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

[Darren Hart]

On Wed, Mar 02, 2022 at 10:32:06AM +0100, Vincent Guittot wrote:
> 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;
> > +       }

Hi Vincent,

I did not have a non-acpi system to test, no. You're right of course,
llc_id is only set by ACPI systems on arm64. We could wrap this in a
CONFIG_ACPI ifdef (or IS_ENABLED), but I think this would be preferable:

+       for_each_possible_cpu(cpu) {
+               if (cpu_topology[cpu].llc_id == -1 ||
+                   cpumask_weight(&cpu_topology[cpu].llc_sibling) > 1)
+                       return;
+       }

Quickly tested on Altra successfully. Would appreciate anyone with non-acpi
arm64 systems who can test and verify this behaves as intended. I will ask
around tomorrow as well to see what I may have access to.

Thanks,

> > +
> > +       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
> >

-- 
Darren Hart
Ampere Computing / OS and Kernel