[PATCH 1/1] arm64: smp: Skip MC sched domain on SoCs with no LLC

[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.

Introduce an alternate sched domain topology for arm64 without the MC
level and test for llc_sibling weight 1 across all CPUs to enable it.

Do this in arch/arm64/kernel/smp.c (as opposed to
arch/arm64/kernel/topology.c) as all the CPU sibling maps are now
populated and we avoid needing to extend the drivers/acpi/pptt.c API to
detect the cluster level being above the cpu llc level. This is
consistent with other architectures and provides a readily extensible
mechanism for other alternate topologies.

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: 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
Signed-off-by: Darren Hart <darren at os.amperecomputing.com>
---
 arch/arm64/kernel/smp.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 27df5c1e6baa..3597e75645e1 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -433,6 +433,33 @@ static void __init hyp_mode_check(void)
 	}
 }
 
+static struct sched_domain_topology_level arm64_no_mc_topology[] = {
+#ifdef CONFIG_SCHED_SMT
+	{ cpu_smt_mask, cpu_smt_flags, SD_INIT_NAME(SMT) },
+#endif
+
+#ifdef CONFIG_SCHED_CLUSTER
+	{ cpu_clustergroup_mask, cpu_cluster_flags, SD_INIT_NAME(CLS) },
+#endif
+
+	{ cpu_cpu_mask, SD_INIT_NAME(DIE) },
+	{ NULL, },
+};
+
+static void __init update_sched_domain_topology(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		if (cpu_topology[cpu].llc_id != -1 &&
+		    cpumask_weight(&cpu_topology[cpu].llc_sibling) > 1)
+			return;
+	}
+
+	pr_info("No LLC siblings, using No MC sched domains topology\n");
+	set_sched_topology(arm64_no_mc_topology);
+}
+
 void __init smp_cpus_done(unsigned int max_cpus)
 {
 	pr_info("SMP: Total of %d processors activated.\n", num_online_cpus());
@@ -440,6 +467,7 @@ void __init smp_cpus_done(unsigned int max_cpus)
 	hyp_mode_check();
 	apply_alternatives_all();
 	mark_linear_text_alias_ro();
+	update_sched_domain_topology();
 }
 
 void __init smp_prepare_boot_cpu(void)
-- 
2.31.1