[PATCH v3 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 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 remove SMT domain if the SMT domain is not
enabled, and adjust the PKG index accordingly if NUMA-in-package
invalidates that level as well.

Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
---
changelog:
v2: fix wording issue as suggested by Thomas [1]
v3: remove pointless memset and adjust PKG index accordingly,
    as suggested by Thomas [2] 

[1]: https://lore.kernel.org/all/87msa2r018.ffs@tglx/
[2]: https://lore.kernel.org/all/875xglntx1.ffs@tglx/

 arch/x86/kernel/smpboot.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 7d202f9785362..4b6daa1545445 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -494,13 +494,29 @@ static struct sched_domain_topology_level x86_topology[] = {
 
 static void __init build_sched_topology(void)
 {
+	bool smt_dropped = false;
+
+	if (cpu_smt_num_threads <= 1) {
+		/*
+		 * SMT level is x86_topology[0].  Shift the array left by one,
+		 */
+		memmove(&x86_topology[0], &x86_topology[1],
+			sizeof(x86_topology) - sizeof(x86_topology[0]));
+		smt_dropped = true;
+	}
+
 	/*
 	 * When there is NUMA topology inside the package invalidate the
 	 * PKG domain since the NUMA domains will auto-magically create the
 	 * right spanning domains based on the SLIT.
 	 */
 	if (x86_has_numa_in_package) {
-		unsigned int pkgdom = ARRAY_SIZE(x86_topology) - 2;
+		unsigned int pkgdom;
+
+		if (smt_dropped)
+			pkgdom = ARRAY_SIZE(x86_topology) - 3;
+		else
+			pkgdom = ARRAY_SIZE(x86_topology) - 2;
 
 		memset(&x86_topology[pkgdom], 0, sizeof(x86_topology[pkgdom]));
 	}
-- 
2.49.0