[HELP] ubacktrace() problem

[Jinhua Luo <luajit dot io at gmail dot com>]

Hi All,

On Debian8 (jessie), I use the sample-bt-off-cpu script to diagnose
the postgresql performance. I already install the debuginfo packages
of kernel, glibc and the postgresql, and I compile the systemtap 2.9
to use.

See https://github.com/openresty/nginx-systemtap-toolkit#sample-bt-off-cpu
for the script.

It fails to unwind some user backtrace (which is a big fraction of the
total samples), and just show some hex adddress, e.g. 0xb77a5d40.

I check that address belongs to the vdso mapping. This issue is not
specific to postgresql, but also for other programs running on the
same system. I try to reinstall the debian8 in a clean box, the issue
is the same.

I am wondering what's wrong with debian8?

And, I am also get confused about the warnings:

WARNING: Too many CFI instuctions

WARNING: DWARF expression stack underflow in CFI

WARNING: no or bad debug frame hdr
WARNING: No binary search table for eh frame, doing slow linear search
for stap_b9bfb422b7bd73acc448d7f775aaacd_37477


I try to run the same script on Ubuntu 14.04 and CentOS 7. They could
work and show me the correct user backtrace. But, the above warnings
exist too! Moreover, the warnings does not necessarily appear at
different runs of the same script! Why systemtap outputs those
warnings on these popular linux distributions? The dwarf info
associated with the kernel, glibc and other programs is not so
qualified? Then what system we need so that we could run systemtap
without any common problems?

Anybody could help me out? Thanks!


Regards,
Jinhua Luo