Re: PATCH [0/4] perf: clean-up of power events API

[Ingo Molnar <mingo@elte.hu>]

* Tejun Heo <tj@kernel.org> wrote:

> Hello,
> 
> On 10/07/2010 05:58 PM, Frederic Weisbecker wrote:
> > I really feel uncomfortable with this tracepoint/ABI problem....
> > Mathieu suggested we start a user library that could handle these
> > changes when they are really necessary.
> > 
> > Thoughts?
> > 
> > (Adding Tejun in Cc).
> 
> Given that tracepoints are supposed to make internal operation 
> visible.  I don't think it's a good idea to make it part of fixed ABI.

Yep, exactly.

OTOH since it exports information we can do disciplined versioning and 
extensions only - i.e. leave the old power events around, add the new 
ones with new distinct names, and phase out the old ones in a kernel 
cycle or two. It's not hard to do.

That way apps can support old kernels too (if they want to), but new 
events as well - and all in a controlled, non-disruptive manner.

More importantly, the kernel wont have cruft and will have no ABI 
restrictions - the only 'restriction' is to treat information in an 
append-only manner (i.e. change the event name if you change it 
materially) - and that's not a big deal here.

The fundamental thing about tracing/instrumentation is that there are no 
deep ABI needs: it's all about analyzing development kernels (and a few 
select versions that get the enterprise treatment) but otherwise the 
half-life of this kind of information is very short.

So we dont want to tie ourselves down with excessive ABIs.

> Maybe some core part can be put in stone but I think things like 
> internal workqueue implementation should be changeable without 
> worrying about ABI issues.

That's most definitely so! There is and will be zero back-coupling from 
workqueue tracepoints to workqueue internals. Dont worry about this.

	Ingo