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

[Vincent Guittot]

On Tue, 1 Mar 2022 at 01:35, Darren Hart <darren at os.amperecomputing.com> 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.
>
> 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 &&

Have you tested it with a non-acpi system ? AFAICT, llc_id is only set
by ACPI system and  llc_id == -1 for others like DT based system

> +                   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
>