Re: [bpf-next PATCH v2 2/4] kbuild: avoid too many execution of scripts/pahole-flags.sh

Jiri Olsa
2023-10-19T08:47:49Z

On Thu, Oct 19, 2023 at 12:19:48AM +0900, Masahiro Yamada wrote:
> scripts/pahole-flags.sh is executed so many times.
>
> You can check how many times it is invoked during the build, as follows:
>
> $ cat <<EOF >> scripts/pahole-flags.sh
> > echo "scripts/pahole-flags.sh was executed" >&2
> > EOF
>
> $ make -s
> scripts/pahole-flags.sh was executed
> scripts/pahole-flags.sh was executed
> scripts/pahole-flags.sh was executed
> scripts/pahole-flags.sh was executed
> scripts/pahole-flags.sh was executed
> [ lots of repeated lines suppressed... ]
>
> This scripts is executed more than 20 times during the kernel build
> because PAHOLE_FLAGS is a recursively expanded variable and exported
> to sub-processes.
>
> With the GNU Make >= 4.4, it is executed more than 60 times because
> exported variables are also passed to other $(shell ) invocations.
> Without careful coding, it is known to cause an exponential fork
> explosion. [1]
>
> The use of $(shell ) in an exported recursive variable is likely wrong
> because $(shell ) is always evaluated due to the 'export' keyword, and
> the evaluation can occur multiple times by the nature of recursive
> variables.
>
> Convert the shell script to a Makefile, which is included only when
> CONFIG_DEBUG_INFO_BTF=y.
>
> [1]: https://savannah.gnu.org/bugs/index.php?64746
>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
> Tested-by: Alan Maguire <alan.maguire@oracle.com>
> Reviewed-by: Nicolas Schier <n.schier@avm.de>
> Tested-by: Miguel Ojeda <ojeda@kernel.org>
> Acked-by: Miguel Ojeda <ojeda@kernel.org>

nice!

Acked-by: Jiri Olsa <jolsa@kernel.org>

thanks,
jirka

> ---
>
> Changes in v2:
> - Fix a typo in commit description
> - Update MAINTAINERS
>
> MAINTAINERS | 2 +-
> Makefile | 4 +---
> scripts/Makefile.btf | 19 +++++++++++++++++++
> scripts/pahole-flags.sh | 30 ------------------------------
> 4 files changed, 21 insertions(+), 34 deletions(-)
> create mode 100644 scripts/Makefile.btf
> delete mode 100755 scripts/pahole-flags.sh
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 35977b269d5e..a08d558b1aaa 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -3742,7 +3742,7 @@ F: net/sched/act_bpf.c
> F: net/sched/cls_bpf.c
> F: samples/bpf/
> F: scripts/bpf_doc.py
> -F: scripts/pahole-flags.sh
> +F: scripts/Makefile.btf
> F: scripts/pahole-version.sh
> F: tools/bpf/
> F: tools/lib/bpf/
> diff --git a/Makefile b/Makefile
> index fed9a6cc3665..eaddec67e5e1 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -513,8 +513,6 @@ LZ4 = lz4c
> XZ = xz
> ZSTD = zstd
>
> -PAHOLE_FLAGS = $(shell PAHOLE=$(PAHOLE) $(srctree)/scripts/pahole-flags.sh)
> -
> CHECKFLAGS := -D__linux__ -Dlinux -D__STDC__ -Dunix -D__unix__ \
> -Wbitwise -Wno-return-void -Wno-unknown-attribute $(CF)
> NOSTDINC_FLAGS :=
> @@ -605,7 +603,6 @@ export KBUILD_RUSTFLAGS RUSTFLAGS_KERNEL RUSTFLAGS_MODULE
> export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
> export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_RUSTFLAGS_MODULE KBUILD_LDFLAGS_MODULE
> export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL KBUILD_RUSTFLAGS_KERNEL
> -export PAHOLE_FLAGS
>
> # Files to ignore in find ... statements
>
> @@ -1002,6 +999,7 @@ KBUILD_CPPFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
> # include additional Makefiles when needed
> include-y := scripts/Makefile.extrawarn
> include-$(CONFIG_DEBUG_INFO) += scripts/Makefile.debug
> +include-$(CONFIG_DEBUG_INFO_BTF)+= scripts/Makefile.btf
> include-$(CONFIG_KASAN) += scripts/Makefile.kasan
> include-$(CONFIG_KCSAN) += scripts/Makefile.kcsan
> include-$(CONFIG_KMSAN) += scripts/Makefile.kmsan
> diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
> new file mode 100644
> index 000000000000..82377e470aed
> --- /dev/null
> +++ b/scripts/Makefile.btf
> @@ -0,0 +1,19 @@
> +# SPDX-License-Identifier: GPL-2.0
> +
> +pahole-ver := $(CONFIG_PAHOLE_VERSION)
> +pahole-flags-y :=
> +
> +# pahole 1.18 through 1.21 can't handle zero-sized per-CPU vars
> +ifeq ($(call test-le, $(pahole-ver), 121),y)
> +pahole-flags-$(call test-ge, $(pahole-ver), 118) += --skip_encoding_btf_vars
> +endif
> +
> +pahole-flags-$(call test-ge, $(pahole-ver), 121) += --btf_gen_floats
> +
> +pahole-flags-$(call test-ge, $(pahole-ver), 122) += -j
> +
> +pahole-flags-$(CONFIG_PAHOLE_HAS_LANG_EXCLUDE) += --lang_exclude=rust
> +
> +pahole-flags-$(call test-ge, $(pahole-ver), 125) += --skip_encoding_btf_inconsistent_proto --btf_gen_optimized
> +
> +export PAHOLE_FLAGS := $(pahole-flags-y)
> diff --git a/scripts/pahole-flags.sh b/scripts/pahole-flags.sh
> deleted file mode 100755
> index 728d55190d97..000000000000
> --- a/scripts/pahole-flags.sh
> +++ /dev/null
> @@ -1,30 +0,0 @@
> -#!/bin/sh
> -# SPDX-License-Identifier: GPL-2.0
> -
> -extra_paholeopt=
> -
> -if ! [ -x "$(command -v ${PAHOLE})" ]; then
> - exit 0
> -fi
> -
> -pahole_ver=$($(dirname $0)/pahole-version.sh ${PAHOLE})
> -
> -if [ "${pahole_ver}" -ge "118" ] && [ "${pahole_ver}" -le "121" ]; then
> - # pahole 1.18 through 1.21 can't handle zero-sized per-CPU vars
> - extra_paholeopt="${extra_paholeopt} --skip_encoding_btf_vars"
> -fi
> -if [ "${pahole_ver}" -ge "121" ]; then
> - extra_paholeopt="${extra_paholeopt} --btf_gen_floats"
> -fi
> -if [ "${pahole_ver}" -ge "122" ]; then
> - extra_paholeopt="${extra_paholeopt} -j"
> -fi
> -if [ "${pahole_ver}" -ge "124" ]; then
> - # see PAHOLE_HAS_LANG_EXCLUDE
> - extra_paholeopt="${extra_paholeopt} --lang_exclude=rust"
> -fi
> -if [ "${pahole_ver}" -ge "125" ]; then
> - extra_paholeopt="${extra_paholeopt} --skip_encoding_btf_inconsistent_proto --btf_gen_optimized"
> -fi
> -
> -echo ${extra_paholeopt}
> --
> 2.40.1
>