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

[Jeremy Linton]

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@xxxxxxx>
> > ---
> >   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@xxxxxxx>

Thanks,