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

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Message-ID: <1965cae22a0.12ab5a70c833868.7155412488566097801@linux.beauty>
Date: Tue, 22 Apr 2025 16:47:18 +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: [PATCH RESEND] x86/smpboot: avoid SMT domain attach/destroy if SMT
 is not enabled

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