Re: [PATCH V2] powerpc/kernel: Add 'ibm, thread-groups' property for CPU allocation

[Michael Bringmann <mwb@linux.vnet.ibm.com>]

On 01/12/2018 08:33 PM, Michael Ellerman wrote:
> Nathan Fontenot <nfont@linux.vnet.ibm.com> writes:
...
>>
>> One thing I don't see addressed in the comments or in the code is
>> migration support. I think we need to update the thread group mask
>> post-migration to reflect the threads per core on the new system.
> 
> Normally I'd agree with you, but I don't see any prospect of the kernel
> surviving if the threads per core changes across a migration. We'll have
> data structures allocated based on the old value and things will
> definitely crash if the value increases. If it shrinks maybe we'd get
> away with it, but either way is dicey.
> 
> If there's an expectation that we'll be able to migrate between systems
> with different settings then we have a much bigger problem.
> 
> cheers
> 
> 

>From what I recall of my tests a few months ago, the device-tree is read in
and flattened before the kernel starts processing the migration state.
I am trying to get access to a couple of P9 systems on which I may test
migration to verify the ordering of events between initializing the kernel
and post-migration processing.

Regards,

-- 
Michael W. Bringmann
Linux Technology Center
IBM Corporation
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mwb@linux.vnet.ibm.com