From: Winston Wen <wentao@uniontech.com>

Add a new kernel hacking option CONFIG_NO_AUTO_INLINE that prevents the
compiler from auto-inlining functions not explicitly marked with the
'inline' keyword.

This enhancement improves function tracer capabilities as it can only
trace functions that haven't been inlined by the compiler.

Previous discussions:

Link: https://lore.kernel.org/all/20181028130945.23581-3-changbin.du@gmail.com/

This patch is modified from commit 917fad29febd ("kernel hacking: add a
config option to disable compiler auto-inlining") which can be founded
in linux-next-history:

Link: https://web.git.kernel.org/pub/scm/linux/kernel/git/next/linux-next-history.git/commit/?id=917fad29febd

Unlike the original implementation, this patch:

1. Make it depends on CC_IS_GCC,
   as Clang 18.1.3 break test_bitmap_const_eval() in lib/test_bitmap.c

2. Make it depends on X86 and LOONGARCH,
   as I haven't test other architectures

3. Removes unnecessary cc-option checks per commit 7d73c3e9c514
   ("Makefile: remove stale cc-option checks").

4. Update help information.

Cc: Changbin Du <changbin.du@gmail.com>
Co-developed-by: Chen Linxuan <chenlinxuan@uniontech.com>
Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
Signed-off-by: Winston Wen <wentao@uniontech.com>
---
 Makefile          |  6 ++++++
 lib/Kconfig.debug | 15 +++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/Makefile b/Makefile
index c91afd55099ea6bc7ed89bc3ef9cf58a498596d4..52da2c27f94167d2d2892a67c065e5df15efbde0 100644
--- a/Makefile
+++ b/Makefile
@@ -1071,6 +1071,12 @@ endif
 # Ensure compilers do not transform certain loops into calls to wcslen()
 KBUILD_CFLAGS += -fno-builtin-wcslen
 
+ifdef CONFIG_NO_AUTO_INLINE
+KBUILD_CFLAGS   += -fno-inline-functions \
+		   -fno-inline-small-functions \
+		   -fno-inline-functions-called-once
+endif
+
 # change __FILE__ to the relative path to the source directory
 ifdef building_out_of_srctree
 KBUILD_CPPFLAGS += $(call cc-option,-ffile-prefix-map=$(srcroot)/=)
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 9fe4d8dfe5782981cc0b2b6614106aac570bd229..2ebb4802886ab4a48841dbdd8973b7d9883548b9 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -436,8 +436,23 @@ config GDB_SCRIPTS
 	  instance. See Documentation/process/debugging/gdb-kernel-debugging.rst
 	  for further details.
 
+
 endif # DEBUG_INFO
 
+config NO_AUTO_INLINE
+	bool "Disable compiler auto-inline optimizations (EXPERIMENTAL)"
+	default n
+	depends on CC_IS_GCC && (X86 || LOONGARCH)
+	help
+	  This will prevent the compiler from optimizing the kernel by
+	  auto-inlining functions not marked with the inline keyword.
+	  With this option, only functions explicitly marked with
+	  "inline" will be inlined. This will allow the function tracer
+	  to trace more functions because it only traces functions that
+	  the compiler has not inlined.
+
+	  If unsure, select N.
+
 config FRAME_WARN
 	int "Warn for stack frames larger than"
 	range 0 8192

-- 
2.48.1

On 4/16/25 2:44 AM, Chen Linxuan via B4 Relay wrote:
> 2. Make it depends on X86 and LOONGARCH,
>     as I haven't test other architectures

That sounds weird to me. Shouldn't this option be made architecture-
independent?

> +config NO_AUTO_INLINE
> +	bool "Disable compiler auto-inline optimizations (EXPERIMENTAL)"
> +	default n
> +	depends on CC_IS_GCC && (X86 || LOONGARCH)

Why "depends on CC_IS_GCC"? Please make sure that both gcc and clang are
supported.

Thanks,

Bart.

On 4/17/25 5:04 PM, Bart Van Assche wrote:
> On 4/16/25 2:44 AM, Chen Linxuan via B4 Relay wrote:
> > 2. Make it depends on X86 and LOONGARCH,
> >     as I haven't test other architectures
> 
> That sounds weird to me. Shouldn't this option be made architecture-
> independent?

It should, but I have only tested it on X86 and LOONGARCH.

> 
> > +config NO_AUTO_INLINE
> > +	bool "Disable compiler auto-inline optimizations (EXPERIMENTAL)"
> > +	default n
> > +	depends on CC_IS_GCC && (X86 || LOONGARCH)
> 
> Why "depends on CC_IS_GCC"? Please make sure that both gcc and clang are
> supported.

I make it depends on CC_IS_GCC because

1. Clang do not have `-fno-inline-small-functions`
   and `-fno-inline-functions-called-once`.

2. If we wrap those options with cc-option,
   Clang 18.1.3 fails to compile test_bitmap_const_eval(),
   with config KASAN and TEST_BITMAP is enabled.

   Comments above test_bitmap_const_eval() says that:

   > /*
   >  * FIXME: Clang breaks compile-time evaluations when KASAN and GCOV are enabled.
   >  * To workaround it, GCOV is force-disabled in Makefile for this configuration.
   >  */

   It seems there are some issues with Clang's compile-time evaluations.

So I think there are some ways to workaround this problem:

1. Make NO_AUTO_INLINE depends on CC_IS_GCC;
2. Make NO_AUTO_INLINE depends on KASAN=n;
3. Disable NO_AUTO_INLINE for TEST_BITMAP
   like how we handle CC_IS_CLANG && KASAN && GCOV in lib/Makefile:

   > ifeq ($(CONFIG_CC_IS_CLANG)$(CONFIG_KASAN),yy)
   > # FIXME: Clang breaks test_bitmap_const_eval when KASAN and GCOV are enabled
   > GCOV_PROFILE_test_bitmap.o := n
   > endif

Which one do you prefer or do you have any other suggestions?

> 
> Thanks,
> 
> Bart.