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

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Message-ID: <85c1501c-f398-bf96-f8b9-383fbb32d12f@huawei.com>
Date: Mon, 19 Aug 2024 15:18:35 +0800
From: Yicong Yang <yangyicong@...wei.com>
To: Dietmar Eggemann <dietmar.eggemann@....com>
CC: <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>, <yangyicong@...ilicon.com>,
	<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>, <xuwei5@...wei.com>,
	<guohanjun@...wei.com>
Subject: Re: [PATCH v5 2/4] arch_topology: Support SMT control for OF based
 system

On 2024/8/16 23:55, Dietmar Eggemann wrote:
> 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.
> 

My system's based on ACPI. For DT part it's emulated and tested on the QEMU VM.

>> 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).

Yes indeed. Was doing this purposely since I think this doing nothing unexpectedly but
only update the max threads recorded in the framework.

>> +}
>> +
>>  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;
>  

This fine with me and this avoid calling cpu_smt_set_num_threads() multiple
times. We can switch to this implementation.

Thanks.

>  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;
>  }
> 
> 
> .
>