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.com
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Yang 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/migrate.c b/mm/migrate.c
index 9d2642a3401862d3f557e80fc3a3fa146596698b..f50087d3ebf2959deed0c77e5f64018651750947 100644 (file)
--- a/mm/migrate.c
+++ b/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;