RE: [PATCH 1/1] perf/x86/intel/uncore: Add support for Intel SKL client uncore

From: Thomas Gleixner
Date: Thu Apr 21 2016 - 04:05:03 EST

On Wed, 20 Apr 2016, Liang, Kan wrote:
> > On Wed, 20 Apr 2016, Liang, Kan wrote:
> > > > The stop of the box1 events disables the whole machinery on that
> > > > node and therefor the box0 event is wreckaged as well. Hmm?
> > > >
> > > Right. How about check the SKL_UNC_PERF_GLOBAL_CTL in enable_event?
> > > If it's cleared, we can reset it there. The drawback is that there
> > > will be an extra rdmsrl and a possible wrmsrl.
> >
> > Well, that does not buy anything as you cannot disable the thing at all, unless
> > you have refcounting. And that refcounting needs to be in the 'type'
> > struct and that would probably be some real pain to implement.
> >
> > The question is whether we need enable/disable at all. If the type is
> > initialized we enable it and on exit we disable it. Ditto on cpu hotplug - which
> > is also used for init to enable all nodes.
> >
> > So if there is no drawback in letting the thing enabled if no events are armed,
> > then we really can do w/o the enable/disable_box callbacks.
> >
> There is no drawback in letting the thing enabled, but PERF_GLOBAL_CTL could
> be disabled after Package C7. I add the enable/disable thing to try to
> workaround it.

I don't see how that solves it. If a counter is active, then C7 will stop it
and you wont get anything useful from it after returning from C7. Or does an
active counter prevent C7?

> I once did the test on a SKL laptop. If the machine goes idle for a while,
> then the uncore counter will always return 0. For fixing it, we have to
> re-enable PERF_GLOBAL_CTL.

Hmm, but that does only help for new events after returning from C7, right?

> I think I made a typo in previous reply. I mean we can check it or just
> force rewrite the PERF_GLOBAL_CTL in enable_box. We don't need disable_box
> since there is no drawback in letting the thing enabled.

Sure, but then you can just unconditionally enable it. IOW, leave the enable
callback as is.