On 13.01.2019 17:14, Russell King - ARM Linux wrote: > On Sun, Jan 13, 2019 at 04:41:52PM +0100, Stefan Agner wrote: >> 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, > > This looks rather horrid - the upshot of this means that the function > tracer becomes unavailable on EABI unless you know that you must change > the unwinder from its default. > > Before the change to the choice statement, people could select the > function graph tracer, and the correct unwinder would be selected > for them. That's knowledge that people never required before, and > I think it's really quite unfair to require them to know this to use > the function graph tracer.
It is not ideal I agree. Searching for the symbol FUNCTION_GRAPH_TRACER in menuconfig immediately shows what dependency are missing. Also, we used to have the same situation with THUMB2_KERNEL already: You had to know that you need to disable Thumb2 Kernel in order to see FUNCTION_GRAPH_TRACER. > > Maybe someone can put some effort into getting the function graph > tracer working with non-framepointer kernels... but as the above > currently stands, I really don't like the patch and I'd much rather > revert the original change to fix this regression. I am all for that effort. Using Thumb2 on Arm32 is also getting more popular. Is known what is exactly missing/what effort would be required? -- Stefan