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Subject: Re: [RFC PATCH 3/3 v2] perf: Update perf tool to monitor uncore
 events
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Stephane Eranian <eranian@google.com>
Cc: Andi Kleen <andi@firstfloor.org>, Lin Ming <ming.m.lin@intel.com>,
        Ingo Molnar <mingo@elte.hu>, lkml <linux-kernel@vger.kernel.org>,
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        Arjan van de Ven <arjan@infradead.org>
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Date: Sun, 21 Nov 2010 18:45:29 +0100
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On Sun, 2010-11-21 at 13:22 +0100, Stephane Eranian wrote:
> On Sun, Nov 21, 2010 at 1:09 PM, Andi Kleen <andi@firstfloor.org> wrote:
> >
> >>
> >>              samples  pcnt function               DSO
> >>              _______ _____ ______________________
> >> ____________________________________
> >>
> >>                 8.00 18.6% kallsyms_expand_symbol [kernel.kallsyms]
> >
> > Reporting a symbol for an uncore event seems highly misleading.
> > After all the uncore counter has no idea for which core the event was,
> > so there isn't really any instruction pointer to report.
> > The event could be event caused by a PCI device or similar.
> >
> > For per function monitoring of uncore events one has to use
> > OFFCORE_RESPONSE, like I implemented recently.
> >
> > I would suggest to not report any symbol names for uncore events.
> > Doing so just will confuse users.
> >
> > In fact I suspect uncore events are only really useful
> > with "stat", but not with "top", or if they are used in top
> > then the symbol reporting should be disabled.
> >
> I agree, uncore should only be used for counting on a
> per-cpu basis. You can leave the perf tool as is, but
> that opens up the risk of misinterpretation by many users,
> or you restrict this in the tool directly which is the better
> solution in my mind.

I would argue against restricting the tool, print a warning perhaps. I
mean, give the user all the rope he needs and tell him how to tie the
knot is the unix way, right? :-)