Question about num_possible_cpus() and cpu_possible_mask

[Michael Kelley]

Question:  Is there any intention to guarantee that the cpu_possible_mask is
"dense", in that all bit positions 0 thru (nr_cpu_ids - 1) are set, with no
"holes"? If that were true, then num_possible_cpus() would be equal to
nr_cpu_ids.

x86 always sets up cpu_possible_mask as dense, as does ARM64 with ACPI.
But it appears there are errors cases on ARM64 with DeviceTree where this
is not the case. I haven't looked at other architectures.

There's evidence both ways:
1) A somewhat recent report[1] on SPARC where cpu_possible_mask
   isn't dense, and there's code assuming that it is dense. This report
   got me thinking about the question.
  
2) setup_nr_cpu_ids() in kernel/smp.c is coded to *not* assume it is dense

3) But there are several places throughout the kernel that do something like
   the following, which assumes they are dense:

	array = kcalloc(num_possible_cpus(), sizeof(<some struct>), GFP_KERNEL);
	....
	index into "array" with smp_processor_id()

On balance, I'm assuming that there's no requirement for cpu_possible_mask
to be dense, and code like #3 above is technically wrong. It should be
using nr_cpu_ids instead of num_possible_cpus(), which is also faster.
We get away with it 99.99% of the time because all (or almost all?)
architectures populate cpu_possible_mask as dense.

There are 6 places in Hyper-V specific code that do #3. And it works because
Hyper-V code only runs on x86 and ARM64 where cpu_possible_mask is
always dense. But in the interest of correctness and robustness against
future changes, I'm planning to fix the Hyper-V code.

There are also a few other places throughout the kernel with the same
problem, and I may look at fixing those as well.

Or maybe my assumptions are off-base. Any thoughts or guidance before
I start submitting patches?

Thanks,

Michael

[1] https://lore.kernel.org/lkml/20240621033005.6mccm7waduelb4m5@xxxxxxxx/