On Tue, 8 Mar 2022 at 11:30, Will Deacon <will@xxxxxxxxxx> wrote: > > 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@xxxxxxx> > > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > > Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx> > > Cc: Catalin Marinas <catalin.marinas@xxxxxxx> > > Cc: Will Deacon <will@xxxxxxxxxx> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > > Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx> > > Cc: Barry Song <song.bao.hua@xxxxxxxxxxxxx> > > Cc: Valentin Schneider <valentin.schneider@xxxxxxx> > > Cc: D. Scott Phillips <scott@xxxxxxxxxxxxxxxxxxxxxx> > > Cc: Ilkka Koskinen <ilkka@xxxxxxxxxxxxxxxxxxxxxx> > > Cc: <stable@xxxxxxxxxxxxxxx> # 5.16.x > > Suggested-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx> > > Signed-off-by: Darren Hart <darren@xxxxxxxxxxxxxxxxxxxxxx> > > --- > > 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? I would not say that I'm happy because this solution skews the core cpu mask in order to abuse the scheduler so that it will remove a wrong but useless level when it will build its domains. But this works so as long as the maintainer are happy, I'm fine > > Will
