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



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