Re: [PATCH v5 21/21] kbuild: implement CONFIG_TRIM_UNUSED_KSYMS without recursion

[Sami Tolvanen]

On Mon, May 15, 2023 at 2:39 PM Nicolas Pitre <nico@xxxxxxxxxxx> wrote:
>
> On Mon, 15 May 2023, Masahiro Yamada wrote:
>
> > When CONFIG_TRIM_UNUSED_KSYMS is enabled, Kbuild recursively traverses
> > the directory tree to determine which EXPORT_SYMBOL to trim. If an
> > EXPORT_SYMBOL turns out to be unused by anyone, Kbuild begins the
> > second traverse, where some source files are recompiled with their
> > EXPORT_SYMBOL() tuned into a no-op.
> >
> > Linus stated negative opinions about this slowness in commits:
> >
> >  - 5cf0fd591f2e ("Kbuild: disable TRIM_UNUSED_KSYMS option")
> >  - a555bdd0c58c ("Kbuild: enable TRIM_UNUSED_KSYMS again, with some guarding")
> >
> > We can do this better now. The final data structures of EXPORT_SYMBOL
> > are generated by the modpost stage, so modpost can selectively emit
> > KSYMTAB entries that are really used by modules.
> >
> > Commit 2cce989f8461 ("kbuild: unify two modpost invocations") is another
> > ground-work to do this in a one-pass algorithm. With the list of modules,
> > modpost sets sym->used if it is used by a module. modpost emits KSYMTAB
> > only for symbols with sym->used==true.
> >
> > BTW, Nicolas explained why the trimming was implemented with recursion:
> >
> >   https://lore.kernel.org/all/2o2rpn97-79nq-p7s2-nq5-8p83391473r@xxxxxxxxxxx/
> >
> > Actually, we never achieved that level of optimization where the chain
> > reaction of trimming comes into play because:
> >
> >  - CONFIG_LTO_CLANG cannot remove any unused symbols
> >  - CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is enabled only for vmlinux,
> >    but not modules
>
> I did achieve it using LTO with gcc back then. See the section called
> "The tree that hides the forest" of https://lwn.net/Articles/746780/ for
> example results.

Clang can do similar optimizations, but not in relocatable links where
the linker must obviously preserve all the globals. A while ago there
was a suggestion of adding an option to LLD that allows one to pass a
list of symbols to preserve in relocatable LTO links, which would
allow us to better optimize vmlinux.o. However, I haven't had a chance
to look into this deeper than this proof of concept:

https://reviews.llvm.org/D142163

> > If deeper trimming is required, we need to revisit this, but I guess
> > that is unlikely to happen.
>
> Would have been nicer to keep this possibility as an option. The code is
> already there and working as intended. The build cost is intrinsic to
> the approach of course. The actual bug is to impose that cost onto
> people who didn't explicitly ask for it.
>
> But I'm no longer fighting this battle.

I agree, this looks like a reasonable solution for now.

Sami