linux-kernel - Re: [RFC][PATCH v1 3/3] cpufreq: intel_pstate: Set asymmetric CPU capacity on hybrid systems

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Date: Fri, 3 May 2024 20:29:51 +0200
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>, x86 Maintainers
 <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
 Linux PM <linux-pm@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Peter Zijlstra <peterz@...radead.org>,
 Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Ricardo Neri <ricardo.neri@...el.com>, Tim Chen <tim.c.chen@...el.com>
Subject: Re: [RFC][PATCH v1 3/3] cpufreq: intel_pstate: Set asymmetric CPU
 capacity on hybrid systems

On 03/05/2024 05:32, Ricardo Neri wrote:
> On Thu, May 02, 2024 at 12:42:54PM +0200, Dietmar Eggemann wrote:
>> On 25/04/2024 21:06, Rafael J. Wysocki wrote:
>>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>>
>>> Make intel_pstate use the HWP_HIGHEST_PERF values from
>>> MSR_HWP_CAPABILITIES to set asymmetric CPU capacity information
>>> via the previously introduced arch_set_cpu_capacity() on hybrid
>>> systems without SMT.
>>
>> Are there such systems around? My i7-13700K has P-cores (CPU0..CPU15)
>> with SMT.
> 
> We have been experimenting with nosmt in the kernel command line.

OK.

[...]

>>> If the given system is hybrid and non-SMT, the new code disables ITMT
>>> support in the scheduler (because it may get in the way of asymmetric CPU
>>> capacity code in the scheduler that automatically gets enabled by setting
>>> asymmetric CPU capacity) after initializing all online CPUs and finds
>>> the one with the maximum HWP_HIGHEST_PERF value.  Next, it computes the
>>> capacity number for each (online) CPU by dividing the product of its
>>> HWP_HIGHEST_PERF and SCHED_CAPACITY_SCALE by the maximum HWP_HIGHEST_PERF.
>>
>> SO either CAS at wakeup and in load_balance or SIS at wakeup and ITMT in
>> load balance.
> 
> May I know what CAS and SIS stand for?

Capacity Aware Scheduling and Select_Idle_Sibling().

Either   select_idle_sibling() -> select_idle_capacity()         (1)

or       select_idle_sibling() -> select_idle_cpu() /* nosmt */  (2)

So my system with now 'nosmt' goes (1).