Re: [PATCH] x86/platform/uv: Abort UV initialization when reduced nr_cpus requires it

[Dave Hansen]

On 7/11/23 13:26, Steve Wahl wrote:
> When nr_cpus is set to a smaller number than actually present, there
> is some node-to-socket mapping info we won't get access to in

First of all, no "we's" in commit messages.

> https://www.kernel.org/doc/html/next/process/maintainer-tip.html#changelog

> build_socket_tables().  This could later result in using a -1 value
> for some array indexing, and eventual kernel page faults.
> 
> To avoid this, if any unfilled table entries are found, print a
> warning message, and resume initializing, acting as if this is not a
> UV system.  UV features will be unavailable, but we will not cause
> kernel dumps.
> 
> This is a condition we expect only in platform debugging situations,
> not in day-to-day operation.

This seems like a hack.

The real problem is that you've got an online Linux NUMA node with no
CPUs.  uv_system_init_hub() (probably) goes and does:

>         for_each_node(nodeid)
>                 __uv_hub_info_list[nodeid] = uv_hub_info_list_blade[uv_node_to_blade_id(nodeid)];

But the node=>blade lookup uses socket numbers.  No CPUs means no socket
numbers.  You _have_ the blade information _somewhere_.  Is there really
no other way to map it to a NUMA node than using the CPU apicid?