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

[Stefan Agner <stefan@agner.ch>]

On 15.04.2019 14:28, Arnd Bergmann wrote:
> On Sun, Apr 14, 2019 at 3:35 PM Russell King - ARM Linux admin
> <linux@armlinux.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.

Function Tracer is currently blocked by buggy mcount implemention on
Clang side. I do have a hacked up version which works with the buggy
Clang implementation, but not something we want to merge IMHO. see also:
https://bugs.llvm.org/show_bug.cgi?id=33845

> 
> 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

I think due to the fact above, we should add  && !CC_IS_CLANG here too.

>>         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.

Looks sensible to me too.

Note that a similar issue came up a while ago on the mailing list:
https://marc.info/?l=linux-arm-kernel&m=154739414703313&w=2
[added Jeremy]

Unfortunately this never really materialized in a mergeable patch.

--
Stefan