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

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Message-ID: <87a5asghj2.fsf@oracle.com>
Date: Tue, 11 Feb 2025 15:58:57 -0800
From: Stephen Brennan <stephen.s.brennan@...cle.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>,
        Andrii Nakryiko
 <andrii@...nel.org>,
        Nicolas Schier <nicolas@...sle.eu>, Kees Cook
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        Sami Tolvanen <samitolvanen@...gle.com>,
        Eduard
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        Kent Overstreet
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        Alexei Starovoitov <ast@...nel.org>, Song
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Subject: Re: [PATCH 2/2] btf: Add the option to include global variable types

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/

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.

As a separate note for this patch series, we discovered that variables
declared twice, where one is declared "__weak", will result in two DWARF
variable declarations, and thus two BTF variables. This trips up the BTF
validation code. So this series as it is cannot move forward. I'm
submitting a fix to dwarves today.

Thanks,
Stephen