Re: [2/8,v3] NUMA Hotplug Emulator: infrastructure of NUMA hotplug emulation

[Shaohui Zheng <shaohui.zheng@intel.com>]

On Thu, Nov 18, 2010 at 01:24:52PM -0800, David Rientjes wrote:
> On Thu, 18 Nov 2010, Shaohui Zheng wrote:
> 
> > in our draft patch, we re-setup nr_node_ids when CONFIG_ARCH_MEMORY_PROBE enabled 
> > and mem=XXX was specified in grub. we set nr_node_ids as MAX_NUMNODES + 1, because
> >  we do not know how many nodes will be hot-added through memory/probe interface. 
> >  it might be a little wasting of memory.
> > 
> 
> nr_node_ids need not be set to anything different at boot, the 
> MEM_GOING_ONLINE callback should be used for anything (like the slab 
> allocators) where a new node is introduced and needs to be dealt with 
> accordingly; this is how regular memory hotplug works, we need no 
> additional code in this regard because it's emulated.  If a subsystem 
> needs to change in response to a new node going online and doesn't as a 
> result of using your emulator, that's a bug and either needs to be fixed 
> or prohibited from use with CONFIG_MEMORY_HOTPLUG.
> 
> (See the MEM_GOING_ONLINE callback in mm/slub.c, for instance, which deals 
> only with the case of node hotplug.)

nr_node_ids is the possible node number. when we do regular memory online,
it is oline to a possible node, and it is already counted in to nr_node_ids.

if you increment nr_node_ids dynamically when node online, it causes a lot of
problems. Many data are initialized according to nr_node_ids. That is our
experience when we debug the emulator.

mm/page_alloc.c:
/*
 * Figure out the number of possible node ids.
 */
static void __init setup_nr_node_ids(void)
{
	unsigned int node;
	unsigned int highest = 0;

	for_each_node_mask(node, node_possible_map)
		highest = node;
	nr_node_ids = highest + 1;
}

There is no conflict between emulator and CONFIG_MEMORY_HOTPLUG. A real node can be
 onlined because we already set it as _possible_; if emulator is enabled, all the 
nodes were marked as _possbile_ node, the real ndoe is also included in.

-- 
Thanks & Regards,
Shaohui