Re: printing kernel backtrace

[fche at redhat dot com (Frank Ch. Eigler)]

Hi -


n.borisov wrote:

> I've started experimenting with systemtamp and I'd like to print the
> callstack leading to the probed function. 

Welcome!

> To that effect I have created the following very simple tapset:

(Just a terminology nit: a "tapset" is a set of script fragments for
automated reuse, kind of like libc.  Yours is simply a script.)

> probe module("dm_thin_pool").function("do_waker") {
>     print_syms(callers(-1))
>     printf("finished backtrace\n");
> }

print_backtrace()  would have been equivalent.


> However the output I get is something like that:
>
>  0xffffffffa0152c80 : do_waker+0x0/0x40 [dm_thin_pool]
>  0xffffffff810a5025 : 0xffffffff810a5025
>  0xffffffffa0152c80 : do_waker+0x0/0x40 [dm_thin_pool]
>  0xffffffffa0152c80 : do_waker+0x0/0x40 [dm_thin_pool]
>  [...]
>  0xffffffffa0152c80 : do_waker+0x0/0x40 [dm_thin_pool]
>  0xffffffffa0152c8 : 0xffffffffa0152c8
> finished backtrace

That doesn't look quite right, does it.  I'd expect stap to produce
some messages about missing unwind/symbol information, but it's not
happening right this time.

The problem is probably that the unwind/symbol info for only your
module was included.  Try running your script again, but add

   -d kernel

to the stap command line.


- FChE