linux-kernel - Re: [PATCH 1/2] perf/x86/rapl: Add support for Intel Meteor Lake

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Date:   Fri, 6 Jan 2023 14:45:22 +0000
From:   "Zhang, Rui" <rui.zhang@...el.com>
To:     "mingo@...nel.org" <mingo@...nel.org>,
        "bp@...en8.de" <bp@...en8.de>
CC:     "Hansen, Dave" <dave.hansen@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "kan.liang@...ux.intel.com" <kan.liang@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] perf/x86/rapl: Add support for Intel Meteor Lake

On Fri, 2023-01-06 at 12:33 +0100, Ingo Molnar wrote:
> * Borislav Petkov <bp@...en8.de> wrote:
> 
> > On Fri, Jan 06, 2023 at 11:56:18AM +0100, Ingo Molnar wrote:
> > > It's a trade-off in any case: there's a point where quirk flags
> > > or even 
> > > feature flags become harder to read and harder to maintain than
> > > cleanly 
> > > separated per model driver functions.
> > 
> > Yeah, no, singular: a synthetic feature *flag*: X86_FEATURE_RAPL.
> > 
> > cpu/intel.c can set it and driver can test it.
> > 
> > Everything else inside the driver.
> > 
> > Until Intel can get their act together and actually do a CPUID bit
> > like AMD. :-P
> > 
> > But when you think about it, whether the model matching happens in
> > the driver or
> > in cpu/intel.c doesn't matter a whole lot.
> > 
> > All that matters is, they should finally give it a CPUID bit.
> 
> The other thing that matters here are the RAPL *incompatibilities*
> between 
> model variants, which are significant AFAICS.
> 
> With a CPUID we get a kind of semi-compatible hardware interface with
> well 
> defined semantics & expansion.

Agreed.
> 
> With 'non-architectural', per-model RAPL features we get very little
> of 
> that...

Exactly.

The main purpose of the model list in RAPL PMU code and the intel_rapl
driver is to differentiate the model-specific behavior, say,
some models use standard energy unit retrieved from MSR
some models use a fixed energy unit for Dram Domain
and
some models use a fixed energy unit for Psys Domain
etc.

> 
> Which is why it's a trade-off that is hard to judge in advance: maybe
> we 
> can simplify the code via a synthethic CPUID[s], maybe it will just
> be 
> another zoo of per-model feature flags...

Agreed.

> Likely won't be able to tell for sure until we see patches.
> 
Yeah, let me cook up a RFC series later and we can continue with that.

thanks,
rui