[LTP] [PATCH] build: Parallelize subdirectory recursion in generic_trunk_target

[LTP] [PATCH] build: Parallelize subdirectory recursion in generic_trunk_target

[Andrea Cervesato]

From: Andrea Cervesato <andrea.cervesato@mailbox.org>

Currently, generic_trunk_target traverses $(SUBDIRS) using a serial shell
'for' loop. This serializes directory traversal and prevents 'make -jN'
from building sibling directories in parallel.

Replace the shell loop with target-based recursion:
1. Map each subdirectory to a phony target: <dir>-all, <dir>-install,
   and <dir>-check.
2. Make 'all', 'install', and 'check' depend on these targets so GNU Make
   can dispatch them concurrently across available job slots.
3. In each recipe, strip the suffix with $(patsubst %-<action>,%,$@) to
   extract the directory name and invoke $(MAKE) -C <dir>.
4. Add explicit ordering dependencies where subdirectories produce
   libraries required by sibling directories (tools before testcases,
   kernel/lib before syscalls and mem, rpc-tirpc before basic_tests).

This cuts full build time by over 20% on multi-core systems.

Signed-off-by: Andrea Cervesato <andrea.cervesato@suse.com>
---
The following results show how this optimization moves compile times
really far from what we have now. With a single threaded build, LTP
would take approximately 12 minutes to compile. With a multi-threaded
build on 16 CPUs, this drops down to ~46 seconds.

time make -j16

Executed in   45.96 secs    fish           external
   usr time  519.11 secs    0.24 millis  519.11 secs
   sys time  167.13 secs    1.08 millis  167.13 secs
---
 Makefile                            |  2 ++
 include/mk/generic_trunk_target.inc | 35 ++++++++++++++++++++++++++++-------
 testcases/kernel/Makefile           |  3 +++
 testcases/network/rpc/Makefile      |  3 +++
 4 files changed, 36 insertions(+), 7 deletions(-)

diff --git a/Makefile b/Makefile
index 2efbe8c6c..0abb5c6b4 100644
--- a/Makefile
+++ b/Makefile
@@ -63,6 +63,8 @@ CHECK_TARGETS		:= $(addsuffix -check,testcases lib)
 all: $(addsuffix -all,$(COMMON_TARGETS)) Version
 
 $(MAKE_TARGETS): lib-all libs-all
+# tools contains apicmds/tst_kvercmp required by some testcases
+testcases-all: tools-all
 
