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

[Darren Hart]

Ampere Altra defines CPU clusters in the ACPI PPTT. They share a Snoop
Control Unit, but have no shared CPU-side last level cache.

cpu_coregroup_mask() will return a cpumask with weight 1, while
cpu_clustergroup_mask() will return a cpumask with weight 2.

As a result, build_sched_domain() will BUG() once per CPU with:

BUG: arch topology borken
the CLS domain not a subset of the MC domain

The MC level cpumask is then extended to that of the CLS child, and is
later removed entirely as redundant. This sched domain topology is an
improvement over previous topologies, or those built without
SCHED_CLUSTER, particularly for certain latency sensitive workloads.
With the current scheduler model and heuristics, this is a desirable
default topology for Ampere Altra and Altra Max system.

Rather than create a custom sched domains topology structure and
introduce new logic in arch/arm64 to detect these systems, update the
core_mask so coregroup is never a subset of clustergroup, extending it
to cluster_siblings if necessary.

This has the added benefit over a custom topology of working for both
symmetric and asymmetric topologies. It does not address systems where
the cluster topology is above a populated mc topology, but these are not
considered today and can be addressed separately if and when they
appear.

The final sched domain topology for a 2 socket Ampere Altra system is
unchanged with or without CONFIG_SCHED_CLUSTER, and the BUG is avoided:

For CPU0:

CONFIG_SCHED_CLUSTER=y
CLS  [0-1]
DIE  [0-79]
NUMA [0-159]

CONFIG_SCHED_CLUSTER is not set
DIE  [0-79]
NUMA [0-159]

Cc: Sudeep Holla <sudeep.holla at arm.com>
Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael at kernel.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will at kernel.org>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Vincent Guittot <vincent.guittot at linaro.org>
Cc: Barry Song <song.bao.hua at hisilicon.com>
Cc: Valentin Schneider <valentin.schneider at arm.com>
Cc: D. Scott Phillips <scott at os.amperecomputing.com>
Cc: Ilkka Koskinen <ilkka at os.amperecomputing.com>
Cc: <stable at vger.kernel.org> # 5.16.x
Suggested-by: Barry Song <song.bao.hua at hisilicon.com>
Signed-off-by: Darren Hart <darren at os.amperecomputing.com>
---
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.
+	 */
+	if (cpumask_subset(core_mask, &cpu_topology[cpu].cluster_sibling))
+		core_mask = &cpu_topology[cpu].cluster_sibling;
+
 	return core_mask;
 }
 
-- 
2.31.1