%.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> --- tools/testing/selftests/bpf/Makefile | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile index 53cc13b92ee2..7660d19b66c2 100644 --- a/tools/testing/selftests/bpf/Makefile +++ b/tools/testing/selftests/bpf/Makefile @@ -399,10 +399,14 @@ $(HOST_BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile) \ DESTDIR=$(HOST_SCRATCH_DIR)/ prefix= all install_headers endif +# vmlinux.h is first dumped to a temprorary file and then compared to +# the previous version. This helps to avoid unnecessary re-builds of +# $(TRUNNER_BPF_OBJS) $(INCLUDE_DIR)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL) | $(INCLUDE_DIR) ifeq ($(VMLINUX_H),) $(call msg,GEN,,$@) - $(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@ + $(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $(INCLUDE_DIR)/.vmlinux.h.tmp + $(Q)cmp -s $(INCLUDE_DIR)/.vmlinux.h.tmp $@ || mv $(INCLUDE_DIR)/.vmlinux.h.tmp $@ else $(call msg,CP,,$@) $(Q)cp "$(VMLINUX_H)" $@ -- 2.34.1
