[PATCH v3 10/17] perf build: Pre-generate BPF skeleton tooling during umbrella prepare phase

[Ian Rogers]

Currently, BPF skeleton generation rules (bpf_skel.mak) are evaluated as
part of util/Build. However, because LIBPERF_UTIL_IN explicitly depends
on the top-level static libbpf archive, Make completely blocked the
execution of bpftool bootstrap and skeleton generation until libbpf
finished compiling midway through the build.

Since bpftool bootstrap compiles its own independent copy of libbpf.a, it
does not depend on the top-level libbpf target.

Decouple early skeleton tooling generation by attaching bpf-skel-prepare
to the umbrella prepare target, exporting CONFIG_PERF_BPF_SKEL to ensure
accurate feature propagation. This allows Make to compile bpftool and
dump vmlinux.h in the background at build startup, eliminating the initial
sub-make startup bottleneck before BPF object compilation while keeping
100% of tooling rules perfectly encapsulated in bpf_skel.mak. Provide an
empty fallback target to ensure builds succeed when BPF skeletons are
disabled.

Tested-by: James Clark <james.clark@xxxxxxxxxx>
Assisted-by: Gemini:gemini-3.1-pro-preview
Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
---
 tools/perf/Makefile.perf | 8 ++++++--
 tools/perf/bpf_skel.mak  | 8 ++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index 30114a31ca76..a0426e914253 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -275,7 +275,7 @@ ifeq ($(PYLINT),1)
 endif
 
 export srctree OUTPUT RM CC CXX RUSTC CLANG LD AR CFLAGS CXXFLAGS RUST_FLAGS V BISON FLEX AWK LIBBPF READELF
-export HOSTCC HOSTLD HOSTAR HOSTCFLAGS SHELLCHECK MYPY PYLINT
+export HOSTCC HOSTLD HOSTAR HOSTCFLAGS SHELLCHECK MYPY PYLINT CONFIG_PERF_BPF_SKEL
 
 include $(srctree)/tools/build/Makefile.include
 
@@ -629,8 +629,12 @@ endif
 __build-dir = $(subst $(OUTPUT),,$(dir $@))
 build-dir   = $(or $(__build-dir),.)
 
+bpf-skel-prepare:
+	$(Q)$(MAKE) -f bpf_skel.mak bpf-skel-prepare
+
 prepare: $(OUTPUT)PERF-VERSION-FILE \
 	arm64-sysreg-defs \
+	bpf-skel-prepare \
 	$(LIBAPI) \
 	$(LIBPERF) \
 	$(LIBSUBCMD) \
@@ -967,7 +971,7 @@ FORCE:
 
 .PHONY: all install clean config-clean strip install-gtk
 .PHONY: shell_compatibility_test please_set_SHELL_PATH_to_a_more_modern_shell
-.PHONY: .FORCE-PERF-VERSION-FILE TAGS tags cscope FORCE prepare
+.PHONY: .FORCE-PERF-VERSION-FILE TAGS tags cscope FORCE prepare bpf-skel-prepare
 .PHONY: python_perf_target
 
 endif # force_fixdep
diff --git a/tools/perf/bpf_skel.mak b/tools/perf/bpf_skel.mak
index 30924ad140c4..1fbd8dfefa5e 100644
--- a/tools/perf/bpf_skel.mak
+++ b/tools/perf/bpf_skel.mak
@@ -15,6 +15,10 @@ ifeq ($(CONFIG_PERF_BPF_SKEL),y)
 BPFTOOL := $(SKEL_TOOL_TMP_OUT)/bootstrap/bpftool
 VMLINUX_H := $(SKEL_TOOL_OUT)/vmlinux.h
 
+.PHONY: bpf-skel-prepare
+bpf-skel-prepare: $(bpf_skel_deps)
+	@:
+
 define get_sys_includes
 $(shell $(1) $(2) -v -E - </dev/null 2>&1 \
        | sed -n '/<...> search starts here:/,/End of search list./{ s| \(/.*\)|-idirafter \1|p }') \
@@ -93,6 +97,10 @@ $(SKEL_OUT)/%.skel.h: $(SKEL_TMP_OUT)/%.bpf.o $(BPFTOOL)
 
 .PRECIOUS: $(SKEL_TMP_OUT)/%.bpf.o
 
+else # CONFIG_PERF_BPF_SKEL
+.PHONY: bpf-skel-prepare
+bpf-skel-prepare:
+	@:
 endif # CONFIG_PERF_BPF_SKEL
 
 clean:
-- 
2.54.0.563.g4f69b47b94-goog