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

[Jiri Olsa <jolsa@redhat.com>]

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

jirka