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

From: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
To: Masahiro Yamada <masahiroy@kernel.org>, linux-kbuild@vger.kernel.org
Cc: linux-kernel@vger.kernel.org,
	"Alan Maguire" <alan.maguire@oracle.com>,
	"Nicolas Schier" <n.schier@avm.de>,
	"Miguel Ojeda" <ojeda@kernel.org>,
	"Alex Gaynor" <alex.gaynor@gmail.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Alice Ryhl" <aliceryhl@google.com>,
	"Andreas Hindborg" <a.hindborg@samsung.com>,
	"Andrii Nakryiko" <andrii@kernel.org>,
	"Benno Lossin" <benno.lossin@proton.me>,
	"Björn 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>,
	"Nathan Chancellor" <nathan@kernel.org>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	"Nicolas Schier" <nicolas@fjasle.eu>,
	"Song Liu" <song@kernel.org>,
	"Stanislav Fomichev" <sdf@google.com>,
	"Wedson Almeida Filho" <wedsonaf@gmail.com>,
	"Yonghong Song" <yonghong.song@linux.dev>,
	bpf@vger.kernel.org, rust-for-linux@vger.kernel.org
Subject: Re: [bpf-next PATCH v2 2/4] kbuild: avoid too many execution of scripts/pahole-flags.sh
Date: Sun, 22 Oct 2023 21:33:58 -0300	[thread overview]
Message-ID: <4c926cff-a3f2-445e-9ca2-9effad423cb7@gmail.com> (raw)
In-Reply-To: <20231018151950.205265-2-masahiroy@kernel.org>

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@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>
> ---
> [...]
> @@ -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@gmail.com>