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

Mathieu Desnoyers
2006-09-14T15:19:09Z

* Ingo Molnar (mingo@elte.hu) wrote:
>
> * Roman Zippel <zippel@linux-m68k.org> wrote:
>
> the key point is that we want _zero_ "static tracepoints". Firstly,
> static tracepoints are fundamentally limited:
>
> - they can only be added at the source code level
>
> - modifying them requires a reboot which is not practical in a
> production environment

Not for kernel modules : unload/load is enough.

> - there can only be a limited set of them, while many problems need
> finegrained tracepoints tailored to the problem at hand

Not true with the dynamic facility loading. LTTng can register new events upon
module load/unload.

>
> - conditional tracepoints are typically either nonexistent or very
> limited.
>
Maybe, but it can be useful to have static instrumentation available for those
limited conditional tracepoints.

> But besides the usability problems, the most important problem is that
> static tracepoints add a _constant maintainance overhead_ to the kernel.
> I'm talking from first hand experience: i wrote 'iotrace' (a static
> tracer) in 1996 and have maintained it for many years, and even today
> i'm maintaining a handful of tracepoints in the -rt kernel. I _dont_
> want static tracepoints in the mainline kernel.
>

If the trace points are modified with the code by the ones who make the
original code changes, it lessens the maintainance overhead. Furthermore, if
there is a major change in a code path that requires rethinking the trace
points, the person introducing the change has the best knowledge of what to do
with the trace point. I think that trace point maintainance should be left to
subsystem maintainers, not a centralised task done by distributions once in a
while.

Talking about experience, Karim has maintained the original LTT trace points,
which targeted key kernel event, for years without major trace points changes
between kernel versions. I think he already proved that maintainance of static
trace points in not an issue.

However, I restate that my position is that both static and dynamic
instrumentation of the kernel are a necessity and that a tracer core should be
usable by both.

Mathieu

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