Re: [patch] Performance Counters for Linux, v4

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Mon, 2008-12-15 at 23:11 +1100, Paul Mackerras wrote:
> Ingo Molnar writes:
> 
> > We are pleased to announce the v4 release of our performance counters 
> > subsystem implementation.
> 
> Looking at the code, I am wondering what you are planning to do to
> support machines that have constraints on what sets of events can be
> counted simultaneously.  Currently you have the core code calling
> counter->hw_ops->hw_perf_counter_enable which can't return an error.
> The core expects it to be able to add any counter regardless of what
> event it's counting, subject only to a maximum number of counters.
> I assume you're going to change that.
> 
> I think the core should put together a list of counters and counter
> groups that it would like to have on the PMU simultaneously and then
> make one call to the arch layer to ask if that is possible.  That
> could either return success or failure.  If it returns failure then
> the core needs to ask for something less, or something different.  I'm
> not sure how the core should choose what to ask for instead, though.

I think the constraint set should be applied when we add to a group, if
when we add a counter to the group, the result isn't schedulable
anymore, we should fail the group addition - and thereby the counter
creation.

This would leave us with groups that are always schedulable in an atomic
fashion.

>From what I understand the code RRs groups (co-scheduling groups where
possible) (ungrouped counter is a group of one), this means that with
the above addition you'd have the needed control over things.

If you need things to be atomic, create a single group, if you're fine
with RR time-sharing, create multiple.

This seems to leave a hole where multiple monitors collide and create
multiple groups unaware of each-other - could we plug this hole with a
group attribute?