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

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Message-ID: <CAADnVQKLykG3akdPRTDgHDey9FW1LpixZHjLcj+eG2rhXo7V1w@mail.gmail.com>
Date: Thu, 13 Feb 2025 17:18:02 -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 Tue, Feb 11, 2025 at 3:59 PM Stephen Brennan
<stephen.s.brennan@...cle.com> wrote:
>
> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
> > 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.
>
> Fair enough. I believe I had shared Alan Maguire's proof-of-concept for
> that idea a while back for an older version of this feature:
>
> https://lore.kernel.org/all/20221104231103.752040-10-stephen.s.brennan@oracle.com/

Right vmlinux_extra was discussed in various context, so let's make it happen.

> We can dust that off and include it for a new version of this series.
> I'd be curious of what you'd like to see for kernel modules? A
> three-level tree would be too complex, in my opinion.

What is the use case for vars in kernel modules?

> module BTF size increased by 53.2%.

This is the sum of all mods with vars divided by
the sum of all mods without?
Any outliers there?
I would expect modules to have few global variables.

So before we decide on what to do with vars in mods lets figure out
the need.