linux-kernel - Re: [V3 PATCH] arm64: remove redundant FRAME_POINTER kconfig option and force to select it

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Date:	Tue, 10 Nov 2015 19:43:35 +0800
From:	yalin wang <yalin.wang2010@...il.com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Mark Rutland <mark.rutland@....com>,
	Yang Shi <yang.shi@...aro.org>, linaro-kernel@...ts.linaro.org,
	Will Deacon <will.deacon@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [V3 PATCH] arm64: remove redundant FRAME_POINTER kconfig option and force to select it


> On Nov 10, 2015, at 19:35, Catalin Marinas <catalin.marinas@....com> wrote:
> 
> On Tue, Nov 10, 2015 at 07:09:00PM +0800, yalin wang wrote:
>>> On Nov 10, 2015, at 18:37, Catalin Marinas <catalin.marinas@....com> wrote:
>>> 
>>> On Mon, Nov 09, 2015 at 10:09:55AM -0800, Yang Shi wrote:
>>>> FRAME_POINTER  is defined in lib/Kconfig.debug, it is unnecessary to redefine
>>>> it in arch/arm64/Kconfig.debug. Actually, the one defined in arm64 directory
>>>> is never used.
>>> 
>>> That's not true since the arm64 definition seems to take precedence.
>>> 
>>>> This adds a dependency on DEBUG_KERNEL for building with frame pointers.
>>> 
>>> It doesn't because arm64 selects ARCH_WANT_FRAME_POINTERS.
>>> 
>>>> ARM64 depends on frame pointer to get correct stack backtrace and need
>>>> FRAME_POINTER kconfig option enabled all the time.
>>>> However, currect implementation makes it could be disabled, so force it
>>>> to be selected by ARM64.
>>>> 
>>>> Signed-off-by: Yang Shi <yang.shi@...aro.org>
>>> 
>>> Patch applied but I changed the commit log slightly. Thanks.
>> i have a question,
>> why FRAME_POINTER  config must be enabled ?
>> and i see ARM arch can  disable this config .
>> if i don’t need stack trace dump and the software release is for 
>> final product , don’t need debug stack trace log .
>> is it possible to disable it for performance reason ?
> 
> If you don't need any stack trace, perf etc., in theory you can disable
> the option. However, the aarch64 gcc compiler always generates it (I'm
> not sure whether the AAPCS mandates it). Anyway, the performance impact
> is very small since there are more general purpose registers available
> in AArch64 already.
> 
i just make a test with -fomit-frame-pointer,  seems gcc can generate code without frame pointer,
like ARM arch.
version:
aarch64-linux-gnu-gcc
gcc version 4.9.2 20140904 (prerelease) (crosstool-NG linaro-1.13.1-4.9-2014.09 - Linaro GCC 4.9-2014.09)

why AARCH64 don’t have frame unwind info just like ARM arch?

Thanks








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