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

[Larry Woodman]

On 03/22/2012 02:51 PM, Christoph Lameter wrote:
> On Thu, 22 Mar 2012, 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.
> >
> > But, I'm not against this if he has no objection.
> The use case for this is if you have an app running on nodes 3,4,5 on your
> machine and now you want to shift it to 4,5,6. The expectation is that the
> location of the pages relative to the first node stay the same.
> Application may manage their locality given a range of nodes and each of
> the x .. x+n nodes has their particular purpose.
So to be clear on this, in that case the intention would be move 3 to 4, 
4 to 5 and 5 to 6
to keep the node ordering the same?

Larry
> If you justd copy 3 to 6 then the app may get confused when doing
> additional allocations since different types of information is now stored
> on the "first" node (which is now 4).
>
>
>
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