linux-kernel - Re: [PATCH v10 0/4] Support SMT control on arm64

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Message-ID: <a9e19d83-cfc4-910c-7bca-b2dd17647c5f@huawei.com>
Date: Thu, 26 Dec 2024 19:50:36 +0800
From: Yicong Yang <yangyicong@...wei.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
CC: <yangyicong@...ilicon.com>, <linuxppc-dev@...ts.ozlabs.org>,
	<x86@...nel.org>, <linux-kernel@...r.kernel.org>, <morten.rasmussen@....com>,
	<msuchanek@...e.de>, <gregkh@...uxfoundation.org>, <rafael@...nel.org>,
	<jonathan.cameron@...wei.com>, <prime.zeng@...ilicon.com>,
	<linuxarm@...wei.com>, <xuwei5@...wei.com>, <guohanjun@...wei.com>,
	<catalin.marinas@....com>, <will@...nel.org>, <sudeep.holla@....com>,
	<tglx@...utronix.de>, <peterz@...radead.org>, <mpe@...erman.id.au>,
	<linux-arm-kernel@...ts.infradead.org>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>, <pierre.gondois@....com>,
	<dietmar.eggemann@....com>
Subject: Re: [PATCH v10 0/4] Support SMT control on arm64

On 2024/12/26 17:23, Shrikanth Hegde wrote:
> 
> 
> On 12/20/24 13:23, Yicong Yang wrote:
>> From: Yicong Yang <yangyicong@...ilicon.com>
>>
>> The core CPU control framework supports runtime SMT control which
>> is not yet supported on arm64. Besides the general vulnerabilities
>> concerns we want this runtime control on our arm64 server for:
>>
>> - better single CPU performance in some cases
>> - saving overall power consumption
>>
> 
> EAS is disabled when SMT is present.
> I am curious to know how power saving happens here.

EAS shouldn't work on non-asymmetic systems, so it's not the case here.
System wide power consumption comes down from the CPU offlining here.

Thanks.

> 
>> This patchset implements it in the following aspects:
>>
>> - Provides a default topology_is_primary_thread()
>> - support retrieve SMT thread number on OF based system
>> - support retrieve SMT thread number on ACPI based system
>> - select HOTPLUG_SMT for arm64
>>
>> Tests has been done on our ACPI based arm64 server and on ACPI/OF\
>> based QEMU VMs.
>>
> .