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* [PATCH] mm: migrate: Add more comments for selecting target node randomly
@ 2021-11-24 15:16 Baolin Wang
  2021-11-24 19:25 ` Yang Shi
  0 siblings, 1 reply; 2+ messages in thread
From: Baolin Wang @ 2021-11-24 15:16 UTC (permalink / raw)
  To: akpm, ying.huang, dave.hansen
  Cc: ziy, shy828301, baolin.wang, zhongjiang-ali, xlpang, linux-mm,
	linux-kernel

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/
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 mm/migrate.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index 8d39720de4eb..d9af89248196 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1209,6 +1209,14 @@ 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;
-- 
2.27.0


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2021-11-24 15:16 [PATCH] mm: migrate: Add more comments for selecting target node randomly Baolin Wang
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