Re: [mm v2 0/3] Support memory cgroup hotplug

[Balbir Singh <bsingharora@gmail.com>]

On 29/11/16 11:42, Tejun Heo wrote:
> Hello, Balbir.
> 
> On Tue, Nov 29, 2016 at 11:09:26AM +1100, Balbir Singh wrote:
>> On 29/11/16 08:10, Tejun Heo wrote:
>>> On Thu, Nov 24, 2016 at 12:05:12AM +1100, Balbir Singh wrote:
>>>> On my desktop NODES_SHIFT is 6, many distro kernels have it a 9. I've known
>>>> of solutions that use fake NUMA for partitioning and need as many nodes as
>>>> possible.
>>>
>>> It was a crude kludge that people used before memcg.  If people still
>>> use it, that's fine but we don't want to optimize / make code
>>> complicated for it, so let's please put away this part of
>>> justification.
>>
>> Are you suggesting those use cases can be ignored now?
> 
> Don't do that.  When did I say that?  What I said is that it isn't a
> good idea to optimize and complicate the code base for it at this
> point.  It shouldn't a controversial argument given fake numa's
> inherent issues and general lack of popularity.
> 
> Besides, does node hotplug even apply to fake numa?  ISTR it being
> configured statically on the boot prompt.

Good point, not sure if it worked with that setup as well.

> 
>>> NUMA code already has possible detection.  Why not simply make memcg
>>> use those instead of MAX_NUMNODES like how we use nr_cpu_ids instead
>>> of NR_CPUS?
>>
>> nodes_possible_map is set to node_online_map at the moment for ppc64.
>> Which becomes a problem when hotplugging a node that was not already
>> online.
>>
>> I am not sure what you mean by possible detection. node_possible_map
>> is set based on CONFIG_NODE_SHIFT and then can be adjusted by the
>> architecture (if desired). Are you suggesting firmware populate it
>> in?
> 
> That's what we do with cpus.  The kernel is built with high maximum
> limit and the kernel queries the firmware during boot to determine how
> many are actually possible on the system, which in most cases isn't
> too far from what's already on the system.  I don't see why we would
> take a different approach with NUMA nodes.

Agreed for the short term. I think memory hotplug needs a review and we'll
probably get it fixed as we make progress along the way

Balbir

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