Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Ingo Molnar (mingo@elte.hu) wrote:
> 
> * Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
> 
> > Following an advice Christoph gave me this summer, submitting a 
> > smaller, easier to review patch should make everybody happier. Here is 
> > a stripped down version of LTTng : I removed everything that would 
> > make the code review reluctant (especially kernel instrumentation and 
> > kernel state dump module). I plan to release this "core" version every 
> > few LTTng releases and post it to LKML.
> > 
> > Comments and reviews are very welcome.
> 
> i have one very fundamental question: why should we do this 
> source-intrusive method of adding tracepoints instead of the dynamic, 
> unintrusive (and thus zero-overhead) KProbes+SystemTap method?
> 

Hi Ingo,

First, I never said that this tracing infrastructure was tied to static trace
points in any way. My goal is to provide a robust data serialisation mechanism
that could be used both from static and dynamic trace points.

Zero-overhead for static tracepoints can be achieved by compiling them out.

One problem with the KProbes approach is that is limits what can be instrumented
because of its performance impact when active : traps are very costly and can
limit instrumentation of often triggered code paths : scheduler change, traps,
interrupts...

Also, a major issue with dynamic instrumentation is that it will never be useful
to kernel developers who keep current with the git HEAD. Dynamic instrumentation
has to be defined outside of the kernel tree and cannot follow the code changes
quickly enough to be useful for a developer without himself maintaining his own
dynamic instrumentation.

I do not advocate for a particular approach : I think that dynamic
instrumentation is very well suited for distributions which stick to a
particular kernel version for a long time. However, static probes can be very
useful for kernel developers as they can follow the kernel HEAD because they
are part of the code.


Mathieu


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