Hi Ihor,
On 2024-12-11 00:17:02+0000, Ihor Solodrai wrote:
> 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 $(KBUILD_BUILD_TIMESTAMP),y) += --btf_features=reproducible_build
>
> Hi Thomas,
>
> There are a couple of issues with reproducible_build flag which I
> think are worth mentioning here. I don't know all the reasons behind
> adding this now, and it's optional too, so feel free to discard my
> comments.
>
> Currently with this flag, the BTF output is deterministic for a given
> order of DWARF compilation units. So the BTF will be the same for the
> same vmlinux binary. However, if the vmlinux is rebuilt due to an
> incremental change in a source code, my understanding is that there is
> no guarantee that DWARF CUs will be in the same order in the binary.
The goal behind reproducible builds is to produce bit-by-bit idential
binaries. If the CUs are in a different order then that requirement
would have been broken there already.
For an incremental build a full relink with *all* CUs is done, not only
the changed once, so the order should always be the same.
> At the same time, reproducible_build slows down BTF generation by
> 30-50%, maybe more depending on the kernel config.
If a user explicitly requests reproducibility then they should get it,
even if it is slower.
> Hopefully these problems will be solved in upcoming pahole releases.
I don't see it as big problem. This is used for release builds, not
during development.
> Question: why KBUILD_BUILD_TIMESTAMP flag? Isn't it more appropriate
> to use a separate flag for this particular feature?
Adding an additional variable would need to be documented and would
makes the feature harder to use. KBUILD_BUILD_TIMESTAMP already needs to
be set by the user if they are building for reproducibility.
> > ifneq ($(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
> >
> >
>
