linux-kernel - Re: [PATCH 30/46] Kbuild, lto: Add Link Time Optimization support

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Message-ID: <858176b3-a70e-01ce-4a56-96cb031f16c5@suse.cz>
Date:   Tue, 15 Nov 2022 14:31:22 +0100
From:   Martin Liška <mliska@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...nel.org>,
        "Jiri Slaby (SUSE)" <jirislaby@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kbuild@...r.kernel.org, Richard Biener <RGuenther@...e.com>,
        Jan Hubicka <jh@...e.de>, "H . J . Lu" <hjl.tools@...il.com>,
        Don Zickus <dzickus@...hat.com>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        Jiri Slaby <jslaby@...e.cz>
Subject: Re: [PATCH 30/46] Kbuild, lto: Add Link Time Optimization support

On 11/14/22 19:55, Josh Poimboeuf wrote:
> On Mon, Nov 14, 2022 at 12:43:28PM +0100, Jiri Slaby (SUSE) wrote:
>> +++ b/Documentation/kbuild/lto-build.rst
>> @@ -0,0 +1,76 @@
>> +=====================================================
>> +gcc link time optimization (LTO) for the Linux kernel
>> +=====================================================
>> +
>> +Link Time Optimization allows the compiler to optimize the complete program
>> +instead of just each file.
>> +
>> +The compiler can inline functions between files and do various other global
>> +optimizations, like specializing functions for common parameters,
>> +determing when global variables are clobbered, making functions pure/const,
>> +propagating constants globally, removing unneeded data and others.
>> +
>> +It will also drop unused functions which can make the kernel
>> +image smaller in some circumstances, in particular for small kernel
>> +configurations.
>> +
>> +For small monolithic kernels it can throw away unused code very effectively
>> +(especially when modules are disabled) and usually shrinks
>> +the code size.
>> +
>> +Build time and memory consumption at build time will increase, depending
>> +on the size of the largest binary. Modular kernels are less affected.
>> +With LTO incremental builds are less incremental, as always the whole
>> +binary needs to be re-optimized (but not re-parsed)
>> +
>> +Oopses can be somewhat more difficult to read, due to the more aggressive
>> +inlining: it helps to use scripts/faddr2line.
>> +
>> +It is currently incompatible with live patching.
> 
> ... because ?

There's no fundamental reason why live patching can't coexist with -flto.

We removed the sorry message for GCC 13.1 release:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=1a308905c1baf64d0ea4d09d7d92b55e79a2a339
when it comes to -flive-patching=inline-clone option.

But it seems Linux does not utilize the option (based on git grep):
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-flive-patching

That said, I would remove this limitation as LTO can make creation of live patches
more complicated, but fundamentally there's no barrier.

Thanks,
Martin