mm: migrate: add more comments for selecting target node randomly

As Yang Shi suggested [1], it will be helpful to explain why we should
select target node randomly now if there are multiple target nodes.

[1] https://lore.kernel.org/all/CAHbLzkqSqCL+g7dfzeOw8fPyeEC0BBv13Ny1UVGHDkadnQdR=g@mail.gmail.com/

Link: https://lkml.kernel.org/r/c31d36bd097c6e9e69fc0f409c43b78e53e64fc2.1637766801.git.baolin.wang@linux.alibaba.comSigned-off-by: NBaolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: NYang Shi <shy828301@gmail.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: zhongjiang-ali <zhongjiang-ali@linux.alibaba.com>
Cc: Xunlei Pang <xlpang@linux.alibaba.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -1206,6 +1206,14 @@ int next_demotion_node(int node)
 		/*
 		 * If there are multiple target nodes, just select one
 		 * target node randomly.
+		 *
+		 * In addition, we can also use round-robin to select
+		 * target node, but we should introduce another variable
+		 * for node_demotion[] to record last selected target node,
+		 * that may cause cache ping-pong due to the changing of
+		 * last target node. Or introducing per-cpu data to avoid
+		 * caching issue, which seems more complicated. So selecting
+		 * target node randomly seems better until now.
 		 */
 		index = get_random_int() % target_nr;
 		break;