Re: [PATCH v11 3/4] arm64: topology: Support SMT control on ACPI based system

[Yicong Yang]

On 2025/2/25 14:08, Hanjun Guo wrote:
> On 2025/2/18 22:10, Yicong Yang wrote:
>> From: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>>
>> 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
>>
>> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
>> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>> ---
>>   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..6eba1ac091ee 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 {
>> +    unsigned 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)
>>   {
>> +    unsigned 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 +70,34 @@ 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 = xa_load(&hetero_cpu, hetero_id);
>> +            if (!entry) {
>> +                entry = kzalloc(sizeof(*entry), GFP_KERNEL);
>> +                WARN_ON_ONCE(!entry);
>> +
>> +                if (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 +108,31 @@ 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");
>> +
>> +        max_smt_thread_num = max(max_smt_thread_num, entry->thread_num);
>> +        xa_erase(&hetero_cpu, hetero_id);
>> +        kfree(entry);
>> +    }
>> +
>> +    /*
>> +     * Notify the CPU framework of the SMT support. Initialize the
>> +     * max_smt_thread_num to 1 if no SMT support detected. A thread
>> +     * number of 1 can be handled by the framework so we don't need
>> +     * to check max_smt_thread_num to see we support SMT or not.
>> +     */
>> +    if (!max_smt_thread_num)
>> +        max_smt_thread_num = 1;
>> +
>> +    cpu_smt_set_num_threads(max_smt_thread_num, max_smt_thread_num);
>> +    xa_destroy(&hetero_cpu);
>>       return 0;
>>   }
>>   #endif
> 
> Looks good to me,
> 
> Reviewed-by: Hanjun Guo <guohanjun@xxxxxxxxxx>
> 

Thanks a lot for taking a look :)