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Date:   Wed, 24 Jan 2018 11:17:21 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Liang, Kan" <kan.liang@intel.com>
Cc:     Andi Kleen <ak@linux.intel.com>,
        Stephane Eranian <eranian@google.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "acme@kernel.org" <acme@kernel.org>
Subject: Re: [PATCH V5 4/8] perf/x86/intel/uncore: add new data structures
 for free running counters
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On Tue, Jan 23, 2018 at 10:00:58PM +0000, Liang, Kan wrote:
> > On Fri, Jan 19, 2018 at 12:24:17PM -0800, Andi Kleen wrote:
> > > > Oh, think a bit more.
> > > > I think we cannot do the same thing as we did for CPU PMU's fixed
> > counters.
> > > >
> > > > The counters here are free running counters. They cannot be start/stop.
> > >
> > > Yes free running counter have completely different semantics. They
> > > need a separate event code.
> > 
> > The only thing that matters is if they count the same thing or not.
> >
> 
> Hi Peter,
> 
> There is NO event available on the GPs, that is exactly the same as
> the free-running counters.
> 
> For example, the BW free-running counters count the requests associated
> with writes and completions.
> The most similar events on the GPs are DATA_REQ_{OF,BY}_CPU.* events.
> Except that some of their sub-events count requests which not completions.
> There are also other minor differences.
> So we don't have alternative events for the free-running counters.
> I think we have to use 0xff.

OK, but explicitly mention this as the reason for having to invent event
codes. Them being fixed purpose or free running isn't a valid reason for
that.