Hamish Martin [off-list ref] 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