Re: [PATCH dwarves 0/7] Add support for generating BTF for all variables

[Stephen Brennan <stephen.s.brennan@oracle.com>]

Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:
> On Fri, Aug 26, 2022 at 11:54 AM Stephen Brennan
> <stephen.s.brennan@oracle.com> wrote:
[...]
>> Future Work
>> -----------
>>
>> If this proves acceptable, I'd like to follow-up with a kernel patch to
>> add a configuration option (default=n) for generating BTF with all
>> variables, which distributions could choose to enable or not.
>>
>> There was previous discussion[3] about leveraging split BTF or building
>> additional kernel modules to contain the extra variables. I believe with
>> this patch series, it is possible to do that. However, I'd argue that
>> simpler is better here: the advantage for using BTF is having it all
>> available in the kernel/module image. Storing extra BTF on the
>> filesystem would break that advantage, and at that point, you'd be
>> better off using a debuginfo format like CTF, which is lightweight and
>> expected to be found on the filesystem.
>
> With all or nothing approach the distros would have a hard choice
> to make whether to enable that kconfig, increase BTF and consume
> extra memory without any obvious reason or just don't do it.
> Majority probably is not going to enable it.
> So the feature will become a single vendor only and with
> inevitable bit-rot.

I'd intend to support it even if just a single distribution enabled it.
But I do see your concern.

> Whereas with split BTF and extra kernel module approach
> we can enable BTF with all global vars by default.
> The extra module will be shipped by all distros and tools
> like bpftrace might start using it.

Split BTF is currently limited to a single base BTF file. We'd need more
patches for pahole to support multiple --btf_base files: e.g.
vmlinux.btf and vmlinux-variables.btf. There's also the question of
modules: presumably we wouldn't try to have "$MODULE" and
"$MODULE-btf-extra" modules due to the added complexity. I doubt the
space savings would be worth it.

I can look into these concerns, but if possible I would like to proceed
with this series, as it is a separate concern from the exact mechanism
by which we include extra BTF into the kernel.

Thanks,
Stephen