2021-11-24 15:16:54

by Baolin Wang

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 <[email protected]>
---
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

2021-11-24 19:25:52

by Yang Shi

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

On Wed, Nov 24, 2021 at 7:17 AM Baolin Wang
<[email protected]> 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 <[email protected]>
> ---
> 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 <[email protected]>

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