Re: [PATCH RESEND] x86/smpboot: avoid SMT domain attach/destroy if SMT is not enabled

[Thomas Gleixner]

On Tue, Apr 22 2025 at 16:47, Li Chen wrote:
> Currently, the SMT domain is added into sched_domain_topology
> by default if CONFIG_SCHED_SMT is enabled.
>
> If cpu_attach_domain finds that the CPU SMT domain’s cpumask_weight
> is just 1, it will destroy_sched_domain it.

If cpu_attach_domain() ..., it will destroy it.

> On a large machine, such as one with 512 cores, this results in
> 512 redundant domain attach/destroy operations.
>
> We can avoid these unnecessary operations by simply checking

s/We can avoid/Avoid/

> cpu_smt_num_threads and not inserting SMT domain into x86_topology if SMT

the SMT domain 

> is not enabled.
>
> #ifdef CONFIG_SCHED_SMT
> -    x86_topology[i++] = (struct sched_domain_topology_level){
> -        cpu_smt_mask, cpu_smt_flags, SD_INIT_NAME(SMT)
> -    };
> +    if (cpu_smt_num_threads > 1) {
> +        x86_topology[i++] = (struct sched_domain_topology_level){
> +            cpu_smt_mask, cpu_smt_flags, SD_INIT_NAME(SMT)
> +        };
> +    }

Looks about right, though I really detest this coding style. I'm not
blaming you, as you just followed the already existing bad taste...

Thanks,

        tglx