Re: [PATCH v3 1/3] driver/base: Optimize memory block registration to reduce boot time

[Oscar Salvador <osalvador@suse.de>]

On Mon, May 05, 2025 at 09:38:43AM +0200, David Hildenbrand wrote:
> On 05.05.25 09:28, Oscar Salvador wrote:
> > On Mon, May 05, 2025 at 09:16:48AM +0200, David Hildenbrand wrote:
> > > memory hotplug code never calls register_one_node(), unless I am missing
> > > something.
> > > 
> > > During add_memory_resource(), we call __try_online_node(nid, false), meaning
> > > we skip register_one_node().
> > > 
> > > The only caller of __try_online_node(nid, true) is try_online_node(), called
> > > from CPU hotplug code, and I *guess* that is not required.
> > 
> > Well, I guess this is because we need to link the cpus to the node.
> > register_one_node() has two jobs: 1) register cpus belonging to the node
> > and 2) register memory-blocks belonging to the node (if any).
> 
> Ah, via __register_one_node() ...
> 
> I would assume that an offline node
> 
> (1) has no memory
> (2) has no CPUs

That is right.

> When we *hotplug* either memory or CPUs, and we first online the node, there
> is nothing to register. Because if there would be something, the node would
> already be online.

I think I do not understand this, but let us imagine the following
scenario:

- You craft a VM with qemu that has a numa node which is memoryless and cpuless.
  This node will be allocated in free_area_init()->alloc_offline_node_data() but
  it will not be marked online because it does not have any resources.

  Then if you create a cpu device and hotplug in there, this will
  trigger try_online_node() from the cpu callback and go all the way to
  __register_one_node() to link the hotplugged cpu to the corresponding node.

  Now, I do not see an issue with that.
  The only think that makes me go "meh", is that register_one_node()
  calls register_memory_blocks_under_node() blindly when there might not
  be any memory block to register (I am sure we bail out if we see that
  somewhere, but still feels kinda of "wrong"?


-- 
Oscar Salvador
SUSE Labs