Re: [PATCH 8/9] powerpc: Add HOTPLUG_SMT support

Laurent Dufour
2023-06-12T15:21:41Z

On 10/06/2023 23:10:02, Thomas Gleixner wrote:
> On Thu, Jun 01 2023 at 18:19, Laurent Dufour wrote:
>> @@ -435,12 +435,17 @@ void __init cpu_smt_disable(bool force)
>> * The decision whether SMT is supported can only be done after the full
>> * CPU identification. Called from architecture code.
>> */
>> -void __init cpu_smt_check_topology(unsigned int num_threads)
>> +void __init cpu_smt_check_topology(unsigned int num_threads,
>> + unsigned int max_threads)
>> {
>> if (!topology_smt_supported())
>> cpu_smt_control = CPU_SMT_NOT_SUPPORTED;
>>
>> - cpu_smt_max_threads = num_threads;
>> + cpu_smt_max_threads = max_threads;
>> +
>> + WARN_ON(num_threads > max_threads);
>> + if (num_threads > max_threads)
>> + num_threads = max_threads;
>
> This does not work. The call site does:
>
>> + cpu_smt_check_topology(smt_enabled_at_boot, threads_per_core);
>
> smt_enabled_at_boot is 0 when 'smt-enabled=off', which is not what the
> hotplug core expects. If SMT is disabled it brings up the primary
> thread, which means cpu_smt_num_threads = 1.

Thanks, Thomas,
Definitively, a test against smt_enabled_at_boot==0 is required here.

> This needs more thoughts to avoid a completely inconsistent duct tape
> mess.

Despite the test against smt_enabled_at_boot, mentioned above, I can't see
anything else to rework. Am I missing something?

>
> Btw, the command line parser and the variable smt_enabled_at_boot being
> type int allow negative number of threads too... Maybe not what you want.

I do agree, it should an unsigned type.

Thanks,
Laurent.

> Thanks,
>
> tglx
>
>
>
>