linux-kernel - Re: [RFC PATCH kernel] trace: Make FTRACE_MCOUNT_USE

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Date:   Fri, 11 Feb 2022 21:47:22 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alexey Kardashevskiy <aik@...abs.ru>
Cc:     llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC PATCH kernel] trace: Make FTRACE_MCOUNT_USE_RECORDMCOUNT
 configurable

On Fri, 11 Feb 2022 12:43:13 +1100
Alexey Kardashevskiy <aik@...abs.ru> wrote:

> For whatever reason LLVM does not allow LTO (Link Time Optimization) if
> FTRACE_MCOUNT_USE_RECORDMCOUNT is enabled.
> 
> This allows disabling just this option instead of disabling all FTRACE
> options.

What FTRACE options are you talking about?

> 
> Signed-off-by: Alexey Kardashevskiy <aik@...abs.ru>
> ---
> 
> Or disabling FTRACE is the right thing to do if HAS_LTO_CLANG=y?
> 
> Came from arch/Kconfig:
> 
> config HAS_LTO_CLANG
>         def_bool y
>         depends on CC_IS_CLANG && LD_IS_LLD && AS_IS_LLVM
>         depends on $(success,$(NM) --help | head -n 1 | grep -qi llvm)
>         depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm)
>         depends on ARCH_SUPPORTS_LTO_CLANG
>         depends on !FTRACE_MCOUNT_USE_RECORDMCOUNT  <======
>         depends on !KASAN || KASAN_HW_TAGS
>         depends on !GCOV_KERNEL
> 
> 
> ---
>  kernel/trace/Kconfig | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index a5eb5e7fd624..87d82d2b0b0b 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -704,7 +704,8 @@ config FTRACE_MCOUNT_USE_OBJTOOL
>  	depends on FTRACE_MCOUNT_RECORD
>  
>  config FTRACE_MCOUNT_USE_RECORDMCOUNT
> -	def_bool y
> +	bool "Enable FTRACE_MCOUNT_USE_RECORDMCOUNT"
> +	default y

I don't think this does what you think it does.

This is not something that should be user selectable. What exactly are
you trying to accomplish here?

-- Steve


>  	depends on !FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY
>  	depends on !FTRACE_MCOUNT_USE_CC
>  	depends on !FTRACE_MCOUNT_USE_OBJTOOL