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

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Message-ID: <20190528120528.GR2606@hirez.programming.kicks-ass.net>
Date:   Tue, 28 May 2019 14:05:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
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 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?

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

I shouldn've have to reverse engineer patches :/