[PATCH v3 13/17] perf pmu-events: Split big_c_string storage into standalone compilation unit

[Ian Rogers <irogers@google.com>]

Currently, jevents.py emits both the massive 2.8 MB big_c_string literal
and tens of thousands of compact_pmu_event struct arrays into a single
pmu-events.c compilation unit. Compiling this giant file takes ~2.2 seconds
on a single CPU core during Kbuild startup.

Refactor jevents.py to emit big_c_string into a dedicated
pmu-events-string.c compilation unit. This allows Kbuild to compile
pmu-events.o and pmu-events-string.o simultaneously in parallel across
two separate CPU cores, preserving 100% string deduplication and zero
dynamic ELF relocations while cutting C compilation latency in half.

Add pmu-events-string.c to tools/perf/.gitignore to ensure in-tree Kbuild
runs do not leave untracked generated files in the working directory.

To guarantee 100% backward compatibility with GNU Make 4.0+ (avoiding the
Make 4.3+ grouped target &: syntax which causes older Make versions like
4.2.1 to spawn multiple concurrent jevents.py processes during parallel
builds), implement a robust dependency chaining pattern:
  $(PMU_EVENTS_C): $(JEVENTS_DEPS)
  $(PMU_EVENTS_STRING_C): $(PMU_EVENTS_C)
          @:
This ensures jevents.py is invoked exactly once. If jevents.py aborts
early, Make's .DELETE_ON_ERROR: purges pmu-events.c, guaranteeing that
subsequent Make invocations correctly re-execute the script and overwrite
pmu-events-string.c, keeping the build fully self-correcting.

Tested-by: James Clark <james.clark@linaro.org>
Assisted-by: Gemini:gemini-3.1-pro-preview
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/.gitignore            |  1 +
 tools/perf/Makefile.perf         |  4 ++--
 tools/perf/pmu-events/Build      | 16 +++++++++++++++-
 tools/perf/pmu-events/jevents.py | 20 ++++++++++++++++----
 4 files changed, 34 insertions(+), 7 deletions(-)

diff --git a/tools/perf/.gitignore b/tools/perf/.gitignore
index 0f9451a6e39c..3b968c5158b8 100644
--- a/tools/perf/.gitignore
+++ b/tools/perf/.gitignore
@@ -38,6 +38,7 @@ arch/*/include/generated/
 trace/beauty/generated/
 pmu-events/arch/common/common/legacy-cache.json
 pmu-events/pmu-events.c
+pmu-events/pmu-events-string.c
 pmu-events/jevents
 pmu-events/metric_test.log
 pmu-events/empty-pmu-events.log
diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index 01da32cd8988..3be0be5eb43b 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -921,7 +921,7 @@ bpf-skel-clean:
 pmu-events-clean:
 ifeq ($(OUTPUT),)
 	$(call QUIET_CLEAN, pmu-events) $(RM) \
-		pmu-events/pmu-events.c \
+		pmu-events/pmu-events*.c \
 		pmu-events/metric_test.log \
 		pmu-events/test-empty-pmu-events.c \
 		pmu-events/empty-pmu-events.log
@@ -929,7 +929,7 @@ ifeq ($(OUTPUT),)
 		-name 'extra-metricgroups.json' -delete
 else # When an OUTPUT directory is present, clean up the copied pmu-events/arch directory.
 	$(call QUIET_CLEAN, pmu-events) $(RM) -r $(OUTPUT)pmu-events/arch \
-		$(OUTPUT)pmu-events/pmu-events.c \
+		$(OUTPUT)pmu-events/pmu-events*.c \
 		$(OUTPUT)pmu-events/metric_test.log \
 		$(OUTPUT)pmu-events/test-empty-pmu-events.c \
 		$(OUTPUT)pmu-events/empty-pmu-events.log
diff --git a/tools/perf/pmu-events/Build b/tools/perf/pmu-events/Build
index dc1df2d57ddc..95172a2a851f 100644
--- a/tools/perf/pmu-events/Build
+++ b/tools/perf/pmu-events/Build
@@ -1,7 +1,12 @@
 EMPTY_PMU_EVENTS_C = pmu-events/empty-pmu-events.c
 # pmu-events.c will be generated by jevents.py or copied from EMPTY_PMU_EVENTS_C
 PMU_EVENTS_C	=  $(OUTPUT)pmu-events/pmu-events.c
+PMU_EVENTS_STRING_C = $(OUTPUT)pmu-events/pmu-events-string.c
+
 pmu-events-y	+= pmu-events.o
+ifneq ($(NO_JEVENTS),1)
+pmu-events-y	+= pmu-events-string.o
+endif
 
