On 01/24/2018 09:00 PM, Frank Ch. Eigler wrote:
> avi wrote:
>
>> I have a centos installation with 4.14.14 installed [...]
>>    at:       8483   to: rcu_sched
>>   0xffffffffa0335150 : 0xffffffffa0335150
>> [stap_fc3e91a0b756c0a804d73b3d051959b6__1891+0xd150/0x0]
>>   0xffffffffa0336476 : 0xffffffffa0336476
>> [...]
> Ouch.
>
>> Is there something in the kernel configuration I need to turn on
>> (.config taken from elrepo's -ml kernel)? Or a missing dependency
>> while building systemtap?
> Shouldn't be the case.
>
>> Maybe systemtap can't cope with retpolines?
> systemtap's backtrace engine uses dwarf unwind information.  In
> principle, retpolines should be representable properly, but in practice,
> there may be some problems with some compilers [1] or some kernel-side
> asm .cfi annotations.  I don't currently anticipate systemtap having to
> change, but these tools do need to produce correct output.
>
> [1]  https://gcc.gnu.org/ml/gcc/2018-01/msg00160.html
>
> As your kernel seems to be hand-built, and we don't have retpoline
> kernels already in circulation, so it'll be tricky to reproduce the
> problem here.  Running stap with -DDEBUG_UNWIND=3 would let us see its

Attached, let's hope the mailing list allows it.

> point of view.  Try also backtracing from other probe points to see how
> systemic the problem is.  It may also help to let us get hold of your
> kernel & its build vitals (esp. gcc version).

The kernel is available here: 
https://copr.fedorainfracloud.org/coprs/avik/kernel/build/704904/. There 
is a -debuginfo package, but I don't know if it's correct.