Re: [PATCH v10 3/4] arm64: topology: Support SMT control on ACPI based system

[Yicong Yang <yangyicong@huawei.com>]

On 2024/12/24 0:40, Jonathan Cameron wrote:
> On Fri, 20 Dec 2024 15:53:12 +0800
> Yicong Yang <yangyicong@huawei.com> wrote:
> 
>> From: Yicong Yang <yangyicong@hisilicon.com>
>>
>> For ACPI we'll build the topology from PPTT and we cannot directly
>> get the SMT number of each core. Instead using a temporary xarray
>> to record the heterogeneous information (from ACPI_PPTT_ACPI_IDENTICAL)
>> and SMT information of the first core in its heterogeneous CPU cluster
>> when building the topology. Then we can know the largest SMT number
>> in the system. If a homogeneous system's using ACPI 6.2 or later,
>> all the CPUs should be under the root node of PPTT. There'll be
>> only one entry in the xarray and all the CPUs in the system will
>> be assumed identical.
>>
>> The core's SMT control provides two interface to the users [1]:
>> 1) enable/disable SMT by writing on/off
>> 2) enable/disable SMT by writing thread number 1/max_thread_number
>>
>> If a system have more than one SMT thread number the 2) may
>> not handle it well, since there're multiple thread numbers in the
>> system and 2) only accept 1/max_thread_number. So issue a warning
>> to notify the users if such system detected.
>>
>> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/ABI/testing/sysfs-devices-system-cpu#n542
>> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> 
> A few trivial things inline.  Either way it's fine as really just my style
> preferences
> 
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> 
>> ---
>>  arch/arm64/kernel/topology.c | 66 ++++++++++++++++++++++++++++++++++++
>>  1 file changed, 66 insertions(+)
>>
>> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
>> index 1a2c72f3e7f8..85cb18d72a29 100644
>> --- a/arch/arm64/kernel/topology.c
>> +++ b/arch/arm64/kernel/topology.c
>> @@ -15,8 +15,10 @@
>>  #include <linux/arch_topology.h>
>>  #include <linux/cacheinfo.h>
>>  #include <linux/cpufreq.h>
>> +#include <linux/cpu_smt.h>
>>  #include <linux/init.h>
>>  #include <linux/percpu.h>
>> +#include <linux/xarray.h>
>>  
>>  #include <asm/cpu.h>
>>  #include <asm/cputype.h>
>> @@ -37,17 +39,28 @@ static bool __init acpi_cpu_is_threaded(int cpu)
>>  	return !!is_threaded;
>>  }
>>  
>> +struct cpu_smt_info {
>> +	int thread_num;
>> +	int core_id;
>> +};
>> +
>>  /*
>>   * Propagate the topology information of the processor_topology_node tree to the
>>   * cpu_topology array.
>>   */
>>  int __init parse_acpi_topology(void)
>>  {
>> +	int max_smt_thread_num = 0;
>> +	struct cpu_smt_info *entry;
>> +	struct xarray hetero_cpu;
>> +	unsigned long hetero_id;
>>  	int cpu, topology_id;
>>  
>>  	if (acpi_disabled)
>>  		return 0;
>>  
>> +	xa_init(&hetero_cpu);
>> +
>>  	for_each_possible_cpu(cpu) {
>>  		topology_id = find_acpi_cpu_topology(cpu, 0);
>>  		if (topology_id < 0)
>> @@ -57,6 +70,32 @@ int __init parse_acpi_topology(void)
>>  			cpu_topology[cpu].thread_id = topology_id;
>>  			topology_id = find_acpi_cpu_topology(cpu, 1);
>>  			cpu_topology[cpu].core_id   = topology_id;
>> +
>> +			/*
>> +			 * In the PPTT, CPUs below a node with the 'identical
>> +			 * implementation' flag have the same number of threads.
>> +			 * Count the number of threads for only one CPU (i.e.
>> +			 * one core_id) among those with the same hetero_id.
>> +			 * See the comment of find_acpi_cpu_topology_hetero_id()
>> +			 * for more details.
>> +			 *
>> +			 * One entry is created for each node having:
>> +			 * - the 'identical implementation' flag
>> +			 * - its parent not having the flag
>> +			 */
>> +			hetero_id = find_acpi_cpu_topology_hetero_id(cpu);
>> +			entry = (struct cpu_smt_info *)xa_load(&hetero_cpu, hetero_id);
> 
> Given xa_load returns a void *,
> 
> 			entry = xa_load(&hetero_cpu, hetero_id);
> 
> should be fine (I haven't checked local style, so feel free to ignore if
> local style is to cast anyway).  Maybe drag the definition of entry into
> a more local scope as well.
> 

sure. will get rid of the cast and checked it won't violate the local style.

> 
>> +			if (!entry) {
>> +				entry = kzalloc(sizeof(*entry), GFP_KERNEL);
>> +				WARN_ON(!entry);
>> +
>> +				entry->core_id = topology_id;
>> +				entry->thread_num = 1;
>> +				xa_store(&hetero_cpu, hetero_id,
>> +					 entry, GFP_KERNEL);
>> +			} else if (entry->core_id == topology_id) {
>> +				entry->thread_num++;
>> +			}
>>  		} else {
>>  			cpu_topology[cpu].thread_id  = -1;
>>  			cpu_topology[cpu].core_id    = topology_id;
>> @@ -67,6 +106,33 @@ int __init parse_acpi_topology(void)
>>  		cpu_topology[cpu].package_id = topology_id;
>>  	}
>>  
>> +	/*
>> +	 * This should be a short loop depending on the number of heterogeneous
>> +	 * CPU clusters. Typically on a homogeneous system there's only one
>> +	 * entry in the XArray.
>> +	 */
>> +	xa_for_each(&hetero_cpu, hetero_id, entry) {
>> +		if (entry->thread_num != max_smt_thread_num && max_smt_thread_num)
>> +			pr_warn_once("Heterogeneous SMT topology is partly supported by SMT control\n");
>> +
>> +		if (entry->thread_num > max_smt_thread_num)
>> +			max_smt_thread_num = entry->thread_num;
> 
> As with DT, maybe min is more informative?
> 
> 		max_smt_thread_num = min(max_smt_thread_num, entry->thread_num);
> 
> I don't care strongly about it though.

either's ok with me, will keep it consistent with DT.

Thanks.