Re: [PATCH 1/3] powerpc/pseries: Simplify cpu readd to use drc_index

[Nathan Lynch <nathanl@linux.ibm.com>]

Tyrel Datwyler <tyreld@linux.vnet.ibm.com> writes:

> On 05/16/2019 12:17 PM, Nathan Lynch wrote:
>> Tyrel Datwyler <tyreld@linux.ibm.com> writes:
>>> The current dlpar_cpu_readd() takes in a cpu_id and uses that to look up
>>> the cpus device_node so that we can get at the ibm,my-drc-index
>>> property. The only user of cpu readd is an OF notifier call back. This
>>> call back already has a reference to the device_node and therefore can
>>> retrieve the drc_index from the device_node.
>> 
>> dlpar_cpu_readd is a hack to try to change the CPU-node relationship at
>> runtime without destabilizing the system. It doesn't accomplish that and
>> it should just be removed (and I'm working on that).
>> 
>
> I will politely disagree. We've done exactly this from userspace for
> years. My experience still suggests that memory affinity is the
> problem area, and that the work to push this all into the kernel
> originally was poorly tested.

Kernel implementation details aside, how do you change the cpu-node
relationship at runtime without breaking NUMA-aware applications? Is
this not a fundamental issue to address before adding code like this?