x86/smpboot: avoid SMT domain attach/destroy if SMT is not enabled

JIRA: https://issues.redhat.com/browse/RHEL-118964

commit f79c9aa
Author: Li Chen <chenl311@chinatelecom.cn>
Date:   Thu Jul 10 18:57:10 2025 +0800

    x86/smpboot: avoid SMT domain attach/destroy if SMT is not enabled

    Currently, the SMT domain is added into sched_domain_topology by default.

    If cpu_attach_domain() finds that the CPU SMT domain’s cpumask_weight
    is just 1, it will destroy it.

    On a large machine, such as one with 512 cores, this results in
    512 redundant domain attach/destroy operations.

    Avoid these unnecessary operations by simply checking
    cpu_smt_num_threads and skip SMT domain if the SMT domain is not
    enabled.

    Suggested-by: K Prateek Nayak <kprateek.nayak@amd.com>
    Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
    Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
    Link: https://lore.kernel.org/r/20250710105715.66594-5-me@linux.beauty

Signed-off-by: Phil Auld <pauld@redhat.com>

Author: auldp

arch/x86/kernel/smpboot.c

@@ -518,6 +518,8 @@ static struct sched_domain_topology_level x86_topology[] = {
 
 static void __init build_sched_topology(void)
 {
+	struct sched_domain_topology_level *topology = x86_topology;
+
 	/*
 	 * When there is NUMA topology inside the package invalidate the
 	 * PKG domain since the NUMA domains will auto-magically create the
@@ -528,7 +530,15 @@ static void __init build_sched_topology(void)
 
 		memset(&x86_topology[pkgdom], 0, sizeof(x86_topology[pkgdom]));
 	}
-	set_sched_topology(x86_topology);
+
+	/*
+	 * Drop the SMT domains if there is only one thread per-core
+	 * since it'll get degenerated by the scheduler anyways.
+	 */
+	if (cpu_smt_num_threads <= 1)
+		++topology;
+
+	set_sched_topology(topology);
 }
 
 void set_cpu_sibling_map(int cpu)