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

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Date:   Wed, 16 Feb 2022 12:19:14 +1100
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Steven Rostedt <rostedt@...dmis.org>
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>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [RFC PATCH kernel] trace: Make FTRACE_MCOUNT_USE_RECORDMCOUNT
 configurable



On 2/12/22 13:47, Steven Rostedt wrote:
> 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?

LTO_CLANG_THIN => HAS_LTO_CLANG => !FTRACE_MCOUNT_USE_RECORDMCOUNT =>

FTRACE [=y] && !FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY [=n] && 
!FTRACE_MCOUNT_USE_CC [=n] && !FTRACE_MCOUNT_USE_OBJTOOL [=n] && 
FTRACE_MCOUNT_RECORD [=y]

A bunch.


> 
>>
>> 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.

Sounds like it.

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

I am trying to

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index b779603978e1..91c122224f83 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -153,6 +153,8 @@ config PPC
         select ARCH_WANT_IRQS_OFF_ACTIVATE_MM
         select ARCH_WANT_LD_ORPHAN_WARN
         select ARCH_WEAK_RELEASE_ACQUIRE
+       select ARCH_SUPPORTS_LTO_CLANG          if PPC64
+       select ARCH_SUPPORTS_LTO_CLANG_THIN     if PPC64


to get LTO working on powerpc64le with minimal change to 
ppc64le_defconfig which has all these FTRACE_xxx enabled.


> 
> -- Steve
> 
> 
>>   	depends on !FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY
>>   	depends on !FTRACE_MCOUNT_USE_CC
>>   	depends on !FTRACE_MCOUNT_USE_OBJTOOL
>