Re: [PATCH v8 13/13] arm64: topology: divorce MC scheduling domain from core_siblings

[Jeremy Linton <jeremy.linton@arm.com>]

Hi,

Thanks for taking a look at this.

On 05/01/2018 09:33 AM, Sudeep Holla wrote:
> 
> 
> On 26/04/18 00:31, Jeremy Linton wrote:
>> Now that we have an accurate view of the physical topology
>> we need to represent it correctly to the scheduler. Generally MC
>> should equal the LLC in the system, but there are a number of
>> special cases that need to be dealt with.
>>
>> In the case of NUMA in socket, we need to assure that the sched
>> domain we build for the MC layer isn't larger than the DIE above it.
>> Similarly for LLC's that might exist in cross socket interconnect or
>> directory hardware we need to assure that MC is shrunk to the socket
>> or NUMA node.
>>
>> This patch builds a sibling mask for the LLC, and then picks the
>> smallest of LLC, socket siblings, or NUMA node siblings, which
>> gives us the behavior described above. This is ever so slightly
>> different than the similar alternative where we look for a cache
>> layer less than or equal to the socket/NUMA siblings.
>>
>> The logic to pick the MC layer affects all arm64 machines, but
>> only changes the behavior for DT/MPIDR systems if the NUMA domain
>> is smaller than the core siblings (generally set to the cluster).
>> Potentially this fixes a possible bug in DT systems, but really
>> it only affects ACPI systems where the core siblings is correctly
>> set to the socket siblings. Thus all currently available ACPI
>> systems should have MC equal to LLC, including the NUMA in socket
>> machines where the LLC is partitioned between the NUMA nodes.
>>
>> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
>> ---
>>   arch/arm64/include/asm/topology.h |  2 ++
>>   arch/arm64/kernel/topology.c      | 32 +++++++++++++++++++++++++++++++-
>>   2 files changed, 33 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/include/asm/topology.h b/arch/arm64/include/asm/topology.h
>> index 6b10459e6905..df48212f767b 100644
>> --- a/arch/arm64/include/asm/topology.h
>> +++ b/arch/arm64/include/asm/topology.h
>> @@ -8,8 +8,10 @@ struct cpu_topology {
>>   	int thread_id;
>>   	int core_id;
>>   	int package_id;
>> +	int llc_id;
>>   	cpumask_t thread_sibling;
>>   	cpumask_t core_sibling;
>> +	cpumask_t llc_siblings;
>>   };
>>   
>>   extern struct cpu_topology cpu_topology[NR_CPUS];
>> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
>> index bd1aae438a31..20b4341dc527 100644
>> --- a/arch/arm64/kernel/topology.c
>> +++ b/arch/arm64/kernel/topology.c
>> @@ -13,6 +13,7 @@
>>   
>>   #include <linux/acpi.h>
>>   #include <linux/arch_topology.h>
>> +#include <linux/cacheinfo.h>
>>   #include <linux/cpu.h>
>>   #include <linux/cpumask.h>
>>   #include <linux/init.h>
>> @@ -214,7 +215,19 @@ EXPORT_SYMBOL_GPL(cpu_topology);
>>   
>>   const struct cpumask *cpu_coregroup_mask(int cpu)
>>   {
>> -	return &cpu_topology[cpu].core_sibling;
>> +	const cpumask_t *core_mask = cpumask_of_node(cpu_to_node(cpu));
>> +
>> +	/* Find the smaller of NUMA, core or LLC siblings */
>> +	if (cpumask_subset(&cpu_topology[cpu].core_sibling, core_mask)) {
>> +		/* not numa in package, lets use the package siblings */
>> +		core_mask = &cpu_topology[cpu].core_sibling;
>> +	}
>> +	if (cpu_topology[cpu].llc_id != -1) {
>> +		if (cpumask_subset(&cpu_topology[cpu].llc_siblings, core_mask))
>> +			core_mask = &cpu_topology[cpu].llc_siblings;
>> +	}
>> +
>> +	return core_mask;
>>   }
>>   
>>   static void update_siblings_masks(unsigned int cpuid)
>> @@ -226,6 +239,9 @@ static void update_siblings_masks(unsigned int cpuid)
>>   	for_each_possible_cpu(cpu) {
>>   		cpu_topo = &cpu_topology[cpu];
>>   
>> +		if (cpuid_topo->llc_id == cpu_topo->llc_id)
>> +			cpumask_set_cpu(cpu, &cpuid_topo->llc_siblings);
>> +
> 
> Would this not result in cpuid_topo->llc_siblings = cpu_possible_mask
> on DT systems where llc_id is not set/defaults to -1 and still pass the
> condition. Does it make sense to add additional -1 check ?
(see comment in Morton's thread)

> 
>>   		if (cpuid_topo->package_id != cpu_topo->package_id)
>>   			continue;
>>   
>> @@ -291,6 +307,10 @@ static void __init reset_cpu_topology(void)
>>   		cpu_topo->core_id = 0;
>>   		cpu_topo->package_id = -1;
>>   
>> +		cpu_topo->llc_id = -1;
>> +		cpumask_clear(&cpu_topo->llc_siblings);
>> +		cpumask_set_cpu(cpu, &cpu_topo->llc_siblings);
>> +
>>   		cpumask_clear(&cpu_topo->core_sibling);
>>   		cpumask_set_cpu(cpu, &cpu_topo->core_sibling);
>>   		cpumask_clear(&cpu_topo->thread_sibling);
>> @@ -311,6 +331,8 @@ static int __init parse_acpi_topology(void)
>>   	is_threaded = read_cpuid_mpidr() & MPIDR_MT_BITMASK;
>>   
>>   	for_each_possible_cpu(cpu) {
>> +		int i;
>> +
>>   		topology_id = find_acpi_cpu_topology(cpu, 0);
>>   		if (topology_id < 0)
>>   			return topology_id;
>> @@ -325,6 +347,14 @@ static int __init parse_acpi_topology(void)
>>   		}
>>   		topology_id = find_acpi_cpu_topology_package(cpu);
>>   		cpu_topology[cpu].package_id = topology_id;
>> +
>> +		i = acpi_find_last_cache_level(cpu);
>> +
>> +		if (i > 0) {
>> +			topology_id = find_acpi_cpu_cache_topology(cpu, i);
>> +			if (topology_id > 0)
>> +				cpu_topology[cpu].llc_id = topology_id;
>> +		}
> 
> [nit] s/topology_id/cache_id/ or s/topology_id/cache_topology_id/ ?

Sure.

> 
> Otherwise looks fine to me. You can add with above things fixed.
> 
> Acked-by: Sudeep Holla <sudeep.holla@arm.com>
> 

Thanks,