linux-kernel - Re: [PATCH 2/4] kernel hacking: new config NO_AUTO_INLINE to disable compiler auto-inline optimizations

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Message-ID: <62bcdf3d-87be-407f-b971-5a53dbb1e4b8@arm.com>
Date:   Thu, 18 Oct 2018 17:57:56 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Du Changbin <changbin.du@...il.com>, yamada.masahiro@...ionext.com,
        michal.lkml@...kovi.net, tglx@...utronix.de, mingo@...hat.com,
        linux@...linux.org.uk, akpm@...ux-foundation.org
Cc:     linux-kbuild@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        Changbin Du <changbin.du@...el.com>,
        linux-sparse@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/4] kernel hacking: new config NO_AUTO_INLINE to disable
 compiler auto-inline optimizations

On 18/10/18 17:25, Du Changbin wrote:
> From: Changbin Du <changbin.du@...el.com>
> 
> 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 <changbin.du@...el.com>
> Acked-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
>   Makefile          |  6 ++++++
>   lib/Kconfig.debug | 25 +++++++++++++++++++++++++
>   2 files changed, 31 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index e8b599b4dcde..757d6507cb5c 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -749,6 +749,12 @@ 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
>   ifdef CONFIG_FTRACE_MCOUNT_RECORD
>     # gcc 5 supports generating the mcount tables directly
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 4966c4fbe7f7..0f9b4fa78b1c 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -211,6 +211,31 @@ 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"
> +	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

This part doesn't look like it belongs in this patch, and judging by the 
commit message in 771c035372a0 wouldn't be welcome back anyway.

Robin.

> +	bool "Enable __deprecated logic"
> +	default y
> +	help
> +	  Enable the __deprecated logic in the kernel build.
> +	  Disable this to suppress the "warning: 'foo' is deprecated
> +	  (declared at kernel/power/somefile.c:1234)" messages.
> +
>   config ENABLE_MUST_CHECK
>   	bool "Enable __must_check logic"
>   	default y
>