On 02/22/2017 07:25 PM, Michael Ellerman wrote:

Hamish Martin [off-list ref] writes:

quoted

This patch series adds the ability to configure the THREAD_SHIFT value a=

nd

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thereby alter the stack size on powerpc systems. We are particularly int=

erested

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in configuring for a 32k stack on PPC64.

...

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For instance for a 70 frame stack, the architecture overhead just for th=

e stack

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frames is:
   70 * 16 bytes =3D 1120 bytes for PPC32, and
   70 * 112 bytes =3D 7840 bytes for PPC64.
So a simple doubling of the PPC32 stack size leaves us with a shortfall =

of 5600

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bytes (7840 - (2 * 1120)). In the example the stack frame overhead for P=

PC32 is

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1120/8192 =3D 13.5% of the stack space, whereas for PPC64 it is 7840/163=

84 =3D

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47.8% of the space.

The aim of this series is to provide the ability for users to configure =

for

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larger stacks without altering the defaults in a way that would impact e=

xisting

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users. However, given the inequity between the PPC32 and PPC64 stacks wh=

en

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taking into account the respective minimum stack frame sizes, we believe
consideration should be given to having a large default. We would apprec=

iate

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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=20
we have a stack that is reasonable and would cause no problems on PPC32.=20
This seems like a good compromise.
I think I was most keen to hear from you guys about whether it was a=20
flat out crazy idea, or if it would open up a huge can of hidden worms.=20
 From your response that seems not to be the case.

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