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

Pierre Gondois
2025-03-04T08:34:44Z

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.