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

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 8038286454cf1bc22377546aa9362bf952aac886..412c813cd26820558c99477ff03af6e59eadeba3 100644 (file)
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -492,6 +492,8 @@ 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
@@ -502,7 +504,15 @@ 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)