linux-kernel - Re: [PATCH 2/9] perf/x86/intel: Basic support for metrics counters

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Date:   Tue, 28 May 2019 14:20:53 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     acme@...nel.org, mingo@...hat.com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, jolsa@...nel.org, eranian@...gle.com,
        alexander.shishkin@...ux.intel.com, ak@...ux.intel.com
Subject: Re: [PATCH 2/9] perf/x86/intel: Basic support for metrics counters



On 5/28/2019 8:05 AM, Peter Zijlstra wrote:
> On Tue, May 21, 2019 at 02:40:48PM -0700, kan.liang@...ux.intel.com wrote:
>> From: Andi Kleen <ak@...ux.intel.com>
>>
>> Metrics counters (hardware counters containing multiple metrics)
>> are modeled as separate registers for each TopDown metric events,
>> with an extra reg being used for coordinating access to the
>> underlying register in the scheduler.
>>
>> This patch adds the basic infrastructure to separate the scheduler
>> register indexes from the actual hardware register indexes. In
>> most cases the MSR address is already used correctly, but for
>> code using indexes we need a separate reg_idx field in the event
>> to indicate the correct underlying register.
> 
> That doesn't parse. What exactly is the difference between reg_idx and
> idx? AFAICT there is a fixed relation like:
> 
>    reg_idx = is_metric_idx(idx) ? INTEL_PMC_IDX_FIXED_SLOTS : idx;
> 
> Do we really need a variable for that?

It may save the calculation. But, right, a variable is not necessary.

> 
> Also, why do we need that whole enabled_events[] array. Do we really not
> have that information elsewhere?

No. We don't have a case that several events share a counter at the same 
time. We don't need to check if other events are enabled when we try to 
disable a counter. So we don't save such information.
But we have to do it for metrics events.

Thanks,
Kan
> 
> I shouldn've have to reverse engineer patches :/
>