Thread (11 messages) flat view 11 messages, 5 authors, 2021-10-18

Re: [PATCH 2/3] arm64: vdso32: lazily invoke COMPAT_CC

From: Nick Desaulniers <ndesaulniers@google.com>
Date: 2021-10-18 20:34:47
Also in: lkml, llvm 

On Sat, Oct 16, 2021 at 7:20 AM Masahiro Yamada [off-list ref] wrote:
On Fri, Oct 15, 2021 at 5:59 AM Nick Desaulniers
[off-list ref] wrote:
quoted
On Tue, Oct 12, 2021 at 8:03 PM Masahiro Yamada [off-list ref] wrote:
quoted
On Wed, Oct 13, 2021 at 8:46 AM Nick Desaulniers
[off-list ref] wrote:
quoted
When running the following command without arm-linux-gnueabi-gcc in
one's $PATH, the following warning is observed:

$ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make -j72 LLVM=1 mrproper
make[1]: arm-linux-gnueabi-gcc: No such file or directory

This is because KCONFIG is not run for mrproper, so CONFIG_CC_IS_CLANG
is not set, and we end up eagerly evaluating various variables that try
to invoke CC_COMPAT.

This is a similar problem to what was observed in
commit 3ec8a5b33dea ("kbuild: do not export LDFLAGS_vmlinux")

Cc: Masahiro Yamada <masahiroy@kernel.org>
Reported-by: Lucas Henneman <redacted>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

There are two ways to fix it:

  [1]: sink the error message to /dev/null
        (as in commit dc960bfeedb01cf832c5632ed1f3daed4416b142)
  [2]: use a recursively-expanded variable as you did.


"Simple variable (:=) vs recursive variable (=)" is a trade-off.

Please be careful about the cost when you try the [2] approach.



Simple variables are immediately expanded while parsing Makefile.
There are 7 call-sites for cc32-option, hence
the compiler is invoked 7 times for building vdso32,
0 times for cleaning.
(Since 57fd251c789647552d32d2fc51bedd4f90d70f9f,
try-run is no-op for 'make clean').




Recursive variables are expanded every time they are used.

IIUC, if_changed expands the command line 3 times.
There are 2 objects (note.o and vgettimeofday.o)
There are 7 call-sites for cc32-option.

So, the compiler is invoked 42 (3 * 2 * 7) times
for building vdso32.
With this patch applied:
$ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make LLVM=1 -j72
clean defconfig
$ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make LLVM=1 -j72
arch/arm64/kernel/vdso32/ V=1 | tr -s ' ' | cut -d ' ' -f 2 | grep
clang | wc -l
55
$ find arch/arm64/kernel/vdso32/ -name \*.o | xargs rm
$ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make LLVM=1 -j72
arch/arm64/kernel/vdso32/ V=1 | tr -s ' ' | cut -d ' ' -f 2 | grep
clang | wc -l
2

Prior to this series:
$ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make LLVM=1 -j72
clean defconfig
$ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make LLVM=1 -j72
arch/arm64/kernel/vdso32/ V=1 | tr -s ' ' | cut -d ' ' -f 2 | grep
clang | wc -l
55
$ find arch/arm64/kernel/vdso32/ -name \*.o | xargs rm
$ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make LLVM=1 -j72
arch/arm64/kernel/vdso32/ V=1 | tr -s ' ' | cut -d ' ' -f 2 | grep
clang | wc -l
2

With patch 3 applied, we can drop CROSS_COMPILE_COMPAT, and we now get:
$ ARCH=arm64 make LLVM=1 -j72 clean defconfig
ARCH=arm64 make LLVM=1 -j72 arch/arm64/kernel/vdso32/ V=1 | tr -s ' '
| cut -d ' ' -f 2 | grep clang | wc -l
44
$ find arch/arm64/kernel/vdso32/ -name \*.o | xargs rm
$ ARCH=arm64 make LLVM=1 -j72 arch/arm64/kernel/vdso32/ V=1 | tr -s '
' | cut -d ' ' -f 2 | grep clang | wc -l
2

Please confirm; perhaps my pipeline missed some invocations? Or was
there a different target you were referring to?





It is pointless to check the build commands.

I am talking about how many times $(call cc32-option, ) is evaluated.
Of course V=1 doesn't print the cc-option invocations! /s
quoted hunk ↗ jump to hunk

How about adding the following debug code?

