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Commit0b9efc991f("linux: use BR2_MAKE", 2023-04-10) replaced $(MAKE) with $(BR2_MAKE) in a number of recipes. As a consequence, the child make is unable to discover the job server, in some cases. In those cases, we get a warning such as: > warning: jobserver unavailable: using -j1. Add `+' to parent make rule. See [1] and [2]. Falling back to single job can make build considerably longer. This longer build time issue can be reproduced in specific conditions. This situation happens when: 1. The top GNU Make is using a "pipe" jobserver. This is the default when GNU Make <= 4.3 is used (and v4.3 is the version inside the current Buildroot Docker reference image). Make > 4.3 changed the default jobserver style to "fifo". See [3][4]. With Make > 4.3, the issue can be reproduced by calling "make --jobserver-style=pipe ...". 2. The root filesystem is an initramfs linked into the Kernel (i.e. using the config BR2_TARGET_ROOTFS_INITRAMFS=y) 3. Buildroot per-package directories is used (i.e. using the config BR2_PER_PACKAGE_DIRECTORIES=y) 4. The build is made in parallel, with 2 or more jobs. For example: make -j$(nproc) Overall, the issue can be reproduced with the commands: utils/docker-run cat >.config <<EOF BR2_aarch64=y BR2_TOOLCHAIN_EXTERNAL=y BR2_PER_PACKAGE_DIRECTORIES=y BR2_LINUX_KERNEL=y BR2_LINUX_KERNEL_USE_ARCH_DEFAULT_CONFIG=y BR2_TARGET_ROOTFS_INITRAMFS=y EOF make olddefconfig make -j$(nproc) The Linux Kernel is built once (with a fake empty initramfs cpio image), when build log is showing ">>> linux 7.2.6 Building". Then, at the end of the Buildroot build, once the CPIO filesystem is complete, it is integrated inside the Kernel with an extra "make" invocation when the build log shows ">>> Rebuilding kernel with initramfs". This second kernel "make" is not expected to rebuild the whole kernel, since compiled objects from the first compilation are still here. However, in the described conditions, the second kernel is fully rebuilt. This is an undesired behaviour. The Make jobserver issue adds up to that: this second full kernel is rebuilt with only one job, which can significantly increase the build time. Running the previous example on a host with 128 CPUs: without this change, build takes 1h5m, with this change, build takes 7m. Note: using GNU Make >= 4.4 (with a fifo jobserver style by default) or removing per-package directories no longer produces the issue. For reference, running the example, without this change and without per-package directories on the same host, the build takes 10 mins. This commit improves the situation by prefixing the recipe with "+", to inform the parent Make that $(BR2_MAKE) can deal with the job server. This will give a chance to do jobs in parallel, in general. Note: the pkg-generic.mk infra already has '+' for _BUILD_CMDS, which is why other $(BR2_MAKE) invocations in linux.mk does not need this '+'. See [5]. [1] https://www.gnu.org/software/make/manual/html_node/Error-Messages.html [2] https://www.gnu.org/software/make/manual/html_node/MAKE-Variable.html [3] https://www.gnu.org/software/make/manual/html_node/Options-Summary.html#index-_002d_002djobserver_002dstyle [4] https://cgit.git.savannah.gnu.org/cgit/make.git/commit/?id=7ad2593b2d2bb5b9332f4444d8bf93ac6f958bc6 [5]069b33a30eCc: Arnout Vandecappelle <arnout@mind.be> Cc: Oleg Lyovin <ovlevin@sberdevices.ru> Cc: buildroot@buildroot.org Signed-off-by: Laszlo Ersek <laszlo.ersek@arm.com> [Julien: extend commit log] Signed-off-by: Julien Olivain <ju.o@free.fr> (cherry picked from commit86a56dcc17) Signed-off-by: Raphaël Mélotte <raphael.melotte@mind.be>
Buildroot is a simple, efficient and easy-to-use tool to generate embedded Linux systems through cross-compilation. The documentation can be found in docs/manual. You can generate a text document with 'make manual-text' and read output/docs/manual/manual.text. Online documentation can be found at https://buildroot.org/docs.html To build and use the buildroot stuff, do the following: 1) run 'make menuconfig' 2) select the target architecture and the packages you wish to compile 3) run 'make' 4) wait while it compiles 5) find the kernel, bootloader, root filesystem, etc. in output/images You do not need to be root to build or run buildroot. Have fun! Buildroot comes with a basic configuration for a number of boards. Run 'make list-defconfigs' to view the list of provided configurations. Please feed suggestions, bug reports, insults, and bribes back to the buildroot mailing list: buildroot@buildroot.org You can also find us on #buildroot on OFTC IRC. If you would like to contribute patches, please read https://buildroot.org/manual.html#submitting-patches