Re: [RFC][PATCH 1/5] [PATCH 1/5] events: Add EVENT_FS the event filesystem

Steven Rostedt
2010-11-17T15:17:04Z

On Wed, 2010-11-17 at 16:03 +0100, Ingo Molnar wrote:
> * Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > Are these events now going to be labeled as stable? Is every tracepoint we have,
> > much have the same data? Linus specifically said at Kernel Summit that he wants
> > absolutely NO modules to have a stable tracepoint.
>
> I think you are worrying about the wrong things.
>
> I think Arjan's complaints at the KS stemmed from prior sporadic declarations on
> lkml that there is no tracepoint ABI _at all_, and that powertop/latencytop could
> break anytime.
>
> But in reality i strongly disagree with such declarations, and tracepoint data that
> is used by PowerTop/timechart/latencytop or perf is and was an ABI, simple as that -
> and i've been enforcing that for two years. (We have so few good instrumentation
> tools that we _really_ dont want to break them.)
>
> At that point, realizing that we have an ABI for existing tools, i think it's
> fundamentally misguided to go out on a limb trying to put barriers in the way of
> other tools that do not even exist to begin with ...
>
> Our real problem with tracing is lack of relevance, lack of utility, lack of
> punch-through analytical power.
>
> Trying to create a sandbox to _reduce utility_ is like the last step, and a really
> optional step, when we have such variety that we want some control over it. It's
> always expensive, it always reduces the tool space as collateral damage.
>
> So please dont think of sysfs or eventfs as a tool to restrict. Think of it as a
> tool to _organize_.
>
> Again, i'd _LOVE_ to have the 'problem' of us having so many tools that analyze
> application and kernel behavior in such a rich way that they use tracepoints that
> were not supposed to be 'stable'.
>
> I simply dont see the 'problem' that is being solved here. We had a stable ABI and
> we didnt break sysprof or powertop/latencytop in the past and wont break it in the
> future either.

What about a tool that picks up tracepoints that were only used by a
developer for in-field debugging, and then that tracepoint disappears
because of a design change. Is it OK for that tool to break with it?

Do all tools that use tracepoints require a "check" feature?

I guess the problem is that creators of the tools to analyze the kernel
have no idea of what they can count on and what they can't. Do we need a
process to have these tool creators request to developers to "keep this
tracepoint"?

-- Steve