(Everytime cc32-option is evaluated, a file "dummy-cc32-option-<PID>"
is created)



diff --git a/arch/arm64/kernel/vdso32/Makefile
b/arch/arm64/kernel/vdso32/Makefile
index 89299a26638b..e40365f5bc38 100644
--- a/arch/arm64/kernel/vdso32/Makefile
+++ b/arch/arm64/kernel/vdso32/Makefile
@@ -26,9 +26,9 @@ LD_COMPAT ?= $(CROSS_COMPILE_COMPAT)ld
 endif

 cc32-option = $(call try-run,\
-        $(CC_COMPAT) $(1) -c -x c /dev/null -o "$$TMP",$(1),$(2))
+        $(CC_COMPAT) $(1) -c -x c /dev/null -o "$$TMP"; touch
dummy-cc32-option-$$$$,$(1),$(2))
 cc32-disable-warning = $(call try-run,\
-       $(CC_COMPAT) -W$(strip $(1)) -c -x c /dev/null -o
"$$TMP",-Wno-$(strip $(1)))
+       $(CC_COMPAT) -W$(strip $(1)) -c -x c /dev/null -o "$$TMP";
touch dummy-cc32-option-$$$$,-Wno-$(strip $(1)))
heh, I usually add `($info $$VAR is [${VAR}]); \` debugging statements
to these. Touching files is another neat trick.

 # We cannot use the global flags to compile the vDSO files, the main reason
 # being that the 32-bit compiler may be older than the main (64-bit) compiler





Without this patch:

masahiro@grover:~/ref/linux$ rm dummy-cc32-*
masahiro@grover:~/ref/linux$ make -s LLVM=1 ARCH=arm64
CROSS_COMPILE_COMPAT=arm-linux-gnueabi- defconfig clean
arch/arm64/kernel/vdso32/ -j8
masahiro@grover:~/ref/linux$ ls -1 dummy-cc32-*
dummy-cc32-disable-warning-765530
dummy-cc32-option-765495
dummy-cc32-option-765500
dummy-cc32-option-765505
dummy-cc32-option-765510
dummy-cc32-option-765515
dummy-cc32-option-765520
dummy-cc32-option-765525




With this patch:



masahiro@grover:~/ref/linux$ rm dummy-cc32-*
masahiro@grover:~/ref/linux$ make -s LLVM=1 ARCH=arm64
CROSS_COMPILE_COMPAT=arm-linux-gnueabi- defconfig clean
arch/arm64/kernel/vdso32/ -j8
masahiro@grover:~/ref/linux$ ls -1 dummy-cc32-*
dummy-cc32-disable-warning-768908
dummy-cc32-disable-warning-768949
dummy-cc32-disable-warning-768990
dummy-cc32-disable-warning-769035
dummy-cc32-disable-warning-769076
dummy-cc32-disable-warning-769117
dummy-cc32-option-768871
dummy-cc32-option-768878
dummy-cc32-option-768883
dummy-cc32-option-768888
dummy-cc32-option-768893
dummy-cc32-option-768898
dummy-cc32-option-768903
dummy-cc32-option-768914
dummy-cc32-option-768919
dummy-cc32-option-768924
dummy-cc32-option-768929
dummy-cc32-option-768934
dummy-cc32-option-768939
dummy-cc32-option-768944
dummy-cc32-option-768955
dummy-cc32-option-768960
dummy-cc32-option-768965
dummy-cc32-option-768970
dummy-cc32-option-768975
dummy-cc32-option-768980
dummy-cc32-option-768985
dummy-cc32-option-768998
dummy-cc32-option-769005
dummy-cc32-option-769010
dummy-cc32-option-769015
dummy-cc32-option-769020
dummy-cc32-option-769025
dummy-cc32-option-769030
dummy-cc32-option-769041
dummy-cc32-option-769046
dummy-cc32-option-769051
dummy-cc32-option-769056
dummy-cc32-option-769061
dummy-cc32-option-769066
dummy-cc32-option-769071
dummy-cc32-option-769082
dummy-cc32-option-769087
dummy-cc32-option-769092
dummy-cc32-option-769097
dummy-cc32-option-769102
dummy-cc32-option-769107
dummy-cc32-option-769112






The diff of  the number of expansions:

cc32-option                        7    ->   42
cc32-disable-warning         1   ->    6
Indeed, thanks for confirming.  An unfortunate dichotomy.  Surely
there's another way to avoid cc-option+cc-disable-warning calls for
the `clean` target?  I'll change to just redirecting errors to
/dev/null with your Suggested-by tag, but this seems a bit of an
unfortunate situation.
-- 
Thanks,
~Nick Desaulniers

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