Re: [PATCH bpf-next 2/2] selftests/bpf: do not update vmlinux.h unnecessarily

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2024-08-28 at 17:46 +0000, Ihor Solodrai wrote:
> %.bpf.o objects depend on vmlinux.h, which makes them transitively
> dependent on unnecessary libbpf headers. However vmlinux.h doesn't
> actually change as often.
> 
> When generating vmlinux.h, compare it to a previous version and update
> it only if there are changes.
> 
> Example of build time improvement (after first clean build):
>   $ touch ../../../lib/bpf/bpf.h
>   $ time make -j8
> Before: real  1m37.592s
> After:  real  0m27.310s
> 
> Notice that %.bpf.o gen step is skipped if vmlinux.h hasn't changed.
> 
> Link: https://lore.kernel.org/bpf/CAEf4BzY1z5cC7BKye8=A8aTVxpsCzD=p1jdTfKC7i0XVuYoHUQ@mail.gmail.com
> 
> Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
> ---

Unfortunately, I think that this is a half-measure.
E.g. the following command forces tests rebuild for me:

  touch ../../../../kernel/bpf/verifier.c; \
  make -j22 -C ../../../../; \
  time make test_progs

To workaround this we need to enable reproducible_build option:

    diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
    index b75f09f3f424..8cd648f3e32b 100644
    --- a/scripts/Makefile.btf
    +++ b/scripts/Makefile.btf
    @@ -19,7 +19,7 @@ pahole-flags-$(call test-ge, $(pahole-ver), 125)      += --skip_encoding_btf_inconsis
     else

     # Switch to using --btf_features for v1.26 and later.
    -pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
    +pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs,reproducible_build

     ifneq ($(KBUILD_EXTMOD),)
     module-pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=distilled_base

Question to the mailing list: do we want this?

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