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

[Hamish Martin]

On 02/22/2017 07:25 PM, Michael Ellerman wrote:
> 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
>
Yes, 15 years testing is hard to argue against, but in our case we feel 
we have a stack that is reasonable and would cause no problems on PPC32. 
This seems like a good compromise.
I think I was most keen to hear from you guys about whether it was a 
flat out crazy idea, or if it would open up a huge can of hidden worms. 
 From your response that seems not to be the case.

Thanks for the input, Michael. I'll add the EXPERT dependency and resubmit.