Re: [PATCH v7 1/4] cpu/SMT: Provide a default topology_is_primary_thread()

[Yicong Yang <yangyicong@huawei.com>]

On 2024/10/30 22:55, Thomas Gleixner wrote:
> On Wed, Oct 30 2024 at 20:54, Yicong Yang wrote:
>>  
>> +#ifndef topology_is_primary_thread
>> +#define topology_is_primary_thread topology_is_primary_thread
> 
> Please do not glue defines and functions together w/o a newline in between.
> 

sure, will add a newline here.

>> +static inline bool topology_is_primary_thread(unsigned int cpu)
>> +{
>> +	/*
>> +	 * On SMT hotplug the primary thread of the SMT won't be disabled.
>> +	 * Architectures do have a special primary thread (e.g. x86) need
>> +	 * to override this function. Otherwise just make the first thread
>> +	 * in the SMT as the primary thread.
>> +	 */
>> +	return cpu == cpumask_first(topology_sibling_cpumask(cpu));
> 
> How is that supposed to work? Assume both siblings are offline, then the
> sibling mask is empty and you can't boot the CPU anymore.
> 

For architectures' using arch_topology, topology_sibling_cpumask() will at least
contain the tested CPU itself. This is initialized in
drivers/base/arch_topology.c:reset_cpu_topology(). So it won't be empty here.

Besides we don't need to check topology_is_primary_thread() at boot time:
-> cpu_up(cpu)
     cpu_bootable()
       if (cpu_smt_control == CPU_SMT_ENABLED &&
           cpu_smt_thread_allowed(cpu)) // will always return true if !CONFIG_SMT_NUM_THREADS_DYNAMIC
         return true; // we'll always return here and @cpu is always bootable

Also tested fine in practice.

Thanks.