Messages in this thread | | | Date | Thu, 17 Nov 2022 12:42:56 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 00/46] gcc-LTO support for the kernel |
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On Thu, Nov 17, 2022 at 08:50:59AM +0000, Richard Biener wrote: > On Thu, 17 Nov 2022, Peter Zijlstra wrote: > > > On Mon, Nov 14, 2022 at 08:40:50PM +0100, Ard Biesheuvel wrote: > > > On Mon, 14 Nov 2022 at 12:44, Jiri Slaby (SUSE) <jirislaby@kernel.org> wrote: > > > > > > > > Hi, > > > > > > > > this is the first call for comments (and kbuild complaints) for this > > > > support of gcc (full) LTO in the kernel. Most of the patches come from > > > > Andi. Me and Martin rebased them to new kernels and fixed the to-use > > > > known issues. Also I updated most of the commit logs and reordered the > > > > patches to groups of patches with similar intent. > > > > > > > > The very first patch comes from Alexander and is pending on some x86 > > > > queue already (I believe). I am attaching it only for completeness. > > > > Without that, the kernel does not boot (LTO reorders a lot). > > > > > > > > In our measurements, the performance differences are negligible. > > > > > > > > The kernel is bigger with gcc LTO due to more inlining. > > > > > > OK, so if I understand this correctly: > > > - the performance is the same > > > - the resulting image is bigger > > > - we need a whole lot of ugly hacks to placate the linker. > > > > > > Pardon my cynicism, but this cover letter does not mention any > > > advantages of LTO, so what is the point of all of this? > > > > Seconded; I really hate all the ugly required for the GCC-LTO > > 'solution'. There not actually being any benefit just makes it a very > > simple decision to drop all these patches on the floor. > > I'd say that instead a prerequesite for the series would be to actually > enforce hidden visibility for everything not part of the kernel module > API so the compiler can throw away unused functions. Currently it has > to keep everything because with a shared object there might be external > references to everything exported from individual TUs.
I'm not sure what you're on about; only symbols annotated with EXPORT_SYMBOL*() are accessible from modules (aka DSOs) and those will have their address taken. You can feely eliminate any unused symbol.
> There was a size benefit mentioned for module-less monolithic kernels > as likely used in embedded setups, not sure if that's enough motivation > to properly annotate symbols with visibility - and as far as I understand > all these 'required' are actually such fixes.
I'm not seeing how littering __visible is useful or desired, doubly so for that static hack, that's just a crude work around for GCC LTO being inferior for not being able to read inline asm.
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