Re: [PATCH -mm] do_migrate_pages() calls migrate_to_node() even iftask is already on a correct node

[KOSAKI Motohiro]

CC to Christoph.

> While moving tasks between cpusets I noticed some strange behavior. Specifically if the nodes of the destination
> cpuset are a subset of the nodes of the source cpuset do_migrate_pages() will move pages that are already on a node
> in the destination cpuset. The reason for this is do_migrate_pages() does not check whether each node in the source
> nodemask is in the destination nodemask before calling migrate_to_node(). If we simply do this check and skip them
> when the source is in the destination moving we wont move nodes that dont need to be moved.
>
> Adding a little debug printk to migrate_to_node():
>
> Without this change migrating tasks from a cpuset containing nodes 0-7 to a cpuset containing nodes 3-4, we migrate
> from ALL the nodes even if they are in the both the source and destination nodesets:
>
> Migrating 7 to 4
> Migrating 6 to 3
> Migrating 5 to 4
> Migrating 4 to 3
> Migrating 1 to 4
> Migrating 3 to 4
> Migrating 0 to 3
> Migrating 2 to 3

Wait.

This may be non-optimal for cpusets, but maybe optimal migrate_pages, especially
the usecase is HPC. I guess this is intended behavior. I think we need to hear
Christoph's intention.

But, I'm not against this if he has no objection.




> With this change we only migrate from nodes that are not in the destination nodesets:
>
> Migrating 7 to 4
> Migrating 6 to 3
> Migrating 5 to 4
> Migrating 2 to 3
> Migrating 1 to 4
> Migrating 0 to 3
>
> Signed-off-by: Larry Woodman<lwoodman@xxxxxxxxxx>

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