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

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Message-ID: <ec04a084-0e01-afe3-6836-ed33615926ab@huawei.com>
Date: Fri, 27 Dec 2024 15:16:47 +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>, Nysal Jan K.A. <nysal@...ux.ibm.com>
Subject: Re: [PATCH v10 0/4] Support SMT control on arm64

On 2024/12/26 20:28, Shrikanth Hegde wrote:
> 
> 
> On 12/26/24 17:20, Yicong Yang wrote:
>> 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.
> 
> Ok. so this is a symmetric system then?

yes, symmetric.

> 
>> System wide power consumption comes down from the CPU offlining here.
>>
> 
> Ok. So SMT is enabled by default and then at runtime disable it to save power by off-lining the sibling threads?
> 

yes.

> 
> Note: When enabling SMT, current behavior differs when a core is fully offline on different archs. You may want to see which is behavior you need in that case. i.e either online or skip.
> 
> PowerPC change where we are skipping a fully offline core.
> https://lore.kernel.org/all/20240731030126.956210-1-nysal@linux.ibm.com/
> 

Thanks for the information! Currently it's implemented as online and no special need for skip. We may need further
support if skip is required in the future, currently for GENERIC_ARCH_TOPOLOGY an offline CPU's thread sibling
only contains itself so there's no information for checking whether the whole core is offline or not.

Thanks.

>> 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.
>>>>
>>> .
> 
> .