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

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Message-ID: <a7636c4b-f449-4018-a890-08412e3ec779@arm.com>
Date: Fri, 16 Aug 2024 17:55:18 +0200
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Yicong Yang <yangyicong@...wei.com>, catalin.marinas@....com,
 will@...nel.org, sudeep.holla@....com, tglx@...utronix.de,
 peterz@...radead.org, mpe@...erman.id.au,
 linux-arm-kernel@...ts.infradead.org, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com
Cc: linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
 linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org, rafael@...nel.org,
 jonathan.cameron@...wei.com, prime.zeng@...ilicon.com, linuxarm@...wei.com,
 yangyicong@...ilicon.com, xuwei5@...wei.com, guohanjun@...wei.com
Subject: Re: [PATCH v5 2/4] arch_topology: Support SMT control for OF based
 system

On 06/08/2024 10:53, Yicong Yang wrote:
> From: Yicong Yang <yangyicong@...ilicon.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 <yangyicong@...ilicon.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;
 }