Re: backtrace using LTTng in Kernel

* Sagar Borikar ([email protected]) wrote:
>   
> Hello,
> 
> I am consistently using LTTng for tracing the MIPS system. While adding the events, I was bit stuck in finding out which of the kernel memcpy function is consuming lot of CPU. I need to search a kernel call which is having highest probability of stalling the cpu. So in short, I would like to see backtrace in LTTng. Can anyone point me out about using backtracking feature in LTTng if at all it is present?
> 
> Thanks in advance,
> 

Hi Sagar,

You would have to port part of ltt/probes/ltt-probe-stack_arch_i386.c
(there is also a x86_64 implementation) to MIPS. You take the standard
kernel stack trace (see arch/mips/kernel/stacktrace.c) and, instead of
sending the output well formatted to printk, you follow what is done in
LTTng to 1 - calculate the size of the sequence in the first pass and 2
- copy the data into the sequence in the 2nd pass.

Note that the way function calls are put is very important: I use an
address that is on the current thread's stack as a start value; it must
not change between the 1st and the 2nd pass.

If you can do that on recent kernels (2.6.22-rc4-mm2), I could integrate
that to LTTng, that would be great :)

Mathieu

> Sagar
> 

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-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
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