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

[Mike Rapoport <rppt@kernel.org>]

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
> 
> 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.
> 
> In particular, try_offline_node() will only offline a node if
> 
> (A) No present pages: No pages are spanned anymore. This includes
>     offline memory blocks.
> (B) No present CPUs.
> 
> But maybe there is some case that I am missing ...

I actually hoped you and Oscar know how that stuff works :)

I tried to figure what is going on there and it all looks really convoluted.

So, on boot we have 
	cpu_up() ->
		try_online_node() ->
 			bails out because all nodes are online (at least on
			x86 AFAIU, see 1ca75fa7f19d ("arch/x86/mm/numa: Do
                        not initialize nodes twice"))
	node_dev_init()i ->
		register_one_node() ->
			this one can use __register_one_node() and loop
			over memblock regions.

And for the hotplug/unplug path, it seems that
register_memory_blocks_under_node(MEMINIT_EARLY) is superfluous, because if
a node had memory it wouldn't get offlined, and if we are hotplugging an
node with memory and cpus, memory hotplug anyway calls
register_memory_blocks_under_node_hotplug().

So, IMHO, register_one_node() should not call
register_memory_blocks_under_node() at all, but again, I might have missed
something :)
 
> -- 
> Cheers,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.