On Mon, Apr 04, 2022 at 04:40:37PM -0700, 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. 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] > > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > Cc: Sudeep Holla <sudeep.holla@xxxxxxx> > 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: Carl Worth <carl@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 > v4: Rebase on 5.18-rc1 for GregKH to pull. Add IS_ENABLED(CONFIG_SCHED_CLUSTER). A bit more context on the state of review: Several folks reviewed, but I didn't add their Reviewed-by since I added the IS_ENABLED(CONFIG_SCHED_CLUSTER) test since they reviewed it last. This change preserves the stated intent of the change when CONFIG_SCHED_CLUSTER is disabled. Barry Song - Suggested this approach Vincent Guittot - informal review with reservations Sudeep Holla - Acked-by Dietmar Eggemann - informal review (added to Cc, apologies for the omission Dietmar) All but Barry's recommendation captured in the v3 thread: https://lore.kernel.org/linux-arm-kernel/f1deaeabfd31fdf512ff6502f38186ef842c2b1f.1646413117.git.darren@xxxxxxxxxxxxxxxxxxxxxx/ Thanks, > > drivers/base/arch_topology.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c > index 1d6636ebaac5..5497c5ab7318 100644 > --- a/drivers/base/arch_topology.c > +++ b/drivers/base/arch_topology.c > @@ -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; > } > -- Darren Hart Ampere Computing / OS and Kernel
