Re: [PATCH v5 2/4] arch_topology: Support SMT control for OF based system

[Dietmar Eggemann]

On 06/08/2024 10:53, Yicong Yang wrote:
> From: Yicong Yang <yangyic...@hisilicon.com>
> 
> On building the topology from the devicetree, we've already
> gotten the SMT thread number of each core. Update the largest
> SMT thread number to enable the SMT control.

Do we have SMT Device Tree (DT) systems out there? But you right that DT
at least supports SMT.

> Signed-off-by: Yicong Yang <yangyic...@hisilicon.com>
> ---
>  drivers/base/arch_topology.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index 75fcb75d5515..95513abd664f 100644
> --- a/drivers/base/arch_topology.c
> +++ b/drivers/base/arch_topology.c
> @@ -11,6 +11,7 @@
>  #include <linux/cleanup.h>
>  #include <linux/cpu.h>
>  #include <linux/cpufreq.h>
> +#include <linux/cpu_smt.h>
>  #include <linux/device.h>
>  #include <linux/of.h>
>  #include <linux/slab.h>
> @@ -531,6 +532,16 @@ static int __init get_cpu_for_node(struct device_node 
> *node)
>       return cpu;
>  }
>  
> +static void __init update_smt_num_threads(unsigned int num_threads)
> +{
> +     static unsigned int max_smt_thread_num = 1;
> +
> +     if (num_threads > max_smt_thread_num) {
> +             max_smt_thread_num = num_threads;
> +             cpu_smt_set_num_threads(max_smt_thread_num, max_smt_thread_num);
> +     }

This could theoretically (unlikely though) call
cpu_smt_set_num_threads() multiple times (on heterogeneous systems with
different numbers of SMT threads).
> +}
> +
>  static int __init parse_core(struct device_node *core, int package_id,
>                            int cluster_id, int core_id)
>  {
> @@ -561,6 +572,8 @@ static int __init parse_core(struct device_node *core, 
> int package_id,
>               i++;
>       } while (1);
>  
> +     update_smt_num_threads(i);
> +
>       cpu = get_cpu_for_node(core);
>       if (cpu >= 0) {
>               if (!leaf) {

Why not simply do this:

-->8--

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index 75fcb75d5515..806537419715 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -30,6 +30,7 @@ static struct cpumask scale_freq_counters_mask;
 static bool scale_freq_invariant;
 DEFINE_PER_CPU(unsigned long, capacity_freq_ref) = 1;
 EXPORT_PER_CPU_SYMBOL_GPL(capacity_freq_ref);
+static unsigned int max_smt_thread_num = 1;
 
 static bool supports_scale_freq_counters(const struct cpumask *cpus)
 {
@@ -577,6 +578,9 @@ static int __init parse_core(struct device_node *core, int 
package_id,
                return -EINVAL;
        }
 
+       if (max_smt_thread_num < i)
+               max_smt_thread_num = i;
+
        return 0;
 }
 
@@ -673,6 +677,9 @@ static int __init parse_socket(struct device_node *socket)
        if (!has_socket)
                ret = parse_cluster(socket, 0, -1, 0);
 
+       if (max_smt_thread_num > 1)
+               cpu_smt_set_num_threads(max_smt_thread_num, max_smt_thread_num);
+
        return ret;
 }