On Tue, Jan 16, 2024 1:08:20PM +0000, Mark Rutland wrote:

On Mon, Jan 15, 2024 at 03:39:00PM +0000, Russell King (Oracle) wrote:

quoted

On Thu, Dec 14, 2023 at 04:05:56PM -0800, Christoph Lameter (Ampere) wrote:

quoted

+# 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.

The suggestion for >= 256 may have been a zero-index/one-index mixup.

It seems > 256 was chosen as the cutoff simply because it preserves existing behavior.
The patch description seems to imply there was pushback from distro maintainers on just increasing
the default NR_CPUS.

The existing default value of 256 is probably already a strain for smaller ARM systems, to which
x86-64 isn't a reasonable comparison. I'm not sure what the reaction to increasing from 64 to 256
in 2019 was like, but picking a pivot point for CPUMASK_OFFSTACK beyond 256 may skew the balance
even less in favor of smaller ARM systems.

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