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

[Rik van Riel]

On 03/22/2012 02:45 PM, KOSAKI Motohiro wrote:
> 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.

How is the moving of pages that are already in the
intended cpuset better than just leaving them alone?

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

Sure. Christoph?
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