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SubjectRe: [PATCH v8 13/13] arm64: topology: divorce MC scheduling domain from core_siblings
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Hi,

On 05/02/2018 06:49 AM, Morten Rasmussen wrote:
> On Tue, May 01, 2018 at 03:33:33PM +0100, 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 ?
>
> I don't think mask will be used by the current code if llc_id == -1 as
> the user does the check. Is it better to have the mask empty than
> default to cpu_possible_mask? If we require all users to implement a
> check it shouldn't matter.
>

Right.

There is also the other way of thinking about it, which is if you remove
the if llc_id == -1 check in cpu_coregroup_mask() does it make more
sense to have llc_siblings default equal all the cores, or just the one
being requested?

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