Re: [PATCH bpf-next] kbuild, bpf: Enable reproducible BTF generation

[Ihor Solodrai]

On Tuesday, December 10th, 2024 at 3:23 PM, Thomas Weißschuh <linux@xxxxxxxxxxxxxx> wrote:

> 
> 
> Pahole v1.27 added a new BTF generation feature to support
> reproducibility in the face of multithreading.
> Enable it if supported and reproducible builds are requested.
> 
> As unknown --btf_features are ignored, avoid the test for the pahole
> version to keep the line readable.
> 
> Fixes: b4f72786429c ("scripts/pahole-flags.sh: Parse DWARF and generate BTF with multithreading.")
> Fixes: 72d091846de9 ("kbuild: avoid too many execution of scripts/pahole-flags.sh")
> Link: https://lore.kernel.org/lkml/4154d202-5c72-493e-bf3f-bce882a296c6@xxxxxxxxxx/
> Link: https://lore.kernel.org/lkml/20240322-pahole-reprodicible-v1-1-3eaafb1842da@xxxxxxxxxxxxxx/
> Signed-off-by: Thomas Weißschuh linux@xxxxxxxxxxxxxx
> 
> ---
> scripts/Makefile.btf | 1 +
> 1 file changed, 1 insertion(+)
> 
> diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
> index c3cbeb13de503555adcf00029a0b328e74381f13..da23265bc8b3cf43c0a1c89fbc4f53815a290e13 100644
> --- a/scripts/Makefile.btf
> +++ b/scripts/Makefile.btf
> @@ -22,6 +22,7 @@ else
> 
> # Switch to using --btf_features for v1.26 and later.
> pahole-flags-$(call test-ge, $(pahole-ver), 126) = -j$(JOBS) --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
> +pahole-flags-$(if $(KBUILD_BUILD_TIMESTAMP),y) += --btf_features=reproducible_build

Hi Thomas,

There are a couple of issues with reproducible_build flag which I
think are worth mentioning here. I don't know all the reasons behind
adding this now, and it's optional too, so feel free to discard my
comments.

Currently with this flag, the BTF output is deterministic for a given
order of DWARF compilation units. So the BTF will be the same for the
same vmlinux binary. However, if the vmlinux is rebuilt due to an
incremental change in a source code, my understanding is that there is
no guarantee that DWARF CUs will be in the same order in the binary.

At the same time, reproducible_build slows down BTF generation by
30-50%, maybe more depending on the kernel config.

Hopefully these problems will be solved in upcoming pahole releases.

Question: why KBUILD_BUILD_TIMESTAMP flag? Isn't it more appropriate
to use a separate flag for this particular feature?

Thanks.

> 
> ifneq ($(KBUILD_EXTMOD),)
> module-pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=distilled_base
> 
> ---
> base-commit: 7cb1b466315004af98f6ba6c2546bb713ca3c237
> change-id: 20241124-pahole-reproducible-2b879ac8bdab
> 
> Best regards,
> --
> Thomas Weißschuh linux@xxxxxxxxxxxxxx
> 
>