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

Alan Jenkins
2009-09-22T15:08:54Z

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 :-).

Thanks!
Alan