Re: [PATCH] remove empty node at boot time

[Bjorn Helgaas <bjorn.helgaas@hp.com>]

On Tuesday 11 July 2006 00:55, KAMEZAWA Hiroyuki wrote:
> On Mon, 10 Jul 2006 11:03:03 -0600
> Bjorn Helgaas <bjorn.helgaas@hp.com> wrote:
> 
> > On Sunday 09 July 2006 23:19, KAMEZAWA Hiroyuki wrote:
> > > Could you try this patch ? (against 2.6.18-rc1)
> > 
> > Your patch does fix it.  But I'm worried about removing
> > empty nodes at boot-time.  I want to support the following
> > scenario:
> > 
> >   node 0: 1 enabled CPU, 3 disabled CPUs, no local memory
> >   node 1: 4 disabled CPUs, no local memory
> >   node 2: no CPUs, big interleaved memory across nodes 0 & 1
> > 
> > At run-time, I'd like to be able to enable any or all of the
> > 7 disabled CPUs.  If you remove the "empty" node 1 at boot-time,
> > it sounds like I won't be able to enable its CPUs later.
> > 
> 
> Hmm.. in my understanding, all structures for *possible* cpus are allocated
> at boot time. Then, only problem seems that a cpu is tied to 
> not-exisiting-node at boot time.
> (see arch/ia64/kernel/numa.c, build_cpu_to_node_map())
> 
> ==
> void __init build_cpu_to_node_map(void)
> {
>         int cpu, i, node;
> 
>         for(node=0; node < MAX_NUMNODES; node++)
>                 cpus_clear(node_to_cpu_mask[node]);
> 
>         for(cpu = 0; cpu < NR_CPUS; ++cpu) {
>                 node = -1;
>                 for (i = 0; i < NR_CPUS; ++i)
>                         if (cpu_physical_id(cpu) = node_cpuid[i].phys_id) {
>                                 node = node_cpuid[i].nid;
>                                 break;
>                         }
>                 cpu_to_node_map[cpu] = (node >= 0) ? node : 0; 
>                 if (node >= 0)
>                         cpu_set(cpu, node_to_cpu_mask[node]);
>         }
> }
> ==> 
> Then what we have to do here are
> 1. remap cpu to the first existing node at hot-add event
> or
> 2. implement node-hot-add triggered by cpu-hot-add.
>    Because we already have implemented node-hot-add triggered by memory-hotadd
>    we can do it by small effort.

I haven't paid much attention to the memory/cpu/node hotplug stuff,
but (2) sounds reasonable.

> I think above will work for your environment.
> do you have any idea other than "don't remove empty node at boot time" ?
> or reserve empty node is the best way ?