Re: [PATCH] x86/platform/uv: Use alternate source for socket to node data

[Steve Wahl]

On Fri, Sep 01, 2023 at 10:18:20AM -0700, Dave Hansen wrote:
> On 8/7/23 07:17, Steve Wahl wrote:
> > When nr_cpus is set to a smaller number than actually present, the
> > cpu_to_node() mapping information for unused CPUs is not available to
> > build_socket_tables().  This results in an incomplete table and will
> > later cause use of a -1 value for some array indexing, and eventual
> > kernel page faults.
> > 
> > Switch to using the __apicid_to_node array, which still contains all
> > the information mapping apicids to nodes, even for CPUs disabled with
> > a reduced nr_cpus setting.
> 
> Before, the lookup went:
> 
> 	CPU => APICID => SOCKET
> 
> But when the CPU wasn't present, there wasn't a way to start this lookup.
> 
> So, instead of looping over all CPUs, looking up their APICIDs and
> mapping those to sockets, just take CPUs out of the equation entirely.
> 
> Loop over all APICIDs which are mapped to a valid NUMA node.  Then just
> extract the socket-id from the APICID.
> 
> Right?
> 
> That seems sane enough.  It's also way less code than the previous approach.

Yes.  That's it precisely.  And, yes, way less code.

Thanks.

--> Steve Wahl

-- 
Steve Wahl, Hewlett Packard Enterprise