Re: [PATCH] powerpc/64s: Increase default stack size to 32KB

[Rahul Rameshbabu]

On Fri, 15 Dec, 2023 23:44:49 +1100 Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
> There are reports of kernels crashing due to stack overflow while
> running OpenShift (Kubernetes). The primary contributor to the stack
> usage seems to be openvswitch, which is used by OVN-Kubernetes (based on
> OVN (Open Virtual Network)), but NFS also contributes in some stack
> traces.
>
> There may be some opportunities to reduce stack usage in the openvswitch
> code, but doing so potentially require tradeoffs vs performance, and
> also requires testing across architectures.
>
> Looking at stack usage across the kernel (using -fstack-usage), shows
> that ppc64le stack frames are on average 50-100% larger than the
> equivalent function built for x86-64. Which is not surprising given the
> minimum stack frame size is 32 bytes on ppc64le vs 16 bytes on x86-64.
>
> So increase the default stack size to 32KB for the modern 64-bit Book3S
> platforms, ie. pseries (virtualised) and powernv (bare metal). That
> leaves the older systems like G5s, and the AmigaOne (pasemi) with a 16KB
> stack which should be sufficient on those machines.
>
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> ---

We noticed this change is causing assembler issues for us when building
the kernel.

  make ARCH=powerpc KERNELRELEASE=6.8.0-rc2_for_upstream_debug_2024_02_06_20_01 KBUILD_BUILD_VERSION=1
  arch/powerpc/kernel/switch.S: Assembler messages:
  arch/powerpc/kernel/switch.S:249: Error: operand out of range (0x000000000000fe50 is not between 0xffffffffffff8000 and 0x0000000000007fff)
  make[6]: *** [scripts/Makefile.build:361: arch/powerpc/kernel/switch.o] Error 1
  make[5]: *** [scripts/Makefile.build:481: arch/powerpc/kernel] Error 2
  make[5]: *** Waiting for unfinished jobs....
  make[4]: *** [scripts/Makefile.build:481: arch/powerpc] Error 2
  make[4]: *** Waiting for unfinished jobs....
  make[3]: *** [Makefile:1921: .] Error 2

The issue lies with the do_switch_64 macro.

--
Thanks,

Rahul Rameshbabu