Re: [PATCH 02/37] xen: introduce a Kconfig option to configure NUMA nodes number

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.09.2021 14:02, Wei Chen wrote:
> Current NUMA nodes number is a hardcode configuration. This
> configuration is difficult for an administrator to change
> unless changing the code.
> 
> So in this patch, we introduce this new Kconfig option for
> administrators to change NUMA nodes number conveniently.
> Also considering that not all architectures support NUMA,
> this Kconfig option only can be visible on NUMA enabled
> architectures. Non-NUMA supported architectures can still
> use 1 as MAX_NUMNODES.

Do you really mean administrators here? To me command line options
are for administrators, but build decisions are usually taken by
build managers of distros.

> --- a/xen/arch/Kconfig
> +++ b/xen/arch/Kconfig
> @@ -17,3 +17,14 @@ config NR_CPUS
>         For CPU cores which support Simultaneous Multi-Threading or similar
>         technologies, this the number of logical threads which Xen will
>         support.
> +
> +config NR_NUMA_NODES
> +     int "Maximum number of NUMA nodes supported"
> +     range 1 4095

How was this upper bound established? Seeing 4095 is the limit of the
number of CPUs, do we really expect a CPU per node on such huge
systems? And did you check that whichever involved data types and
structures are actually suitable? I'm thinking e.g. of things like ...

> --- a/xen/include/asm-x86/numa.h
> +++ b/xen/include/asm-x86/numa.h
> @@ -3,8 +3,6 @@
>  
>  #include <xen/cpumask.h>
>  
> -#define NODES_SHIFT 6
> -
>  typedef u8 nodeid_t;

... this.

Jan