mm/slub: let number of online CPUs determine the slub page order

The page order of the slab that gets chosen for a given slab cache depends
on the number of objects that can be fit in the slab while meeting other
requirements.  We start with a value of minimum objects based on
nr_cpu_ids that is driven by possible number of CPUs and hence could be
higher than the actual number of CPUs present in the system.  This leads
to calculate_order() chosing a page order that is on the higher side
leading to increased slab memory consumption on systems that have bigger
page sizes.

Hence rely on the number of online CPUs when determining the mininum
objects, thereby increasing the chances of chosing a lower conservative
page order for the slab.

Vlastimil said:
  "Ideally, we would react to hotplug events and update existing caches
   accordingly. But for that, recalculation of order for existing caches
   would have to be made safe, while not affecting hot paths. We have
   removed the sysfs interface with 32a6f409b6 ("mm, slub: remove
   runtime allocation order changes") as it didn't seem easy and worth
   the trouble.

   In case somebody wants to start with a large order right from the
   boot because they know they will hotplug lots of cpus later, they can
   use slub_min_objects= boot param to override this heuristic. So in
   case this change regresses somebody's performance, there's a way
   around it and thus the risk is low IMHO"

Link: https://lkml.kernel.org/r/20201118082759.1413056-1-bharata@linux.ibm.com
Signed-off-by: Bharata B Rao <bharata@linux.ibm.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slub.c

@@ -3431,7 +3431,7 @@ static inline int calculate_order(unsigned int size)
 	 */
 	min_objects = slub_min_objects;
 	if (!min_objects)
-		min_objects = 4 * (fls(nr_cpu_ids) + 1);
+		min_objects = 4 * (fls(num_online_cpus()) + 1);
 	max_objects = order_objects(slub_max_order, size);
 	min_objects = min(min_objects, max_objects);