[PATCH v3 05/17] perf build: Decouple pmu-events from prepare umbrella target

[Ian Rogers]

Currently, the $(LIBPMU_EVENTS_IN) sub-make depends on the massive
"prepare" umbrella target. Because "prepare" depends on external
libraries (libapi, libperf, etc.) as well as dozens of generated
headers, make completely serializes the launch of the pmu-events
sub-make behind some of those unrelated prerequisites.

Since pmu-events is a large compilation unit, unblock its startup by
binding it directly to only $(LIBPERF) instead of prepare. This allows
background python generation scripts to overlap simultaneously with
the rest of the build.

Testing a parallel build (make -j28 clean all) shows improvements:
  Before:
    real    0m27.642s
    user    2m32.356s
    sys     0m26.683s

  After:
    real    0m22.254s
    user    2m32.810s
    sys     0m24.646s

This reclaims over 5 full seconds of build latency (~19.5% overall
reduction) by elevating average CPU concurrency from ~5.5 active cores
up to ~8 active cores.

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 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index c81797ceec42..c66af4c825fd 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -550,7 +550,7 @@ build := -f $(srctree)/tools/build/Makefile.build dir=. obj
 $(PERF_IN): prepare FORCE
 	$(Q)$(MAKE) $(build)=perf
 
-$(LIBPMU_EVENTS_IN): FORCE prepare
+$(LIBPMU_EVENTS_IN): FORCE $(LIBPERF)
 	$(Q)$(MAKE) -f $(srctree)/tools/build/Makefile.build dir=pmu-events obj=pmu-events
 
 $(LIBPMU_EVENTS): $(LIBPMU_EVENTS_IN)
-- 
2.54.0.563.g4f69b47b94-goog