Re: [PATCH 2/4] kbuild: sort the list of symbols exported by the kernel (__ksymtab)

Sam Ravnborg
2009-09-23T17:38:56Z

On Tue, Sep 22, 2009 at 04:08:55PM +0100, Alan Jenkins wrote:
> Sam Ravnborg wrote:
> > On Tue, Sep 22, 2009 at 02:38:36PM +0100, Alan Jenkins wrote:
> >
> >> modpost of vmlinux.o now extracts the ksymtab sections and outputs
> >> sorted versions of them as .tmp_exports.c. These sorted sections
> >> are linked into vmlinux and the original unsorted sections are
> >> discarded.
> >>
> >> This will allow modules to be loaded faster, resolving symbols using
> >> binary search, without any increase in the memory needed for the
> >> symbol tables.
> >>
> >> This does not affect the building of modules, so hopefully it won't
> >> affect compile times too much.
> >>
> >
> > I do not quite follow you here.
> >
> > With your patch:
> >
> > For vmlinux we define our symbols in sections
> > named *_sorted - but they are not sorted.
> >
> > We than create a small .c file that uses the original sections names
> > which is what is used in the final vmlinux.
> >
>
> Actually it's intended to work the other way round.
>
> EXPORT_SYMBOL() generates symbols in __ksymtab as usual. These get as
> far as vmlinux.o before I start changing anything.
>
> My .c file then uses __EXPORT_SYMBOL_SORTED() to generate symbols in
> __ksymtab_sorted (or __ksymtab_gpl_sorted etc.), based on the symbols in
> vmlinux.o. So the _sorted sections _are_ sorted.
>
> Finally, I changed the vmlinux linker script to effectively rename
> __ksymtab_sorted and discard the original unsorted __ksymtab.
>
> I hope that clears that up.
>
> > Could we replace the content of these sections rather than playing
> > games with the names?
> >
> > Sam
> >
>
> Thinking about it, I guess I could avoid using new section names. I
> could change the vmlinux linker script to discard all __ksymtab
> sections, _except_ those which came from the file .tmp_exports.o. (And
> maybe rename .tmp_exports.o to .tmp_sorted_exports.o to make it more
> obvious).
>
> That would avoid the need to add separate __EXPORT_SYMBOL_NAME() and
> __EXPORT_SYMBOL_SORTED(). OTOH, it means the linker script uses more
> features and is more tightly coupled to the build system. I.e. the
> script references a specific file.
>
> I prefer my original method, but I can change it if you disagree :-).

One of the problems you hit is that we use the same linker script for
all the links we do of the kernel image.
I had a patchset that pulled out the final link from the top-level
Makefile and made it a simple shell script.
I should give that another go which would then be a good basis for
this patchset and a simplied final linking.

Sam