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Message-ID: <0b849052-0eb1-4085-aa65-73b7451cd6ba@arm.com>
Date: Fri, 3 May 2024 20:29:51 +0200
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Subject: Re: [RFC][PATCH v1 3/3] cpufreq: intel_pstate: Set asymmetric CPU
 capacity on hybrid systems
To: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>, x86 Maintainers
 <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
 Linux PM <linux-pm@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
 Peter Zijlstra <peterz@infradead.org>,
 Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
 "Rafael J. Wysocki" <rafael@kernel.org>,
 Ricardo Neri <ricardo.neri@intel.com>, Tim Chen <tim.c.chen@intel.com>
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 <050c561c-487e-4e89-a7b2-9752cebc9f46@arm.com>
 <20240503033242.GA14835@ranerica-svr.sc.intel.com>
From: Dietmar Eggemann <dietmar.eggemann@arm.com>
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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@intel.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).