NUMA topology question wrt. d4edc5b6

Nishanth Aravamudan nacc at
Thu May 22 06:04:51 EST 2014

Hi Srivatsa,

After d4edc5b6 ("powerpc: Fix the setup of CPU-to-Node mappings during
CPU online"), cpu_to_node() looks like:

static inline int cpu_to_node(int cpu)
        int nid;

        nid = numa_cpu_lookup_table[cpu];

         * During early boot, the numa-cpu lookup table might not have been
         * setup for all CPUs yet. In such cases, default to node 0.
        return (nid < 0) ? 0 : nid;

However, I'm curious if this is correct in all cases. I have seen
several LPARs that do not have any CPUs on node 0. In fact, because node
0 is statically set online in the initialization of the N_ONLINE
nodemask, 0 is always present to Linux, whether it is present on the
system. I'm not sure what the best thing to do here is, but I'm curious
if you have any ideas? I would like to remove the static initialization
of node 0, as it's confusing to users to see an empty node (particularly
when it's completely separate in the numbering from other nodes), but
we trip a panic (refer to:


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