linux-kernel - Re: Kconfig dependency issue on function-graph tracer and frame pointer on arm

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Date:   Mon, 15 Apr 2019 14:28:59 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Stefan Agner <stefan@...er.ch>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Kconfig dependency issue on function-graph tracer and frame
 pointer on arm

On Sun, Apr 14, 2019 at 3:35 PM Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:

> The subsequent hunks remove the defaulting of the choice according to
> the function graph tracer - this is not a "hint" where the user can
> still choose either option irrespective of the state of the function
> graph tracer.  They should only be able to select the frame pointer
> option in that case.
>
> Another way forward would be for someone to put the work in to making
> the function graph tracer work without frame pointers.

I think Stefan was already looking into making CONFIG_FUNCTION_TRACER
work with clang. I don't know what the status of that work is, but I
think getting
FUNCTION_GRAPH_TRACER working at the same time would be best.

I never noticed the Kconfig issue here, because I was using a patch to
turn off FUNCTION_TRACER on ARM with clang to make it build, and that
turns off FUNCTION_GRAPH_TRACER in the process.

> So, how about this:
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 850b4805e2d1..9aed25a6019b 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -73,7 +73,7 @@ config ARM
>         select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
>         select HAVE_EXIT_THREAD
>         select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
> -       select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL
> +       select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL && !CC_IS_CLANG
>         select HAVE_FUNCTION_TRACER if !XIP_KERNEL
>         select HAVE_GCC_PLUGINS
>         select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)
> diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
> index 6d6e0330930b..e388af4594a6 100644
> --- a/arch/arm/Kconfig.debug
> +++ b/arch/arm/Kconfig.debug
> @@ -47,8 +47,8 @@ config DEBUG_WX
>
>  choice
>         prompt "Choose kernel unwinder"
> -       default UNWINDER_ARM if AEABI && !FUNCTION_GRAPH_TRACER
> -       default UNWINDER_FRAME_POINTER if !AEABI || FUNCTION_GRAPH_TRACER
> +       default UNWINDER_ARM if AEABI
> +       default UNWINDER_FRAME_POINTER if !AEABI
>         help
>           This determines which method will be used for unwinding kernel stack
>           traces for panics, oopses, bugs, warnings, perf, /proc/<pid>/stack,
> @@ -65,7 +65,7 @@ config UNWINDER_FRAME_POINTER
>
>  config UNWINDER_ARM
>         bool "ARM EABI stack unwinder"
> -       depends on AEABI
> +       depends on AEABI && !FUNCTION_GRAPH_TRACER
>         select ARM_UNWIND
>         help
>           This option enables stack unwinding support in the kernel

This looks good to me in the meantime, at least if there is any
way to get the non-graph FUNCTION_TRACER to build with clang.

       Arnd