[PATCH 0/2] Allow configurable stack size (especially 32k on PPC64)

[Michael Ellerman]

Hamish Martin <hamish.martin at alliedtelesis.co.nz> writes:
> This patch series adds the ability to configure the THREAD_SHIFT value and
> thereby alter the stack size on powerpc systems. We are particularly interested
> in configuring for a 32k stack on PPC64.
...
>
> For instance for a 70 frame stack, the architecture overhead just for the stack
> frames is:
>    70 * 16 bytes = 1120 bytes for PPC32, and
>    70 * 112 bytes = 7840 bytes for PPC64.
> So a simple doubling of the PPC32 stack size leaves us with a shortfall of 5600
> bytes (7840 - (2 * 1120)). In the example the stack frame overhead for PPC32 is
> 1120/8192 = 13.5% of the stack space, whereas for PPC64 it is 7840/16384 =
> 47.8% of the space.
>
> The aim of this series is to provide the ability for users to configure for
> larger stacks without altering the defaults in a way that would impact existing
> users. However, given the inequity between the PPC32 and PPC64 stacks when
> taking into account the respective minimum stack frame sizes, we believe
> consideration should be given to having a large default. We would appreciate
> any input or opinions on this issue.

Thanks for the detailed explanation.

The patches look fine, so I don't see any reason why we wouldn't merge
this. I might make the config option depend on EXPERT, but that's just
cosmetic.


You're right about the difference in stack overhead between 32 & 64-bit.
But I guess on the other hand we've been using 16K stacks on 64-bit for
over 15 years, and although we have had some reports of stack overflow
they're not a common problem.

cheers