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

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Message-ID: <87msa2r018.ffs@tglx>
Date: Fri, 20 Jun 2025 15:54:27 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Li Chen <me@...ux.beauty>, Ingo Molnar <mingo@...hat.com>, Borislav
 Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86
 <x86@...nel.org>, linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND] x86/smpboot: avoid SMT domain attach/destroy if
 SMT is not enabled

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