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

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On Wed, Sep 7, 2022 at 12:07 PM Stephen Brennan
<stephen.s.brennan@xxxxxxxxxx> wrote:
>
> Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes:
> > On Fri, Aug 26, 2022 at 11:54 AM Stephen Brennan
> > <stephen.s.brennan@xxxxxxxxxx> 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.

This thread was dormant for 8 days.
That's a poor example of "intend to support".


> > 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.

Not an option. Sorry.



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