linux-kernel - Re: [PATCH 3/9] cpu/SMT: Store the current/max number of threads

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Message-ID: <bf213e3c-e296-0709-511d-b5c0ee36d9ee@linux.ibm.com>
Date:   Wed, 14 Jun 2023 14:27:40 +0200
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org
Cc:     linux-arch@...r.kernel.org, x86@...nel.org,
        dave.hansen@...ux.intel.com, mingo@...hat.com, bp@...en8.de,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 3/9] cpu/SMT: Store the current/max number of threads

On 13/06/2023 20:53:56, Thomas Gleixner wrote:
> On Tue, Jun 13 2023 at 19:16, Laurent Dufour wrote:
>> On 10/06/2023 23:26:18, Thomas Gleixner wrote:
>>> On Thu, May 25 2023 at 01:56, Michael Ellerman wrote:
>>>>  #ifdef CONFIG_HOTPLUG_SMT
>>>>  enum cpuhp_smt_control cpu_smt_control __read_mostly = CPU_SMT_ENABLED;
>>>> +static unsigned int cpu_smt_max_threads __ro_after_init;
>>>> +unsigned int cpu_smt_num_threads;
>>>
>>> Why needs this to be global? cpu_smt_control is pointlessly global already.
>>
>> I agree that cpu_smt_*_threads should be static.

I spoke too quickly, cpu_smt_num_threads is used in the powerpc code.

When a new CPU is added it used to decide whether a thread has to be
onlined or not, and there is no way to pass it as argument at this time.
In details, it is used in topology_smt_thread_allowed() called by
dlpar_online_cpu() (see patch "powerpc/pseries: Honour current SMT state
when DLPAR onlining CPUs" at the end of this series).

I think the best option is to keep it global.

>>
>> Howwever, regarding cpu_smt_control, it is used in 2 places in the x86 code:
>>  - arch/x86/power/hibernate.c in arch_resume_nosmt()
>>  - arch/x86/kernel/cpu/bugs.c in spectre_v2_user_select_mitigation()
> 
> Bah. I must have fatfingered the grep then.
> 
>> An accessor function may be introduced to read that value in these 2
>> functions, but I'm wondering if that's really the best option.
>>
>> Unless there is a real need to change this through this series, I think
>> cpu_smt_control can remain global.
> 
> That's fine.
> 
> Thanks,
> 
>         tglx