On Wed, Aug 28, 2024 at 3:02 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote: > > 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@xxxxxxxxxxxxxx > > > > Signed-off-by: Ihor Solodrai <ihor.solodrai@xxxxx> > > --- > > 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? I don't think so. Too drastic just to save rebuild time. imo the patch is good enough as-is.
