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

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Message-ID: <CAEf4Bzaxb1npVtH_CnFNrOJQxQF5t82_nZxqbaFLiE-rpk_jBg@mail.gmail.com>
Date:   Tue, 17 Oct 2023 12:56:52 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alex Gaynor <alex.gaynor@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Benno Lossin <benno.lossin@...ton.me>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Gary Guo <gary@...yguo.net>, Hao Luo <haoluo@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Miguel Ojeda <ojeda@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Schier <nicolas@...sle.eu>, Song Liu <song@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Yonghong Song <yonghong.song@...ux.dev>, bpf@...r.kernel.org,
        rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 2/4] kbuild: avoid too many execution of scripts/pahole-flags.sh

On Tue, Oct 17, 2023 at 3:38 AM Masahiro Yamada <masahiroy@...nel.org> 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 exectuted 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@...nel.org>
> ---
>
>  Makefile                |  4 +---
>  scripts/Makefile.btf    | 19 +++++++++++++++++++
>  scripts/pahole-flags.sh | 30 ------------------------------
>  3 files changed, 20 insertions(+), 33 deletions(-)
>  create mode 100644 scripts/Makefile.btf
>  delete mode 100755 scripts/pahole-flags.sh
>
> 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)

What if we just used := here? Wouldn't it avoid unnecessary multiple executions?

I don't make Makefile.btf approach, just curious why := doesn't work,
if it doesn't.

> -
>  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
>