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

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Message-ID: <1978b29a34b.dbd6def8716427.2074630307673917130@linux.beauty>
Date: Fri, 20 Jun 2025 10:27:31 +0800
From: Li Chen <me@...ux.beauty>
To: "Thomas Gleixner" <tglx@...utronix.de>, "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

Gentle ping.

 ---- On Tue, 22 Apr 2025 16:47:18 +0800  Li Chen <me@...ux.beauty> wrote --- 
 > From: Li Chen <chenl311@...natelecom.cn>
 > 
 > 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.
 > 
 > 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
 > cpu_smt_num_threads and not inserting SMT domain into x86_topology if SMT
 > is not enabled.
 > 
 > Signed-off-by: Li Chen <chenl311@...natelecom.cn>
 > ---
 > 
 > [RESEND] because I forgot to add any mailing list previously.
 > 
 > arch/x86/kernel/smpboot.c | 8 +++++---
 > 1 file changed, 5 insertions(+), 3 deletions(-)
 > 
 > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
 > index d6cf1e23c2a3..da6192e1af12 100644
 > --- a/arch/x86/kernel/smpboot.c
 > +++ b/arch/x86/kernel/smpboot.c
 > @@ -485,9 +485,11 @@ static void __init build_sched_topology(void)
 >     int i = 0;
 > 
 > #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)
 > +        };
 > +    }
 > #endif
 > #ifdef CONFIG_SCHED_CLUSTER
 >     x86_topology[i++] = (struct sched_domain_topology_level){
 > --
 > 2.48.1
 > 
 > 

Regards,
Li