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

[Sudeep Holla <sudeep.holla@arm.com>]

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 ?

>  		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/ ?

Otherwise looks fine to me. You can add with above things fixed.

Acked-by: Sudeep Holla <sudeep.holla@arm.com>

-- 
Regards,
Sudeep