/slab.c
index 57f7aa4..a69d0a5 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -3975,10 +3975,8 @@ static int enable_cpucache(struct kmem_c
 	 * to a larger limit. Thus disabled by default.
 	 */
 	shared = 0;
-#ifdef CONFIG_SMP
-	if (cachep->buffer_size <= PAGE_SIZE)
+	if (cachep->buffer_size <= PAGE_SIZE && num_possible_cpus() > 1)
 		shared = 8;
-#endif
 
 #if DEBUG
 	/*