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

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, Oct 31 2024 at 20:17, Yicong Yang wrote:
> On 2024/10/30 22:55, Thomas Gleixner wrote:
>>> +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.

Fair enough. Can you please expand the comment and say:

     The sibling cpumask of a offline CPU contains always the CPU
     itself.

> 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

cpu_smt_control is not guaranteed to have CPU_SMT_ENABLED state, so this
argument is bogus.

> Also tested fine in practice.

I've heard that song before.

What matters is not what you tested. What matters is whether the code is
correct _and_ understandable.

Thanks,

        tglx