Re: [RFC/PATCH] perf x86: Add off-core event constraints for Sandy/IvyBridge micro architecture

[Ingo Molnar <mingo@kernel.org>]

* Jiri Olsa <jolsa@redhat.com> wrote:

> On Mon, Jan 28, 2013 at 06:49:55PM +0100, Stephane Eranian wrote:
> > On Sun, Jan 27, 2013 at 6:33 PM, Jiri Olsa <jolsa@redhat.com> wrote:
> > > hi,
> > > I was looking at the offcore stuff and it looks like we might
> > > be missing some constraints for offcore response events on
> > > Sandy/IvyBridge.
> > >
> > > The table 18.8.5 (Off-core Response Performance Monitoring)
> > > in Intel SDM states PMC0 for 0xb7 and PMC3 for 0xbb, but
> > > there's no other explanation or related description.
> > >
> > > I can't say/ack if the counters looks bad or right with or
> > > without the patch so far.. so just curious ;-)
> > >
> > Those are artificial constraints which should not be there.
> > Remember that offcore_rsp uses an extra MSR which has
> > to be shared by all the counters on the PMU. So a way to
> > handle the sharing of that extra MSR is to impose an
> > artificial constraint on the event itself. If it can only run
> > on one counter, then you get the management of the
> > extra MSR for free, i.e., only one event gets it.
> > 
> > In perf_events, we use a more sophisticated dynamic scheme
> > which does not use this artificial constraint. We can measure
> > the event multiple times and share the extra MSR if possible
> > (same value). Why multiple times you might ask? For instance,
> > with different priv levels.
> > 
> > 
> > Hope this helps.
> 
> nice, thanks a lot for explanation

Would be nice to stick this into the code somewhere appropriate, 
AFAICS this information only lives in older commit logs atm.

Thanks,

	Ingo