patch "topology: make core_mask include at least cluster_siblings" added to driver-core-linus

This is a note to let you know that I've just added the patch titled

topology: make core_mask include at least cluster_siblings

to my driver-core git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core.git
in the driver-core-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From db1e59483dfd8d4e956575302520bb8f7e20c79b Mon Sep 17 00:00:00 2001
From: Darren Hart <[email protected]>
Date: Mon, 11 Apr 2022 13:53:34 -0700
Subject: topology: make core_mask include at least cluster_siblings

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. Only do this if CONFIG_SCHED_CLUSTER is enabled to avoid also changing the topology (MC) when CONFIG_SCHED_CLUSTER is disabled.

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]

---
 drivers/base/arch_topology.c | 9 +++++++++
 1 file changed, 9 insertions(+)

index 1d6636ebaac5..5497c5ab7318 100644
@@ -667,6 +667,15 @@ 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 (IS_ENABLED(CONFIG_SCHED_CLUSTER) &&
+	    cpumask_subset(core_mask, &cpu_topology[cpu].cluster_sibling))
+		core_mask = &cpu_topology[cpu].cluster_sibling;
+
 	return core_mask;
 }
 

2.35.3