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

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Message-ID: <37D7C6CF3E00A74B8858931C1DB2F077058E5CD5@SHSMSX103.ccr.corp.intel.com>
Date:	Thu, 21 Apr 2016 11:55:56 +0000
From:	"Liang, Kan" <kan.liang@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	"peterz@...radead.org" <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	"eranian@...gle.com" <eranian@...gle.com>
Subject: RE: [PATCH 1/1] perf/x86/intel/uncore: Add support for Intel SKL
 client uncore



> 
> 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?

Right, the workaround doesn't cover all cases. It helps for the new events
and the cases that monitoring a busy system. A busy system means it never
enter C7 during the counting.
I will mention it in the changelog of V2. 

> 
> > 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?

Yes.

> 
> > 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.

Will do that in V2.

Thanks,
Kan