On Wed, Feb 16, 2022 at 7:30 PM Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote: > > On Tue, 15 Feb 2022 at 18:32, Darren Hart <darren@xxxxxxxxxxxxxxxxxxxxxx> wrote: > > > > On Tue, Feb 15, 2022 at 06:09:08PM +0100, Vincent Guittot wrote: > > > On Tue, 15 Feb 2022 at 17:46, Will Deacon <will@xxxxxxxxxx> wrote: > > > > > > > > On Tue, Feb 15, 2022 at 08:44:23AM -0800, Darren Hart wrote: > > > > > On Tue, Feb 15, 2022 at 04:38:59PM +0000, Will Decon wrote: > > > > > > On Fri, Feb 11, 2022 at 03:20:51AM +0000, Song Bao Hua (Barry Song) wrote: > > > > > > > > > > > > > > > > > > > > > > -----Original Message----- > > > > > > > > From: Darren Hart [mailto:darren@xxxxxxxxxxxxxxxxxxxxxx] > > > > > > > > Sent: Friday, February 11, 2022 2:43 PM > > > > > > > > To: LKML <linux-kernel@xxxxxxxxxxxxxxx>; Linux Arm > > > > > > > > <linux-arm-kernel@xxxxxxxxxxxxxxxxxxx> > > > > > > > > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>; Will Deacon <will@xxxxxxxxxx>; > > > > > > > > Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Vincent Guittot > > > > > > > > <vincent.guittot@xxxxxxxxxx>; Song Bao Hua (Barry Song) > > > > > > > > <song.bao.hua@xxxxxxxxxxxxx>; Valentin Schneider > > > > > > > > <valentin.schneider@xxxxxxx>; D . Scott Phillips > > > > > > > > <scott@xxxxxxxxxxxxxxxxxxxxxx>; Ilkka Koskinen > > > > > > > > <ilkka@xxxxxxxxxxxxxxxxxxxxxx>; stable@xxxxxxxxxxxxxxx > > > > > > > > Subject: [PATCH] arm64: smp: Skip MC domain for SoCs without shared cache > > > > > > > > > > > > > > > > SoCs such as the Ampere Altra define clusters but have no shared > > > > > > > > processor-side cache. As of v5.16 with CONFIG_SCHED_CLUSTER and > > > > > > > > CONFIG_SCHED_MC, build_sched_domain() will BUG() with: > > > > > > > > > > > > > > > > BUG: arch topology borken > > > > > > > > the CLS domain not a subset of the MC domain > > > > > > > > > > > > > > > > for each CPU (160 times for a 2 socket 80 core Altra system). The MC > > > > > > > > level cpu mask is then extended to that of the CLS child, and is later > > > > > > > > removed entirely as redundant. > > > > > > > > > > > > > > > > This change detects when all cpu_coregroup_mask weights=1 and uses an > > > > > > > > alternative sched_domain_topology equivalent to the default if > > > > > > > > CONFIG_SCHED_MC were disabled. > > > > > > > > > > > > > > > > The final resulting sched domain topology is unchanged with or without > > > > > > > > CONFIG_SCHED_CLUSTER, and the BUG is avoided: > > > > > > > > > > > > > > > > For CPU0: > > > > > > > > > > > > > > > > With CLS: > > > > > > > > CLS [0-1] > > > > > > > > DIE [0-79] > > > > > > > > NUMA [0-159] > > > > > > > > > > > > > > > > Without CLS: > > > > > > > > DIE [0-79] > > > > > > > > NUMA [0-159] > > > > > > > > > > > > > > > > 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 > > > > > > > > Signed-off-by: Darren Hart <darren@xxxxxxxxxxxxxxxxxxxxxx> > > > > > > > > > > > > > > Hi Darrent, > > > > > > > What kind of resources are clusters sharing on Ampere Altra? > > > > > > > So on Altra, cpus are not sharing LLC? Each LLC is separate > > > > > > > for each cpu? > > > > > > > > > > > > > > > --- > > > > > > > > arch/arm64/kernel/smp.c | 32 ++++++++++++++++++++++++++++++++ > > > > > > > > 1 file changed, 32 insertions(+) > > > > > > > > > > > > > > > > diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c > > > > > > > > index 27df5c1e6baa..0a78ac5c8830 100644 > > > > > > > > --- a/arch/arm64/kernel/smp.c > > > > > > > > +++ b/arch/arm64/kernel/smp.c > > > > > > > > @@ -715,9 +715,22 @@ void __init smp_init_cpus(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, }, > > > > > > > > +}; > > > > > > > > + > > > > > > > > void __init smp_prepare_cpus(unsigned int max_cpus) > > > > > > > > { > > > > > > > > const struct cpu_operations *ops; > > > > > > > > + bool use_no_mc_topology = true; > > > > > > > > int err; > > > > > > > > unsigned int cpu; > > > > > > > > unsigned int this_cpu; > > > > > > > > @@ -758,6 +771,25 @@ void __init smp_prepare_cpus(unsigned int