[PATCH V2 2/2] x86/smpboot: avoid SMT domain attach/destroy if SMT is not enabled

[Li Chen <me@linux.beauty>]

From: Li Chen <chenl311@chinatelecom.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 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 not inserting SMT domain into x86_topology if
the SMT domain is not enabled.

Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
---
 arch/x86/kernel/smpboot.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 7d202f9785362..9ff8b10715cc1 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -492,8 +492,24 @@ static struct sched_domain_topology_level x86_topology[] = {
 	{ NULL },
 };
 
+static void __init maybe_remove_smt_level(void)
+{
+	if (cpu_smt_num_threads <= 1) {
+		/*
+		 * SMT level is x86_topology[0].  Shift the array left by one,
+		 * keep the sentinel { NULL } at the end.
+		 */
+		memmove(&x86_topology[0], &x86_topology[1],
+			sizeof(x86_topology) - sizeof(x86_topology[0]));
+		memset(&x86_topology[ARRAY_SIZE(x86_topology) - 1], 0,
+		       sizeof(x86_topology[0]));
+	}
+}
+
 static void __init build_sched_topology(void)
 {
+	maybe_remove_smt_level();
+
 	/*
 	 * When there is NUMA topology inside the package invalidate the
 	 * PKG domain since the NUMA domains will auto-magically create the
-- 
2.49.0