 .PHONY: include-all include-install
 include-install: $(top_builddir)/include/config.h include/mk/config.mk include-all
diff --git a/include/mk/generic_trunk_target.inc b/include/mk/generic_trunk_target.inc
index f9db7896a..6975507e8 100644
--- a/include/mk/generic_trunk_target.inc
+++ b/include/mk/generic_trunk_target.inc
@@ -83,15 +83,36 @@ ifeq ($(strip $(SUBDIRS)),)
 	$(warning CURDIR is: $(CURDIR))
 	$(error SUBDIRS empty -- did you want generic_leaf_target instead?)
 else
-$(RECURSIVE_TARGETS): %: | $(SUBDIRS)
+# Recurse into subdirectories in parallel instead of using a serial shell loop.
+# Each subdirectory is mapped to a phony target (<dir>-all, <dir>-install, etc.).
+# GNU Make can then schedule sibling directories concurrently under -jN.
+all:     $(addsuffix -all,$(SUBDIRS))
+install: $(addsuffix -install,$(SUBDIRS))
+check:   $(addsuffix -check,$(SUBDIRS))
+
+.PHONY: $(addsuffix -all,$(SUBDIRS)) $(addsuffix -install,$(SUBDIRS)) $(addsuffix -check,$(SUBDIRS))
+
+# For each target (e.g. 'open-all'), strip the suffix to get the directory name,
+# build trunk-local targets first, and ensure the subdirectory exists.
+$(addsuffix -all,$(SUBDIRS)): trunk-all | $(SUBDIRS) $(MAKE_DEPS)
 ifdef VERBOSE
-	@set -e; for dir in $(SUBDIRS); do \
-	    $(MAKE) -C $$dir -f "$(abs_srcdir)/$$dir/Makefile" $@; \
-	done
+	$(MAKE) -C "$(patsubst %-all,%,$@)" -f "$(abs_srcdir)/$(patsubst %-all,%,$@)/Makefile" all
 else
-	@set -e; for dir in $(SUBDIRS); do \
-	    $(MAKE) --no-print-directory -C $$dir -f "$(abs_srcdir)/$$dir/Makefile" $@; \
-	done
+	@$(MAKE) --no-print-directory -C "$(patsubst %-all,%,$@)" -f "$(abs_srcdir)/$(patsubst %-all,%,$@)/Makefile" all
+endif
+
+$(addsuffix -install,$(SUBDIRS)): trunk-install | $(SUBDIRS) $(MAKE_DEPS)
+ifdef VERBOSE
+	$(MAKE) -C "$(patsubst %-install,%,$@)" -f "$(abs_srcdir)/$(patsubst %-install,%,$@)/Makefile" install
+else
+	@$(MAKE) --no-print-directory -C "$(patsubst %-install,%,$@)" -f "$(abs_srcdir)/$(patsubst %-install,%,$@)/Makefile" install
+endif
+
+$(addsuffix -check,$(SUBDIRS)): trunk-check | $(SUBDIRS) $(MAKE_DEPS)
+ifdef VERBOSE
+	$(MAKE) -C "$(patsubst %-check,%,$@)" -f "$(abs_srcdir)/$(patsubst %-check,%,$@)/Makefile" check
+else
+	@$(MAKE) --no-print-directory -C "$(patsubst %-check,%,$@)" -f "$(abs_srcdir)/$(patsubst %-check,%,$@)/Makefile" check
 endif
 endif
 
diff --git a/testcases/kernel/Makefile b/testcases/kernel/Makefile
index 29e26742d..3bf5e433e 100644
--- a/testcases/kernel/Makefile
+++ b/testcases/kernel/Makefile
@@ -54,3 +54,6 @@ FILTER_OUT_DIRS		+= containers controllers device-drivers fs io ipc mem \
 endif
 
 include $(top_srcdir)/include/mk/generic_trunk_target.mk
+
+# libkerntest.a in lib/ must be built before syscalls and mem consumers
+syscalls-all mem-all: lib-all
diff --git a/testcases/network/rpc/Makefile b/testcases/network/rpc/Makefile
index 6ecb1a665..8d12bdf1e 100644
--- a/testcases/network/rpc/Makefile
+++ b/testcases/network/rpc/Makefile
@@ -9,3 +9,6 @@ include $(top_srcdir)/include/mk/env_pre.mk
 INSTALL_TARGETS		:= *.sh
 
 include $(top_srcdir)/include/mk/generic_trunk_target.mk
+
+# librpc-tirpc.a in rpc-tirpc/ must be built before basic_tests
+basic_tests-all: rpc-tirpc-all

---
base-commit: 2ae4cfe69b9010018f2e5df6fa4bbba21810d4f2
change-id: 20261020-build_system_optimize-6cd0343f08e1

Best regards,
--  
Andrea Cervesato <andrea.cervesato@suse.com>


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Re: [LTP] build: Parallelize subdirectory recursion in generic_trunk_target

[linuxtestproject.agent]

Hi Andrea,

On 20 Oct 2026, Andrea Cervesato wrote:
> build: Parallelize subdirectory recursion in generic_trunk_target

> diff --git a/testcases/kernel/Makefile b/testcases/kernel/Makefile
> +syscalls-all mem-all: lib-all

Medium: running make -j8 directly in testcases/kernel/syscalls or
testcases/kernel/mem bypasses this parent dependency, allowing sibling
leaves to rebuild the same missing libkerntest.a concurrently.
This races object/archive writes and can cause intermittent link failures.
Build shared prerequisites once at these subtree entry points before
launching their children.

Verdict - Needs revision

---
Note:

The agent can sometimes produce false positives although often its
findings are genuine. If you find issues with the review, please
comment this email or ignore the suggestions.

Regards,
LTP AI Reviewer

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