lib/nodemask: optimize node_random for nodemask with single NUMA node

The most common case for certain node_random usage (demotion nodemask) is
with nodemask weight 1.  We can avoid calling get_random_init() in that
case and always return the only node set in the nodemask.

A simple test as below
  before = rdtsc_ordered();
  for (i= 0; i < 100; i++) {
      rand = node_random(&nmask);
  }
  after = rdtsc_ordered();

Without fix after - before : 16438
With fix after - before : 816

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Aneesh Kumar K.V <[email protected]>
Reviewed-by: "Huang, Ying" <[email protected]>
Acked-by: Wei Xu <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Bharata B Rao <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Hesham Almatary <[email protected]>
Cc: Jagdish Gediya <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Jonathan Cameron <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: SeongJae Park <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

include/linux/nodemask.h

@@ -505,12 +505,21 @@ static inline int num_node_state(enum node_states state)
 static inline int node_random(const nodemask_t *maskp)
 {
 #if defined(CONFIG_NUMA) && (MAX_NUMNODES > 1)
-	int w, bit = NUMA_NO_NODE;
+	int w, bit;
 
 	w = nodes_weight(*maskp);
-	if (w)
+	switch (w) {
+	case 0:
+		bit = NUMA_NO_NODE;
+		break;
+	case 1:
+		bit = first_node(*maskp);
+		break;
+	default:
 		bit = bitmap_ord_to_pos(maskp->bits,
-			get_random_int() % w, MAX_NUMNODES);
+					get_random_int() % w, MAX_NUMNODES);
+		break;
+	}
 	return bit;
 #else
 	return 0;