Re: ARM: config issue with ftrace function graph tracer

[Stefan Agner]

Hi,

On 12.01.2019 02:01, Jeremy Fertic wrote:
> I'm having a problem with the ftrace function graph tracer on a 32 bit arm
> board (orangepi pc). A bisect points to the following commit:
> 
> f9b58e8c7d03 ("ARM: 8800/1: use choice for kernel unwinders")
> 
> Before this commit, if I use sunxi_defconfig and then menuconfig to enable
> FTRACE and FUNCTION_TRACER then the function graph tracer works. With this
> commit, and as of v5.0-rc1, doing the same as above results in a broken
> function graph tracer and often an oops as well. The commit introduces a
> choice group and it looks like it should default to UNWINDER_FRAME_POINTER
> if FUNCTION_GRAPH_TRACER is enabled. FUNCTION_GRAPH_TRACER is enabled by
> default when I enable FUNCTION_TRACER but this has no effect on the choice.
> The choice always defaults to the other option which is UNWINDER_ARM. If I
> manually choose UNWINDER_FRAME_POINTER then the function graph tracer works
> fine.

The default selection is there, but this is made at "make
sunxi_defconfig" time. At this point FUNCTION_GRAPH_TRACER is not
enabled, hence Kconfig uses UNWIDER_ARM. However, when enabling the
FUNCTION_GRAPH_TRACER Kconfig will _not_ reconsider and switch enable
UNWINDER_FRAME_POINTER.

Before that commit, when enabling FUNCTION_GRAPH_TRACER, we simply also
enabled FRAME_POINTER...

I guess we need to make sure that FUNCTION_GRAPH_TRACER depends on the
UNWINDER_FRAME_POINTER choice. There is already a similar dependency
with THUMB2_KERNEL. We can cleanup that dependency since
UNWINDER_FRAME_POINTER already depends on !THUMB2_KERNEL.

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 664e918e2624..a2ac65a8b2cc 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -69,7 +69,7 @@ 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 UNWINDER_FRAME_POINTER
        select HAVE_FUNCTION_TRACER if !XIP_KERNEL
        select HAVE_GCC_PLUGINS
        select HAVE_GENERIC_DMA_COHERENT
diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index 6d6e0330930b..8341649fa71d 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,


> 
> Any idea why the default behaviour has changed?

The rational is stated in the commit message, the main reason was to
disallow UNWINDER_FRAME_POINTER in case Clang is used (since Clang does
not create the expected function prologue).

The choice also aligns with how unwinders are selected on x86.

--
Stefan