linux-kernel - Re: [PATCH v3 0/2] cpufreq/amd-pstate: Set initial min_freq to lowest_nonlinear

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Message-ID: <d7290507-bdb2-4bda-a33b-0a9459b9533b@amd.com>
Date: Mon, 6 Jan 2025 10:13:32 +0530
From: Dhananjay Ugwekar <Dhananjay.Ugwekar@....com>
To: Russell Haley <yumpusamongus@...il.com>,
 Mario Limonciello <mario.limonciello@....com>,
 Hanabishi <i.r.e.c.c.a.k.u.n+kernel.org@...il.com>
Cc: linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
 "Gautham R. Shenoy" <gautham.shenoy@....com>
Subject: Re: [PATCH v3 0/2] cpufreq/amd-pstate: Set initial min_freq to
 lowest_nonlinear_freq

On 1/5/2025 9:07 AM, Russell Haley wrote:
> 
> 
> On 12/8/24 10:35 AM, Mario Limonciello wrote:
>> On 12/8/2024 01:54, Hanabishi wrote:
>>> Hello. Maybe I'm too late on this, but I have some concerns.
>>>
>>> On 10/17/24 05:39, Dhananjay Ugwekar wrote:
>>>> In other systems, power consumption has increased but so has the
>>>> throughput/watt.
>>>
>>> I just want to bring up the fact that this change affects all
>>> governors. It sounds good for the performance governor, but not so
>>> much for the powersave governor.
>>>
>>> So the question is: don't we want the lowest power consumption
>>> possible in the powersave mode? Even if it means decreased efficiency.
>>> Powersave by definition supposed to make battery last as long as
>>> possible no matter what, isn't it?
>>>
>>
>> No, the powersave governor isn't a one stop shop to bring everything to
>> longest battery.
>>
>> By your argument we should set the EPP to "power" by default and "boost"
>> to off by default when the powersave governor is enacted?
>>
>> All of those are far too aggressive for a default behavior.  Setting the
>> lowest nonlinear frequency as the default lowest scaling frequency is
>> about having a good default that balances responsiveness, battery life
>> and performance.
>>
>> Like all knobs anyone that doesn't agree with it can of course modify it
>> from sysfs.
>>
> 
> If the documentation is correct, the lowest_nonlinear_frequency *does*
> result in the lowest battery consumption unless you are running one or
> more threads at 100% utilization until the battery dies. In that case,
> lowest nonlinear frequency should result in greatest number of
> instructions retired when the battery dies. I say instructions retired
> rather than work completed, because "100% until the battery dies" is
> only stress tests, malware, and damn-the-torpedos concurrency frameworks
> that use spinwaits.
> 
> If that is not true, then either the documentation is wrong, or the
> CPU's reporting of its lowest nonlinear frequency is wrong.
> 
> I am puzzled why the CPU even exposes frequencies below
> lowest-nonlinear. They should always be worse than PWM-ing between C0 at
> lowest nonlinear freq and some deeper C-state. 

I dont think we can assume that idling at lowest frequency would *always* be 
worse than going to the shallowest C-state (considering the c-state entry-exit 
latency), in terms of power, performance or tail latencies. This might vary 
between different systems and scenarios. 

Testing software that has
> to run on much slower CPUs, I guess?