linux-kernel - Re: [PATCH 8/9] powerpc: Add HOTPLUG

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Message-ID: <c0fe7b2c-1ce7-3d2f-0175-e61920fc3dee@linux.ibm.com>
Date:   Mon, 12 Jun 2023 17:20:01 +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, dave.hansen@...ux.intel.com,
        x86@...nel.org, mingo@...hat.com, bp@...en8.de,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 8/9] powerpc: Add HOTPLUG_SMT support

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
> 
> 
> 
>