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

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

[Baolin Wang]

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

Re: [PATCH] mm: migrate: Add more comments for selecting target node randomly

[Yang Shi]

On Wed, Nov 24, 2021 at 7:17 AM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
> 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.

Thank you.

Reviewed-by: Yang Shi <shy828301@gmail.com>

>                  */
>                 index = get_random_int() % target_nr;
>                 break;
> --
> 2.27.0
>