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Message-ID: <2247ced8-c4f9-80a7-a0a2-0953abed135f@arm.com>
Date:   Fri, 22 Sep 2023 13:13:35 +0200
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Subject: Re: [PATCH] arch_topology: Support SMT control on arm64
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To:     Yicong Yang <yangyicong@huawei.com>, catalin.marinas@arm.com,
        will@kernel.org, sudeep.holla@arm.com,
        linux-arm-kernel@lists.infradead.org
Cc:     yangyicong@hisilicon.com, gregkh@linuxfoundation.org,
        rafael@kernel.org, jonathan.cameron@huawei.com,
        prime.zeng@hisilicon.com, linuxarm@huawei.com,
        linux-kernel@vger.kernel.org
References: <20230919123319.23785-1-yangyicong@huawei.com>
 <130ce530-a3c1-0918-e04f-cb7d94aef57b@arm.com>
 <80b2556c-b56c-ca23-22c7-70a0a3995701@huawei.com>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
In-Reply-To: <80b2556c-b56c-ca23-22c7-70a0a3995701@huawei.com>
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On 21/09/2023 10:56, Yicong Yang wrote:
> On 2023/9/21 1:08, Dietmar Eggemann wrote:
>> On 19/09/2023 14:33, Yicong Yang wrote:
>>> From: Yicong Yang <yangyicong@hisilicon.com>

[...]

> If you manually disable SMT by offline each CPUs the cpu_smt_control will
> not be updated. It'll updated when using the interface like
> `/sys/devices/system/cpu/smt/control` or cmdline. By these means,
> the framework will use topology_is_primary_thread() to decide which CPU
> in the SMT will keep online:
> 
> // e.g. echo off > /sys/devices/system/cpu/smt/control
> [ kernel/cpu.c ]
> control_store()
>   __store_smt_control()
>     cpuhp_smt_disable()
>       for_each_online_cpu(cpu)
>         if (topology_is_primary_thread(cpu))
>           continue;            <---------- will skip the primary thread
>       [...]
>       cpu_smt_control = CPU_SMT_DISABLED;
> 
> topology_is_primary_thread() checking only applies to the SMT control but
> not to the CPU offline.

I see, make sense. Retested on my SMT4 Arm64 server with 256 CPUs.