linux-kernel - Re: [PATCH 3/3] perf/x86/rapl: Fix psys-energy event on Intel SPR platform

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Date:   Sat, 16 Jan 2021 13:50:10 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Zhang Rui <rui.zhang@...el.com>
Cc:     mingo@...hat.com, acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        kan.liang@...ux.intel.com, ak@...ux.intel.com
Subject: Re: [PATCH 3/3] perf/x86/rapl: Fix psys-energy event on Intel SPR
 platform

On Fri, Jan 15, 2021 at 05:22:08PM +0800, Zhang Rui wrote:
> There are several things special for the RAPL Psys energy counter, on
> Intel Sapphire Rapids platform.
> 1. it contains one Psys master package, and only CPUs on the master
>    package can read valid value of the Psys energy counter, reading the
>    MSR on CPUs in the slave package returns 0.
> 2. The master package does not have to be Physical package 0. And when
>    all the CPUs on the Psys master package are offlined, we lose the Psys
>    energy counter, at runtime.
> 3. The Psys energy counter can be disabled by BIOS, while all the other
>    energy counters are not affected.
> 
> It is not easy to handle all of these in the current RAPL PMU design
> because
> a) perf_msr_probe() validates the MSR on some random CPU, which may either
>    be in the Psys master package or in the Psys slave package.
> b) all the RAPL events share the same PMU, and there is not API to remove
>    the psys-energy event cleanly, without affecting the other events in
>    the same PMU.
> 
> This patch addresses the problems in a simple way.
> 
> First, by setting .no_check bit for RAPL Psys MSR, the psys-energy event
> is always added, so we don't have to check the Psys ENERGY_STATUS MSR on
> master package.
> 
> Then, rapl_not_visible() is removed because
> 1. it is useless for RAPL MSRs with .no_check cleared, because the
>    .is_visible() callbacks is always overridden in perf_msr_probe().
> 2. it is useless for RAPL MSRs with .no_check set, because we actually
>    want the sysfs attributes always be visible for those MSRs.
> 
> With the above changes, we always probe the psys-energy event on Intel SPR
> platform. Difference is that the event counter returns 0 when the Psys
> RAPL Domain is disabled by BIOS, or the Psys master package is offlined.

Maybe I'm too tired, but I cannot follow. How does this cure the fact
that the rapl_cpu_mask might not include that master thing. And how can
software detect what the master thing is to begin with?