Commit f79c9aa4 authored Jul 10, 2025 by Li Chen Committed by Peter Zijlstra Jul 14, 2025

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

parent fbc2010d

arch/x86/kernel/smpboot.c

+11 −1

Original line number	Diff line number	Diff line
		@@ -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)