[v2,3/5] kernel hacking: new config NO_AUTO_INLINE to disable compiler auto-inline optimizations

Commit Message

Du, Changbin May 2, 2018, 1:44 p.m. UTC

From: Changbin Du <[e-mail]>

This patch add a new kernel hacking option NO_AUTO_INLINE. Selecting
this option 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.

Signed-off-by: Changbin Du <[e-mail]>
Cc: Steven Rostedt <[e-mail]>

---
v2: Some grammar updates from Steven.
---
 Makefile          |  6 ++++++
 lib/Kconfig.debug | 18 ++++++++++++++++++
 2 files changed, 24 insertions(+)

Comments

Steven Rostedt May 2, 2018, 2:07 p.m. UTC | #1

On Wed,  2 May 2018 21:44:58 +0800
[e-mail] wrote:

> From: Changbin Du <[e-mail]>
> 
> This patch add a new kernel hacking option NO_AUTO_INLINE. Selecting
> this option 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.
> 
> Signed-off-by: Changbin Du <[e-mail]>
> Cc: Steven Rostedt <[e-mail]>

I'm fine with this patch if others are OK with it too.

Acked-by: Steven Rostedt (VMware) <[e-mail]>

-- Steve

> 
> ---
> v2: Some grammar updates from Steven.
> ---
>  Makefile          |  6 ++++++
>  lib/Kconfig.debug | 18 ++++++++++++++++++
>  2 files changed, 24 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index 619a85a..eb694f6 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -775,6 +775,12 @@ KBUILD_CFLAGS 	+= $(call cc-option, -femit-struct-debug-baseonly) \
>  		   $(call cc-option,-fno-var-tracking)
>  endif
>  
> +ifdef CONFIG_NO_AUTO_INLINE
> +KBUILD_CFLAGS   += $(call cc-option, -fno-inline-functions) \
> +		   $(call cc-option, -fno-inline-small-functions) \
> +		   $(call cc-option, -fno-inline-functions-called-once)
> +endif
> +
>  ifdef CONFIG_FUNCTION_TRACER
>  ifndef CC_FLAGS_FTRACE
>  CC_FLAGS_FTRACE := -pg
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index c40c7b7..ab55801 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -198,6 +198,24 @@ config GDB_SCRIPTS
>  	  instance. See Documentation/dev-tools/gdb-kernel-debugging.rst
>  	  for further details.
>  
> +config NO_AUTO_INLINE
> +	bool "Disable compiler auto-inline optimizations"
> +	default n
> +	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.
> +
> +	  Enabling this function can help debugging a kernel if using
> +	  the function tracer. But it can also change how the kernel
> +	  works, because inlining functions may change the timing,
> +	  which could make it difficult while debugging race conditions.
> +
> +	  If unsure, select N.
> +
>  config ENABLE_WARN_DEPRECATED
>  	bool "Enable __deprecated logic"
>  	default y

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Arnd Bergmann May 2, 2018, 8:27 p.m. UTC | #2

On Wed, May 2, 2018 at 9:44 AM,  <[e-mail]> wrote:
> From: Changbin Du <[e-mail]>
>
> This patch add a new kernel hacking option NO_AUTO_INLINE. Selecting
> this option 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.
>
> Signed-off-by: Changbin Du <[e-mail]>
> Cc: Steven Rostedt <[e-mail]>

Should this be closer to CONFIG_OPTIMIZE_INLINING or
possibly mutually exclusive with it?

       Arnd
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Du, Changbin May 3, 2018, 1 p.m. UTC | #3

On Wed, May 02, 2018 at 04:27:47PM -0400, Arnd Bergmann wrote:
> On Wed, May 2, 2018 at 9:44 AM,  <[e-mail]> wrote:
> > From: Changbin Du <[e-mail]>
> >
> > This patch add a new kernel hacking option NO_AUTO_INLINE. Selecting
> > this option 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.
> >
> > Signed-off-by: Changbin Du <[e-mail]>
> > Cc: Steven Rostedt <[e-mail]>
> 
> Should this be closer to CONFIG_OPTIMIZE_INLINING or
> possibly mutually exclusive with it?
>
They are not related I think. CONFIG_OPTIMIZE_INLINING only has effect on
functions which are explicitly marked as inline.

