On 22/11/16 01:03, Michal Hocko wrote:

On Wed 16-11-16 10:44:58, Balbir Singh wrote:

quoted

In the absence of hotplug we use extra memory proportional to
(possible_nodes - online_nodes) * number_of_cgroups. PPC64 has a patch
to disable large consumption with large number of cgroups. This patch
adds hotplug support to memory cgroups and reverts the commit that
limited possible nodes to online nodes.

I didn't get to read patches yet (I am currently swamped by emails after
longer vacation so bear with me) but this doesn't tell us _why_ we want
this and how much we can actaully save. 

The motivation was 3af229f2071f
(powerpc/numa: Reset node_possible_map to only node_online_map)

In general being dynamic is more

complex and most systems tend to have possible_nodes close to
online_nodes in my experience (well at least on most reasonable
architectures). I would also appreciate some highlevel description of
the implications. E.g. how to we synchronize with the hotplug operations
when iterating node specific data structures.

I agree dynamic is more complex, but I think we'll begin to see a lot
of more of it. The rules are not hard IMHO. From an implication perspective
it means that we need to get/put_online_mem_nodes in certain paths - specifically
mem_cgroup_alloc/free and mem_cgroup_init from what I can see so far

Thanks for the review!

Balbir Singh