linux-kernel - [PATCH] mm: migrate: Add more comments for selecting target node randomly

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Message-Id: <c31d36bd097c6e9e69fc0f409c43b78e53e64fc2.1637766801.git.baolin.wang@linux.alibaba.com>
Date:   Wed, 24 Nov 2021 23:16:33 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     akpm@...ux-foundation.org, ying.huang@...el.com,
        dave.hansen@...ux.intel.com
Cc:     ziy@...dia.com, shy828301@...il.com, baolin.wang@...ux.alibaba.com,
        zhongjiang-ali@...ux.alibaba.com, xlpang@...ux.alibaba.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH] 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/
Signed-off-by: Baolin Wang <baolin.wang@...ux.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