linux-kernel - Re: [RFC PATCH 2/4] cpufreq: acpi-cpufreq: Add processor to the ignore PSD override list

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Message-ID: <87sg88tt5e.fsf@stealth>
Date:   Mon, 14 Dec 2020 22:27:09 +0900
From:   Punit Agrawal <punitagrawal@...il.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Wei Huang <whuang2@....com>, rjw@...ysocki.net, wei.huang2@....com,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        x86@...nel.org
Subject: Re: [RFC PATCH 2/4] cpufreq: acpi-cpufreq: Add processor to the
 ignore PSD override list

Borislav Petkov <bp@...en8.de> writes:

> On Sat, Dec 12, 2020 at 08:36:48AM +0900, Punit Agrawal wrote:
>> To me it suggests, that there are likely more systems from the family
>> that show the characteristic described below.
>
> Until we find a *single* system with a broken BIOS which has those
> objects kaputt and then this heuristic would need an exception.
>
> VS the clear statement from AMD that from zen3 onwards, all BIOS will be
> tested. I hope they boot Linux at least before they ship.

IIUC, this suggests that Linux booting on anything prior to Zen3 is down
to pure luck - I hope that wasn't the case.

>> In all these systems, the override causes this topology information to
>> be ignored - treating each core to be a separate domain. The proposed
>> patch removes the override so that _PSD is taken into account.
>
> You're still not answering my question: what does the coupling of the
> SMT threads bring on those systems? Power savings? Perf improvement?
> Anything palpable or measurable?

At the moment acpi thermals is bust on this and other affected AMD
system I have access to. That'll need fixing before any sensible
measurements can be run.

Tbh, I didn't quite expect the patch to the PSD exclusion list to be so
controversial - especially when a similar change for zen3 had recently
been merged. If you're really not keen on the change, I will carry it
locally for the time being and revisit once the other issues have been
resolved.