 # pmu-events.c file is generated in the OUTPUT directory so it needs a
 # separate rule to depend on it properly
@@ -9,6 +14,10 @@ $(OUTPUT)pmu-events/pmu-events.o: $(PMU_EVENTS_C)
 	$(call rule_mkdir)
 	$(call if_changed_dep,cc_o_c)
 
+$(OUTPUT)pmu-events/pmu-events-string.o: $(PMU_EVENTS_STRING_C)
+	$(call rule_mkdir)
+	$(call if_changed_dep,cc_o_c)
+
 # Message for $(call echo-cmd,cp), possibly remove the src file from
 # the destination to save space in the build log.
 quiet_cmd_cp   = COPY    $(patsubst %$<,%,$@) <- $<
@@ -118,6 +127,7 @@ CUR_OUT_JSON	:= $(shell [ -d $(OUT_DIR) ] && find $(OUT_DIR) -type f)
 
 # Things in the OUTPUT directory but shouldn't be there as computed by
 # OUT_JSON and GEN_JSON.
+
 ORPHAN_FILES	:= $(filter-out $(OUT_JSON) $(GEN_JSON),$(CUR_OUT_JSON))
 
 # Message for $(call echo-cmd,mkd). There is already a mkdir message
@@ -224,6 +234,10 @@ endif
 # and inputs are dependencies.
 $(PMU_EVENTS_C): $(JEVENTS_DEPS)
 	$(call rule_mkdir)
-	$(Q)$(call echo-cmd,gen)$(PYTHON) $(JEVENTS_PY) $(JEVENTS_ARCH) $(JEVENTS_MODEL) $(OUT_DIR) $@
+	$(Q)$(call echo-cmd,gen)$(PYTHON) $(JEVENTS_PY) $(JEVENTS_ARCH) $(JEVENTS_MODEL) \
+            $(OUT_DIR) $(PMU_EVENTS_C) $(PMU_EVENTS_STRING_C)
+
+$(PMU_EVENTS_STRING_C): $(PMU_EVENTS_C)
+	@:
 
 endif # ifeq ($(NO_JEVENTS),1)
diff --git a/tools/perf/pmu-events/jevents.py b/tools/perf/pmu-events/jevents.py
index 3a1bcdcdc685..70a45e62f5d1 100755
--- a/tools/perf/pmu-events/jevents.py
+++ b/tools/perf/pmu-events/jevents.py
@@ -1422,6 +1422,8 @@ such as "arm/cortex-a34".''',
   )
   ap.add_argument(
       'output_file', type=argparse.FileType('w', encoding='utf-8'), nargs='?', default=sys.stdout)
+  ap.add_argument(
+      'output_string_file', type=argparse.FileType('w', encoding='utf-8'), nargs='?', default=None)
   _args = ap.parse_args()
 
   _args.output_file.write(f"""
@@ -1463,10 +1465,20 @@ struct pmu_table_entry {
     ftw(arch_path, [], preprocess_one_file)
 
   _bcs.compute()
-  _args.output_file.write('static const char *const big_c_string =\n')
-  for s in _bcs.big_string:
-    _args.output_file.write(s)
-  _args.output_file.write(';\n\n')
+  if not _args.output_string_file:
+    _args.output_file.write('static const char *const big_c_string =\n')
+    for s in _bcs.big_string:
+      _args.output_file.write(s)
+    _args.output_file.write(';\n\n')
+  else:
+    _args.output_string_file.write('/* SPDX-License-Identifier: GPL-2.0 */\n')
+    _args.output_string_file.write('/* Autogenerated by jevents.py */\n')
+    _args.output_string_file.write('const char big_c_string[] =\n')
+    for s in _bcs.big_string:
+      _args.output_string_file.write(s)
+    _args.output_string_file.write(';\n')
+    _args.output_string_file.close()
+    _args.output_file.write('extern const char big_c_string[];\n\n')
   for arch in archs:
     arch_path = f'{_args.starting_dir}/{arch}'
     ftw(arch_path, [], process_one_file)
-- 
2.54.0.563.g4f69b47b94-goog