max_cpus) > > > > > > > > > > > > > > > > set_cpu_present(cpu, true); > > > > > > > > numa_store_cpu_info(cpu); > > > > > > > > + > > > > > > > > + /* > > > > > > > > + * Only use no_mc topology if all cpu_coregroup_mask weights=1 > > > > > > > > + */ > > > > > > > > + if (cpumask_weight(cpu_coregroup_mask(cpu)) > 1) > > > > > > > > + use_no_mc_topology = false; > > > > > > > > > > > > > > This seems to be wrong? If you have 5 cpus, > > > > > > > Cpu0 has cpu_coregroup_mask(cpu)== 1, cpu1-4 > > > > > > > has cpu_coregroup_mask(cpu)== 4, for cpu0, you still > > > > > > > need to remove MC, but for cpu1-4, you will need > > > > > > > CLS and MC both? > > > > > > > > > > > > What is the *current* behaviour on such a system? > > > > > > > > > > > > > > > > As I understand it, any system that uses the default topology which has > > > > > a cpus_coregroup weight of 1 and a child (cluster, smt, ...) weight > 1 > > > > > will behave as described above by printing the following for each CPU > > > > > matching this criteria: > > > > > > > > > > BUG: arch topology borken > > > > > the [CLS,SMT,...] domain not a subset of the MC domain > > > > > > > > > > And then extend the MC domain cpumask to match that of the child and continue > > > > > on. > > > > > > > > > > That would still be the behavior for this type of system after this > > > > > patch is applied. > > > > > > > > That's what I thought, but in that case applying your patch is a net > > > > improvement: systems either get current or better behaviour. > > > > > > CLUSTER level is normally defined as a intermediate group of the MC > > > level and both levels have the scheduler flag SD_SHARE_PKG_RESOURCES > > > flag > > > > > > In the case of Ampere altra, they consider that CPUA have a CLUSTER > > > level which SD_SHARE_PKG_RESOURCES with another CPUB but the next and > > > larger MC level then says that CPUA doesn't SD_SHARE_PKG_RESOURCES > > > with CPUB which seems to be odd because the SD_SHARE_PKG_RESOURCES has > > > not disappeared Looks like there is a mismatch in topology description > > > > Hi Vincent, > > > > Agree. Where do you think this mismatch exists? > > I think that the problem comes from that the default topology order is > assumed to be : > SMT > CLUSTER shares pkg resources i.e. cache > MC > DIE > NUMA > > but in your case, you want a topology order like : > SMT > MC > CLUSTER shares SCU > DIE > NUMA > > IIUC, the cluster is defined as the 2nd (no SMT) or 3rd (SMT) level in > the PPTT table whereas the MC level is defined as the number of cache > levels. So i would say that you should compare the level to know the > ordering > > Then, there is another point: > In your case, CLUSTER level still has the flag SD_SHARE_PKG_RESOURCES > which is used to define some scheduler internal variable like > sd_llc(sched domain last level of cache) which allows fast task > migration between this cpus in this level at wakeup. In your case the > sd_llc should not be the cluster but the MC with only one CPU. But I > would not be surprised that most of perf improvement comes from this > sd_llc wrongly set to cluster instead of the single CPU I assume this "mistake" is actually what Ampere altra needs while it is wrong but getting right result? Ampere altra has already got both: 1. Load Balance between clusters 2. wake_affine by select sibling cpu which is sharing SCU I am not sure how much 1 and 2 are helping Darren's workloads respectively. > > > > > > I'd describe this as a mismatch between the default assumptions of > > the sched domains construction code (that SD_SHARE_PKG_RESOURCES implies > > a shared cpu-side cache) and SoCs without a shared cpu-side cache. This > > is encoded in properties of the MC level and the requirement that child > > domains be a subset of the parent domain cpumask. > > > > The MC-less topology addresses this in a consistent way with other > > architectures using the provided API for non-default topologies without > > changing these fundamental assumptions and without changing the final > > resulting topology which is correct and matches the topology supplied in > > the ACPI PPTT. > > > > > > -- > > Darren Hart > > Ampere Computing / OS and Kernel Thanks Barry
