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

[Will Deacon]

On Fri, Mar 04, 2022 at 09:01:36AM -0800, Darren Hart wrote:
> 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;
> +

Sudeep, Vincent, are you happy with this now?

Will