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

[Pierre Gondois <pierre.gondois@arm.com>]

On 2/28/25 14:56, Sudeep Holla wrote:
> On Tue, Feb 18, 2025 at 10:10:17PM +0800, Yicong Yang 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
>>
>> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
>> Signed-off-by: Yicong Yang <yangyicong@hisilicon.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..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");
> 
> Ditto as previous patch about handling no threaded cores with threaded cores
> in the system. I am not sure if that is required but just raising it here.
> 
>> +
>> +		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;
>> +
> 
> Ditto as previous patch, can get rid if it is default 1.
> 

On non-SMT platforms, not calling cpu_smt_set_num_threads() leaves
cpu_smt_num_threads uninitialized to UINT_MAX:

smt/active:0
smt/control:-1

If cpu_smt_set_num_threads() is called:
active:0
control:notsupported

So it might be slightly better to still initialize max_smt_thread_num.

Otherwise I tested the patches on arm64 ACPI smt platforms and it worked
well, so for all the patches (if there are no other major modifications):
Reviewed-by: Pierre Gondois <pierre.gondois@arm.com>

Regards,
Pierre