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

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

On 2/28/25 20:06, Sudeep Holla wrote:
> On Fri, Feb 28, 2025 at 06:51:16PM +0100, Pierre Gondois wrote:
>>
>>
>> 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.
>>
> 
> Sure, what I meant is to have max_smt_thread_num set to 1 by default is
> that is what needed anyways and the above code does that now.
> 
> Why not start with initialised to 1 instead ?
> Of course some current logic needs to change around testing it for zero.
> 

I think there would still be a way to check against the default value.
If we have:
unsigned int max_smt_thread_num = 1;

then on a platform with 2 threads, the detection condition would trigger:
xa_for_each(&hetero_cpu, hetero_id, entry) {
     if (entry->thread_num != max_smt_thread_num && max_smt_thread_num)     <---- (entry->thread_num=2) and (max_smt_thread_num=1)
         pr_warn_once("Heterogeneous SMT topology is partly
                       supported by SMT control\n");

so we would need an additional variable:
bool is_initialized = false;