Re: [PATCH v2 0/5] cpumask: cleanup nr_cpu_ids vs nr_cpumask_bits mess

[Valentin Schneider]

On 06/09/22 16:38, Peter Zijlstra wrote:
> On Tue, Sep 06, 2022 at 01:06:47PM +0100, Valentin Schneider wrote:
>
>>   #define nr_cpumask_bits nr_cpu_ids
>
> That assumes the CPU space is dense; is this so? That is, you can have 4
> CPUs while the highest cpu number is vastly larger than 4.
>
> It's uncommon, but not unheard of I think. ISTR some BIOSes leaving
> holes in the CPU space when there were empty CPU sockets on the
> motherboard.

I'd assume this would be visible in the cpu_possible_mask and thus be
properly reflected in nr_cpu_ids. Otherwise that would already break with
CONFIG_CPUMASK_OFFSTACK=y, I think.