linux-kernel - [PATCH v4 4/4] x86/smpboot: avoid SMT domain attach/destroy if SMT is not enabled

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Message-ID: <20250706030636.397197-5-me@linux.beauty>
Date: Sun,  6 Jul 2025 11:06:32 +0800
From: Li Chen <me@...ux.beauty>
To: Li Chen  <me@...ux.beauty>," Thomas Gleixner "<tglx@...utronix.de>," Ingo Molnar "<mingo@...hat.com>," Borislav Petkov "<bp@...en8.de>," Dave Hansen "<dave.hansen@...ux.intel.com>,<x86@...nel.org>," H . Peter Anvin "<hpa@...or.com>," Rafael J . Wysocki "<rafael.j.wysocki@...el.com>," Peter Zijlstra "<peterz@...radead.org>," Sohil Mehta "<sohil.mehta@...el.com>," Brian Gerst "<brgerst@...il.com>," Patryk Wlazlyn " <patryk.wlazlyn@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Li Chen <chenl311@...natelecom.cn>,
	Sohil Mehta <sohil.mehta@...el.com>,
	Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>,
	K Prateek Nayak <kprateek.nayak@....com>
Subject: [PATCH v4 4/4] 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 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.

Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
Tested-by: K Prateek Nayak <kprateek.nayak@....com>
Signed-off-by: Li Chen <chenl311@...natelecom.cn>
---
 arch/x86/kernel/smpboot.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index cd70e5322462a..8c1960a455bfb 100644
--- 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)
-- 
2.49.0