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/20220818131042.113280-11-aneesh.kumar@linux.ibm.comSigned-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Acked-by: Wei Xu <weixugc@google.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Bharata B Rao <bharata@amd.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Hesham Almatary <hesham.almatary@huawei.com>
Cc: Jagdish Gediya <jvgediya.oss@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Tim Chen <tim.c.chen@intel.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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;