[106/146] mm: migrate: add more comments for selecting target node randomly

Commit Message

Andrew Morton Jan. 14, 2022, 10:08 p.m. UTC

From: Baolin Wang <[e-mail]>
Subject: 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=[e-mail]/

Link: https://lkml.kernel.org/r/c31[e-mail]
Signed-off-by: Baolin Wang <[e-mail]>
Reviewed-by: Yang Shi <[e-mail]>
Cc: "Huang, Ying" <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Zi Yan <[e-mail]>
Cc: zhongjiang-ali <[e-mail]>
Cc: Xunlei Pang <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/migrate.c |    8 ++++++++
 1 file changed, 8 insertions(+)

Patch

--- a/mm/migrate.c~mm-migrate-add-more-comments-for-selecting-target-node-randomly
+++ a/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;