linux-kernel - Re: [PATCH bpf-next] scripts/gen-btf.sh: Disable LTO when generating initial .o file

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Message-ID: <CAADnVQLDNHQtoOuLNdwt5yZ1qnTEqcruUpZfPQMhfTHNN6XX0A@mail.gmail.com>
Date: Tue, 6 Jan 2026 14:01:39 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Ihor Solodrai <ihor.solodrai@...ux.dev>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, 
	Yonghong Song <yonghong.song@...ux.dev>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, clang-built-linux <llvm@...ts.linux.dev>
Subject: Re: [PATCH bpf-next] scripts/gen-btf.sh: Disable LTO when generating
 initial .o file

On Tue, Jan 6, 2026 at 1:53 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> On Mon, Jan 05, 2026 at 05:06:49PM -0800, Ihor Solodrai wrote:
> > I got curious and did a little experiment. Basically, I ran perf stat
> > on this part of gen-btf.sh:
> >
> >       echo "" | ${CC} ${CLANG_FLAGS} ${KBUILD_CFLAGS} -c -x c -o ${btf_data} -
> >       ${OBJCOPY} --add-section .BTF=${ELF_FILE}.BTF \
> >               --set-section-flags .BTF=alloc,readonly ${btf_data}
> >       ${OBJCOPY} --only-section=.BTF --strip-all ${btf_data}
> >
> > Replacing ${CC} command with:
> >
> >       ${OBJCOPY} --strip-all "${ELF_FILE}" ${btf_data} 2>/dev/null
> >
> > for comparison.
> >
> > TL;DR is that using ${CC} is:
> >   * about 1.5x faster than GNU objcopy --strip-all .tmp_vmlinux1
> >   * about 16x (!) faster than llvm-objcopy --strip-all .tmp_vmlinux1
> >
> > With obvious caveats that this is a particular machine (Threadripper
> > PRO 3975WX), toolchain etc:
> >   * clang version 21.1.7
> >   * gcc (GCC) 15.2.1 20251211
> >
> > This is bpf-next (a069190b590e) with BPF CI-like kconfig.
>
> Oof, that difference between GNU and LLVM's objcopy implementations...
> At the same time, it was only a little over a second for llvm-objcopy.
> Maybe that gets worse if more is built into the kernel to the point
> where it is untenable but maybe it is worth the reduced complexity? That
> said, my patch is pretty simple (and a follow up for KBUILD_CPPFLAGS if
> needed would be equally simple), your testing demonstrates that there
> is some performance improvement, and I cannot imagine there being any
> other bugs of this nature in this area going forward. I have no real
> strong opinion, I just need my builds to finish :)

Pls resend both patches? Or squash as one ?
Sounds like the current one is incomplete.