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

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Date:   Sun, 22 Oct 2023 21:33:58 -0300
From:   Martin Rodriguez Reboredo <yakoyoku@...il.com>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        linux-kbuild@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Alan Maguire <alan.maguire@...cle.com>,
        Nicolas Schier <n.schier@....de>,
        Miguel Ojeda <ojeda@...nel.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>,
        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: [bpf-next PATCH v2 2/4] kbuild: avoid too many execution of
 scripts/pahole-flags.sh

On 10/18/23 12:19, 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@...nel.org>
> Reviewed-by: Alan Maguire <alan.maguire@...cle.com>
> Tested-by: Alan Maguire <alan.maguire@...cle.com>
> Reviewed-by: Nicolas Schier <n.schier@....de>
> Tested-by: Miguel Ojeda <ojeda@...nel.org>
> Acked-by: Miguel Ojeda <ojeda@...nel.org>
> ---
> [...]
> @@ -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

Would've used a tab, for legibility sake.

>   include-$(CONFIG_KASAN)		+= scripts/Makefile.kasan
>   include-$(CONFIG_KCSAN)		+= scripts/Makefile.kcsan
>   include-$(CONFIG_KMSAN)		+= scripts/Makefile.kmsan
> [...]
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>