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

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

* 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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