linux-kernel - Re: [PATCH 3/5] kernel hacking: new config NO_AUTO_INLINE to disable compiler atuo-inline optimizations

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Date:   Tue, 1 May 2018 10:54:20 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     changbin.du@...el.com
Cc:     yamada.masahiro@...ionext.com, michal.lkml@...kovi.net,
        tglx@...utronix.de, mingo@...hat.com, akpm@...ux-foundation.org,
        x86@...nel.org, lgirdwood@...il.com, broonie@...nel.org,
        arnd@...db.de, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH 3/5] kernel hacking: new config NO_AUTO_INLINE to
 disable compiler atuo-inline optimizations

On Tue,  1 May 2018 21:00:12 +0800
changbin.du@...el.com wrote:

> From: Changbin Du <changbin.du@...el.com>
> 
> This patch add a new kernel hacking option NO_AUTO_INLINE. Selecting
> this option will make compiler not auto-inline kernel functions. By
> enabling this option, all the kernel functions (including static ones)
> will not be optimized out except those marked as inline or always_inline.
> This is useful when you are using ftrace to understand the control flow
> of kernel code or tracing some static functions.

I'm not against this patch, but it's up to others if this gets included
or not.

> 
> Signed-off-by: Changbin Du <changbin.du@...el.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> ---
>  Makefile          |  6 ++++++
>  lib/Kconfig.debug | 13 +++++++++++++
>  2 files changed, 19 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..90f35ad 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -198,6 +198,19 @@ config GDB_SCRIPTS
>  	  instance. See Documentation/dev-tools/gdb-kernel-debugging.rst
>  	  for further details.
>  
> +config NO_AUTO_INLINE
> +	bool "Disable compiler atuo-inline optimizations"

typo: s/atuo/auto/

> +	default n
> +	help
> +	  This will make compiler not auto-inline kernel functions for
> +	  optimization. By enabling this option, all the kernel functions
> +	  (including static ones) will not be optimized out except those
> +	  marked as inline or always_inline. This is useful when you are
> +	  using ftrace to understand the control flow of kernel code or
> +	  tracing some static functions.

Some grammar updates:

	  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.

> +
> +	  Use only if you want to debug the kernel.

The proper way to say the above is:

	  If unsure, select N

-- Steve

> +
>  config ENABLE_WARN_DEPRECATED
>  	bool "Enable __deprecated logic"
>  	default y