RE: Proposed systemtap access to perfmon hardware

["Stone, Joshua I" <joshua.i.stone@intel.com>]

fche@redhat.com wrote:
> joshua wrote:
>>  [...]
>>> If they are started by default, where exactly are they running?
>>> Beginning of begin probe? End of begin probe?
>> 
>> I think the perfmon setup needs to happen before all begin probes, so
>> that the handle can be accessed within begin probes.  [...]
> 
> How important would that be?  At this time, we don't provide any
> ordering guarantees amongst begin/end probes.  If a begin probe would
> have to manipulate the handle, why not put that right into the
> perfctr.*.setup probe directly?

Perhaps it isn't that important - I didn't consider putting the work in
the ".setup" probe.  This is fine as long as the setup probe has the
guarantee that the counters haven't started yet (regardless of its
position in the begin stage)

It might be nice to have a similar shutdown/cleanup/whatever probe that
guarantees that the counters have been stopped.  This is superfluous if
we guarantee that the counters will be stopped before running any end
probes, but I haven't seen discussion on that point yet.  We should also
define when the handle be invalidated - this could be at the conclusion
of the cleanup probe, if it exists, or after all end probes have
completed.


Josh