linux-kernel - Re: [PATCH] cpufreq: intel_pstate: Register when ACPI PCCH is present

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Date:   Wed, 18 Jul 2018 12:51:34 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Andreas Herrmann <aherrmann@...e.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: intel_pstate: Register when ACPI PCCH is present

On Wed, Jul 18, 2018 at 12:43 PM, Andreas Herrmann <aherrmann@...e.com> wrote:
> On Tue, Jul 17, 2018 at 08:06:54PM +0200, Rafael J. Wysocki wrote:
>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>
>> Currently, intel_pstate doesn't load if _PSS is not present on
>> HP Proliant systems, because it expects the firmware to take over
>> CPU performance scaling in that case.  However, if ACPI PCCH is
>> present, the firmware expects the kernel to use it for CPU
>> performance scaling and the pcc-cpufreq driver is loaded for that.
>>
>> Unfortunately, the firmware interface used by that driver is not
>> scalable for fundamental reasons, so pcc-cpufreq is way suboptimal
>> on systems with more than just a few CPUs.  In fact, it is better to
>> avoid using it at all.
>>
>> For this reason, modify intel_pstate to look for ACPI PCCH if _PSS
>> is not present and register if it is there.  Also prevent the
>> pcc-cpufreq driver from trying to initialize if intel_pstate has
>> been registered already.
>>
>> Fixes: fbbcdc0744da (intel_pstate: skip the driver if ACPI has power mgmt option)
>> Reported-by: Andreas Herrmann <aherrmann@...e.com>
>> Reviewed-by: Andreas Herrmann <aherrmann@...e.com>
>> Acked-by: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>> ---
>>
>> This is a replacement for https://patchwork.kernel.org/patch/10530091/
>>
>> In addition to the intel_pstate changes in the above, it also
>> prevents pcc-cpufreq from trying to initialize (which will fail
>> ultimately, but may confuse the firmware in the meantime).
>>
>> Andreas, please test this one and let me know if it still works for you.
>
> Done that (with system in "Dynamic Power Savings Mode"). It still
> works and system was stable.

Thanks!

> FYI, as a sniff test I've run a kernbench test. I now repeat the test
> with system switched to "OS control mode". Just in case -- if there
> was interference with platform code while system was in "Dynamic Power
> Savings Mode" (significantly affecting performance) I might spot it
> this way.

I'd rather not expect performance to be affected by this, but power
very well may be affected.

Anyway, good idea!

Cheers,
Rafael