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

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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@xxxxxxxxxx>
Reviewed-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
Tested-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
Reviewed-by: Nicolas Schier <n.schier@xxxxxx>
Tested-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Acked-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
---
[...]
@@ -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@xxxxxxxxx>




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