>        Arnd

Steven Rostedt May 3, 2018, 2:16 p.m. UTC | #4

On Thu, 3 May 2018 21:00:13 +0800
"Du, Changbin" <[e-mail]> wrote:

> On Wed, May 02, 2018 at 04:27:47PM -0400, Arnd Bergmann wrote:
> > On Wed, May 2, 2018 at 9:44 AM,  <[e-mail]> wrote:  
> > > From: Changbin Du <[e-mail]>
> > >
> > > This patch add a new kernel hacking option NO_AUTO_INLINE. Selecting
> > > this option 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.
> > >
> > > Signed-off-by: Changbin Du <[e-mail]>
> > > Cc: Steven Rostedt <[e-mail]>  
> > 
> > Should this be closer to CONFIG_OPTIMIZE_INLINING or
> > possibly mutually exclusive with it?
> >  
> They are not related I think. CONFIG_OPTIMIZE_INLINING only has effect on
> functions which are explicitly marked as inline.
> 

Agreed, as OPTIMIZE_INLINING is to make functions marked inline not to
be inlined. And I just noticed (doing a git grep), that that config is
now only available in arch/x86. Maybe it always was, but sparc
undefines it for vclock_gettime.c.

$ git grep OPTIMIZE_INLIN
arch/sparc/vdso/vdso32/vclock_gettime.c:#undef  CONFIG_OPTIMIZE_INLINING
arch/x86/Kconfig.debug:config OPTIMIZE_INLINING
arch/x86/configs/i386_defconfig:CONFIG_OPTIMIZE_INLINING=y
arch/x86/configs/x86_64_defconfig:CONFIG_OPTIMIZE_INLINING=y
arch/x86/entry/vdso/vdso32/vclock_gettime.c:#undef CONFIG_OPTIMIZE_INLINING
include/linux/compiler-gcc.h:    !defined(CONFIG_OPTIMIZE_INLINING) || (__GNUC__ < 4)
kernel/configs/tiny.config:CONFIG_OPTIMIZE_INLINING=y

-- Steve
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Patch

diff --git a/Makefile b/Makefile
index 619a85a..eb694f6 100644
--- a/Makefile
+++ b/Makefile
@@ -775,6 +775,12 @@  KBUILD_CFLAGS 	+= $(call cc-option, -femit-struct-debug-baseonly) \
 		   $(call cc-option,-fno-var-tracking)
 endif
 
+ifdef CONFIG_NO_AUTO_INLINE
+KBUILD_CFLAGS   += $(call cc-option, -fno-inline-functions) \
+		   $(call cc-option, -fno-inline-small-functions) \
+		   $(call cc-option, -fno-inline-functions-called-once)
+endif
+
 ifdef CONFIG_FUNCTION_TRACER
 ifndef CC_FLAGS_FTRACE
 CC_FLAGS_FTRACE := -pg
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index c40c7b7..ab55801 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -198,6 +198,24 @@  config GDB_SCRIPTS
 	  instance. See Documentation/dev-tools/gdb-kernel-debugging.rst
 	  for further details.
 
+config NO_AUTO_INLINE
+	bool "Disable compiler auto-inline optimizations"
+	default n
+	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.
+
+	  Enabling this function can help debugging a kernel if using
+	  the function tracer. But it can also change how the kernel
+	  works, because inlining functions may change the timing,
+	  which could make it difficult while debugging race conditions.
+
+	  If unsure, select N.
+
 config ENABLE_WARN_DEPRECATED
 	bool "Enable __deprecated logic"
 	default y