On Sun, Nov 11, 2007 at 04:46:26PM +0100, Andi Kleen wrote:
>
> > My thoughts go more into the direction that we have hundreds of similar
> > cases where e.g. a VFS function might currently only by used by OCFS2
> > and therefore be dead code for most users, and the only maintainable
> > solution will be to solve these at the compiler and/or linker level.
>
> -ffunction-sections can mostly do it, but only for non modular kernels
>
> One problem is that EXPORT_SYMBOL always creates a reference to the function
> even when nothing uses it.
>
> We would need a weak EXPORT_SYMBOL and some way to check references
> over main kernel and modules. I suppose it could be done as part of modpost
> and then generating a custom linker script that only includes the function
> sections referenced by anybody. But to make this work it would require
> putting all the EXPORT_SYMBOLs into own sections too, but I suppose
> that would be possible.
>
> In the past we had trouble that the explicit linker scripts mentioning every
> function section made the linker very slow, but perhaps that's fixed now.
I don't remember the technical details, but Denys Vlasenko posted some
patches that implemented -ffunction-sections while retaining exports.
Another approach would be calling gcc with "-combine -fwhole-program".
David Woodhouse once sent a whacky patch that implemented this on a
per-module basis retaining exports.
Much harder, but with the biggest possible savings, would be a
CONFIG_MODULES=n kernel built with one gcc call.
>...
> The question is if it would be still have a large enough user base without the
> distribution kernels. If it would be only used by a few users I don't think
> the maintenance overhead would be worth it.
>...
You forget the big embedded userbase - some even stay at kernel 2.4
since kernel 2.6 is much bigger.
> -Andi
cu
Adrian
--
"Is there not promise of rain?" Ling Tan asked suddenly out
of the darkness. There had been need of rain for many days.
"Only a promise," Lao Er said.
Pearl S. Buck - Dragon Seed
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