Re: [PATCH] powerpc/64s: introduce CONFIG_MAXSMP to test very large SMP

Excerpts from Michael Ellerman's message of November 8, 2021 3:28 pm:

Nicholas Piggin [off-list ref] writes:

quoted

Similarly to x86, add MAXSMP that should help flush out problems with
vary large SMP and other values associated with very big systems.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/Kconfig                   | 8 ++++++++
 arch/powerpc/platforms/Kconfig.cputype | 5 +++--
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index b8f6185d3998..d585fcfa456f 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -64,6 +64,13 @@ config NEED_PER_CPU_EMBED_FIRST_CHUNK
 config NEED_PER_CPU_PAGE_FIRST_CHUNK
 	def_bool y if PPC64
 
+config MAXSMP
+	bool "Enable Maximum number of SMP Processors and NUMA Nodes"
+	depends on SMP && DEBUG_KERNEL && PPC_BOOK3S_64
+	help
+	  Enable maximum number of CPUS and NUMA Nodes for this architecture.
+	  If unsure, say N.

As evidenced by the kernel robot report, I think we need to exclude this
from allyesconfig.

Because our max is 16K, larger than the 8K on x86, we are going to be
constantly hitting stack usage errors in driver code. Getting those
fixed tends to take time, because the driver authors don't see the
warnings when they build for other arches, and because the fixes go via
driver trees.

Yeah I realised after I hit send. Surprisingly there weren't too many
but agree going ahead of x86 would always come with annoyances and at
least would have to fix existing tree.

Making MAXSMP depend on !COMPILE_TEST should do the trick.

I'll do that. Or maybe make it 8192 if COMPILE_TEST otherwise 16384.

The reason for 16K is if we bump the deault at some point we might go to 
8K, in which case it would be good to have a test above it to catch
marginal cases.

Thanks,
Nick