Re: [PATCH v3] topology: make core_mask include at least cluster_siblings

[Dietmar Eggemann <dietmar.eggemann@xxxxxxx>]

On 08/03/2022 12:04, Vincent Guittot wrote:
> On Tue, 8 Mar 2022 at 11:30, Will Deacon <will@xxxxxxxxxx> wrote:

[...]

>>> ---
>>> v1: Drop MC level if coregroup weight == 1
>>> v2: New sd topo in arch/arm64/kernel/smp.c
>>> v3: No new topo, extend core_mask to cluster_siblings
>>>
>>>  drivers/base/arch_topology.c | 8 ++++++++
>>>  1 file changed, 8 insertions(+)
>>>
>>> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
>>> index 976154140f0b..a96f45db928b 100644
>>> --- a/drivers/base/arch_topology.c
>>> +++ b/drivers/base/arch_topology.c
>>> @@ -628,6 +628,14 @@ const struct cpumask *cpu_coregroup_mask(int cpu)
>>>                       core_mask = &cpu_topology[cpu].llc_sibling;
>>>       }
>>>
>>> +     /*
>>> +      * For systems with no shared cpu-side LLC but with clusters defined,
>>> +      * extend core_mask to cluster_siblings. The sched domain builder will
>>> +      * then remove MC as redundant with CLS if SCHED_CLUSTER is enabled.

IMHO, if core_mask weight is 1, MC will be removed/degenerated anyway.

This is what I get on my Ampere Altra (I guess I don't have the ACPI
changes which would let to a CLS sched domain):

# cat /sys/kernel/debug/sched/domains/cpu0/domain*/name
DIE
NUMA
root@oss-altra01:~# zcat /proc/config.gz | grep SCHED_CLUSTER
CONFIG_SCHED_CLUSTER=y

>>> +      */
>>> +     if (cpumask_subset(core_mask, &cpu_topology[cpu].cluster_sibling))
>>> +             core_mask = &cpu_topology[cpu].cluster_sibling;
>>> +
>>
>> Sudeep, Vincent, are you happy with this now?
> 
> I would not say that I'm happy because this solution skews the core
> cpu mask in order to abuse the scheduler so that it will remove a
> wrong but useless level when it will build its domains.
> But this works so as long as the maintainer are happy, I'm fine

I do not have any better idea than this tweak here either in case the
platform can't provide a cleaner setup.

Maybe the following is easier to read but then we use
'&cpu_topology[cpu].llc_sibling' in cpu_coregroup_mask() already ...

@@ -617,6 +617,7 @@ EXPORT_SYMBOL_GPL(cpu_topology);
 const struct cpumask *cpu_coregroup_mask(int cpu)
 {
        const cpumask_t *core_mask = cpumask_of_node(cpu_to_node(cpu));
+       const cpumask_t *cluster_mask = cpu_clustergroup_mask(cpu);

        /* Find the smaller of NUMA, core or LLC siblings */
        if (cpumask_subset(&cpu_topology[cpu].core_sibling, core_mask)) {
@@ -628,6 +629,9 @@ const struct cpumask *cpu_coregroup_mask(int cpu)
                        core_mask = &cpu_topology[cpu].llc_sibling;
        }

+       if (cpumask_subset(core_mask, cluster_mask))
+               core_mask = cluster_mask;
+
        return core_mask;
 }

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>