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@arm.com Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: "Rafael J. Wysocki" rafael@kernel.org Cc: Catalin Marinas catalin.marinas@arm.com Cc: Will Deacon will@kernel.org Cc: Peter Zijlstra peterz@infradead.org Cc: Vincent Guittot vincent.guittot@linaro.org Cc: Barry Song song.bao.hua@hisilicon.com Cc: Valentin Schneider valentin.schneider@arm.com Cc: D. Scott Phillips scott@os.amperecomputing.com Cc: Ilkka Koskinen ilkka@os.amperecomputing.com Cc: stable@vger.kernel.org # 5.16.x Suggested-by: Barry Song song.bao.hua@hisilicon.com Signed-off-by: Darren Hart darren@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; }