Re: [PATCH v3 2/2] x86/smpboot: avoid SMT domain attach/destroy if SMT is not enabled

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello Li,

On 6/25/2025 9:15 AM, Li Chen wrote:
> 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;
> +	}
> +

Instead of this memmove, wouldn't just passing the "&x86_topology[1]"
to set_sched_topology() when "cpu_smt_num_threads <= 1"  work?

Something like below:

(Only tested on a QEMU based VM)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 3818b16c19e1..1793248d28cd 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -494,6 +494,8 @@ 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
@@ -504,7 +506,15 @@ 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)


>   	/*
>   	 * 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]));
>   	}

-- 
Thanks and Regards,
Prateek