linux-kernel - Re: [PATCH 2/2] btf: Add the option to include global variable types

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Message-ID: <CAADnVQLiyezBW34dhkwZw+mWmkFAYMZUdHbOa4uYCdPbgS10SQ@mail.gmail.com>
Date: Fri, 7 Feb 2025 15:50:31 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Stephen Brennan <stephen.s.brennan@...cle.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Nicolas Schier <nicolas@...sle.eu>, Kees Cook <kees@...nel.org>, KP Singh <kpsingh@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Sami Tolvanen <samitolvanen@...gle.com>, 
	Eduard Zingerman <eddyz87@...il.com>, linux-arch <linux-arch@...r.kernel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Kent Overstreet <kent.overstreet@...ux.dev>, 
	Pasha Tatashin <pasha.tatashin@...een.com>, Jiri Olsa <jolsa@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>, Jann Horn <jannh@...gle.com>, 
	Ard Biesheuvel <ardb@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, Hao Luo <haoluo@...gle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, 
	Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Arnd Bergmann <arnd@...db.de>, Nathan Chancellor <nathan@...nel.org>, linux-debuggers@...r.kernel.org, 
	Alexei Starovoitov <ast@...nel.org>, Song Liu <song@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH 2/2] btf: Add the option to include global variable types

On Thu, Feb 6, 2025 at 5:21 PM Stephen Brennan
<stephen.s.brennan@...cle.com> wrote:
> When the feature was implemented in pahole, my measurements indicated
> that vmlinux BTF size increased by about 25.8%, and module BTF size
> increased by 53.2%. Due to these increases, the feature is implemented
> behind a new config option, allowing users sensitive to increased memory
> usage to disable it.
>

...
> +config DEBUG_INFO_BTF_GLOBAL_VARS
> +       bool "Generate BTF type information for all global variables"
> +       default y
> +       depends on DEBUG_INFO_BTF && PAHOLE_VERSION >= 128
> +       help
> +         Include type information for all global variables in the BTF. This
> +         increases the size of the BTF information, which increases memory
> +         usage at runtime. With global variable types available, runtime
> +         debugging and tracers may be able to provide more detail.

This is not a solution.
Even if it's changed to 'default n' distros will enable it
like they enable everything and will suffer a regression.

We need to add a new module like vmlinux_btf.ko that will contain
this additional BTF data. For global vars and everything else we might need.

pw-bot: cr