On Mon, Jan 15, 2024 at 03:39:00PM +0000, Russell King (Oracle) wrote:
> On Thu, Dec 14, 2023 at 04:05:56PM -0800, Christoph Lameter (Ampere) wrote:
> > +# Determines the placement of cpumasks.
> > +#
> > +# With CPUMASK_OFFSTACK the cpumasks are dynamically allocated.
> > +# Useful for machines with lots of core because it avoids increasing
> > +# the size of many of the data structures in the kernel.
> > +#
> > +# If this is off then the cpumasks have a static sizes and are
> > +# embedded within data structures.
> > +#
> > +config CPUMASK_OFFSTACK
> > + def_bool y
> > + depends on NR_CPUS > 256
>
> Should that be ">= 256" ?
I don't think that ">= 256" makes sense. Note that since the cpumasks are
arrays of unsigned long, they're chunked into groups of 64 bits:
2 to 64 cpus: 1 x unsigned long => 8 bytes
65 to 128 cpus: 2 x unsigned long => 16 bytes
129 to 192 cpus: 3 x unsigned long => 24 bytes
193 to 256 cpus: 4 x unsigned long => 32 bytes
257 to 320 cpus: 5 x unsigned long => 40 bytes
... and so if a mask for 256 CPUs is too big to go in the stack, so is any mask
for 193+ CPUs, and so ">= 256" should be clamped down to ">= 193" or "> 192".
The boundary should be just after a multiple of 64.
How did we choose 256 specifically? I note that x86-64 allows 512 CPUs before
requiring CPUMASK_OFFSTACK, and I see that powerpc selects CPUMASK_OFFSTACK
when NR_CPUS >= 8192.
Mark.
