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

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Message-ID: <04c9757c-98e9-4c5d-9759-8a9ac9d6a803@linux.ibm.com>
Date: Thu, 26 Dec 2024 17:58:00 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Yicong Yang <yangyicong@...wei.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 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?

> 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?


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