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Date:   Tue, 17 Oct 2023 14:29:57 +0200
From:   Nicolas Schier <n.schier@avm.de>
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Alice Ryhl <aliceryhl@google.com>,
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        Andrii Nakryiko <andrii@kernel.org>,
        Benno Lossin <benno.lossin@proton.me>,
        =?utf-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
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        Jiri Olsa <jolsa@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Miguel Ojeda <ojeda@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nicolas Schier <nicolas@fjasle.eu>, Song Liu <song@kernel.org>,
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        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Yonghong Song <yonghong.song@linux.dev>, bpf@vger.kernel.org,
        rust-for-linux@vger.kernel.org
Subject: Re: [PATCH 2/4] kbuild: avoid too many execution of
 scripts/pahole-flags.sh
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Mail-Followup-To: Masahiro Yamada <masahiroy@kernel.org>,
        linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
        Alex Gaynor <alex.gaynor@gmail.com>,
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        Alice Ryhl <aliceryhl@google.com>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        Andrii Nakryiko <andrii@kernel.org>,
        Benno Lossin <benno.lossin@proton.me>,
        =?utf-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Daniel Borkmann <daniel@iogearbox.net>, Gary Guo <gary@garyguo.net>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Miguel Ojeda <ojeda@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
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        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Yonghong Song <yonghong.song@linux.dev>, bpf@vger.kernel.org,
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References: <20231017103742.130927-1-masahiroy@kernel.org>
 <20231017103742.130927-2-masahiroy@kernel.org>
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> Subject: kbuild: avoid too many execution of scripts/pahole-flags.sh
executions?

On Tue, Oct 17, 2023 at 07:37:40PM +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 exectuted more than 20 times during the kernel build

executed

> 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>
> ---

Thanks, looks good to me!

Reviewed-by: Nicolas Schier <n.schier@avm.de>