linux-kernel - Re: [PATCH v11 3/4] arm64: topology: Support SMT control on ACPI based system

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Message-ID: <2fdea4f6-db98-4dc7-947f-e19ee54d2c3c@arm.com>
Date: Tue, 4 Mar 2025 09:25:02 +0100
From: Pierre Gondois <pierre.gondois@....com>
To: Yicong Yang <yangyicong@...wei.com>, Sudeep Holla <sudeep.holla@....com>
Cc: yangyicong@...ilicon.com, catalin.marinas@....com, will@...nel.org,
 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, dietmar.eggemann@....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,
 sshegde@...ux.ibm.com
Subject: Re: [PATCH v11 3/4] arm64: topology: Support SMT control on ACPI
 based system



On 3/3/25 15:40, Yicong Yang wrote:
> On 2025/3/3 19:16, Sudeep Holla wrote:
>> On Mon, Mar 03, 2025 at 10:56:12AM +0100, Pierre Gondois wrote:
>>> On 2/28/25 20:06, Sudeep Holla wrote:
>>>>>>
>>>>>> Ditto as previous patch, can get rid if it is default 1.
>>>>>>
>>>>>
>>>>> On non-SMT platforms, not calling cpu_smt_set_num_threads() leaves
>>>>> cpu_smt_num_threads uninitialized to UINT_MAX:
>>>>>
>>>>> smt/active:0
>>>>> smt/control:-1
>>>>>
>>>>> If cpu_smt_set_num_threads() is called:
>>>>> active:0
>>>>> control:notsupported
>>>>>
>>>>> So it might be slightly better to still initialize max_smt_thread_num.
>>>>>
>>>>
>>>> Sure, what I meant is to have max_smt_thread_num set to 1 by default is
>>>> that is what needed anyways and the above code does that now.
>>>>
>>>> Why not start with initialised to 1 instead ?
>>>> Of course some current logic needs to change around testing it for zero.
>>>>
>>>
>>> I think there would still be a way to check against the default value.
>>> If we have:
>>> unsigned int max_smt_thread_num = 1;
>>>
>>> then on a platform with 2 threads, the detection condition would trigger:
>>> xa_for_each(&hetero_cpu, hetero_id, entry) {
>>>      if (entry->thread_num != max_smt_thread_num && max_smt_thread_num)     <---- (entry->thread_num=2) and (max_smt_thread_num=1)
>>>          pr_warn_once("Heterogeneous SMT topology is partly
>>>                        supported by SMT control\n");
>>>
>>> so we would need an additional variable:
>>> bool is_initialized = false;
>>
>> Sure, we could do that or skip the check if max_smt_thread_num == 1 ?
>>
>> I mean
>> 	if (entry->thread_num != max_smt_thread_num && max_smt_thread_num != 1)
>>

I think it will be problematic if we parse:
- first a CPU with 1 thread
- then a CPU with 2 threads

in that case we should detect the 'Heterogeneous SMT topology',
but we cannot because we don't know whether max_smt_thread_num=1
because 1 is the default value or we found a CPU with one thread.