linux-kernel - Re: [PATCH V5 4/8] perf/x86/intel/uncore: add new data structures for free running counters

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Message-ID: <20180119171929.GZ2228@hirez.programming.kicks-ass.net>
Date:   Fri, 19 Jan 2018 18:19:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...el.com>
Cc:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>,
        "eranian@...gle.com" <eranian@...gle.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [PATCH V5 4/8] perf/x86/intel/uncore: add new data structures
 for free running counters

On Fri, Jan 19, 2018 at 03:15:00PM +0000, Liang, Kan wrote:
> > 
> > On Thu, Jan 18, 2018 at 05:43:10PM +0000, Liang, Kan wrote:
> > > In the uncore document, there is no event-code assigned to free running
> > counters.
> > > Some events need to be defined to indicate the free running counters.
> > > The events are encoded as event-code + umask-code.
> > >
> > > The event-code for all free running counters is 0xff, which is the
> > > same as the fixed counters.
> > 
> > Is it possible to count the same things using the generic counters?
> > 
> 
> Yes, there are events for generic counters to count bandwidth and
> utilization.
> 
> The reasons of introducing free running counters are
> - To provide highly valuable information (bandwidth and utilization)
>    which most of the customers are interested in
> - To save on the precious generic counters

_IF_ the exact same counters are available on the GPs then we must use the
same event code for them and use event scheduling to place them on
fixed/free-running counters when possible.

That's what we do for the CPU PMU's fixed counters too.

Don't invent magic event codes just because.