arch_topology: Limit span of cpu_clustergroup_mask()

Currently the cluster identifier is not set on DT based platforms.
The reset or default value is -1 for all the CPUs. Once we assign the
cluster identifier values correctly, the cluster_sibling mask will be
populated and returned by cpu_clustergroup_mask() to contribute in the
creation of the CLS scheduling domain level, if SCHED_CLUSTER is
enabled.

To avoid topologies that will result in questionable or incorrect
scheduling domains, impose restrictions regarding the span of clusters,
as presented to scheduling domains building code: cluster_sibling should
not span more or the same CPUs as cpu_coregroup_mask().

This is needed in order to obtain a strict separation between the MC and
CLS levels, and maintain the same domains for existing platforms in
the presence of CONFIG_SCHED_CLUSTER, where the new cluster information
is redundant and irrelevant for the scheduler.

While previously the scheduling domain builder code would have removed MC
as redundant and kept CLS if SCHED_CLUSTER was enabled and the
cpu_coregroup_mask() and cpu_clustergroup_mask() spanned the same CPUs,
now CLS will be removed and MC kept.

Link: https://lore.kernel.org/r/20220704101605.1318280-18-sudeep.holla@arm.com
Cc: Darren Hart <darren@os.amperecomputing.com>
Tested-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/base/arch_topology.c

@@ -686,6 +686,14 @@ const struct cpumask *cpu_coregroup_mask(int cpu)
 
 const struct cpumask *cpu_clustergroup_mask(int cpu)
 {
+	/*
+	 * Forbid cpu_clustergroup_mask() to span more or the same CPUs as
+	 * cpu_coregroup_mask().
+	 */
+	if (cpumask_subset(cpu_coregroup_mask(cpu),
+			   &cpu_topology[cpu].cluster_sibling))
+		return get_cpu_mask(cpu);
+
 	return &cpu_topology[cpu].cluster_sibling;
 }