On Wed, 6 Jan 2021 at 09:35, Barry Song <song.bao.hua@xxxxxxxxxxxxx> wrote: > > ARM64 server chip Kunpeng 920 has 6 clusters in each NUMA node, and each > cluster has 4 cpus. All clusters share L3 cache data, but each cluster > has local L3 tag. On the other hand, each clusters will share some > internal system bus. This means cache coherence overhead inside one cluster > is much less than the overhead across clusters. > > This patch adds the sched_domain for clusters. On kunpeng 920, without > this patch, domain0 of cpu0 would be MC with cpu0~cpu23 with ; with this > patch, MC becomes domain1, a new domain0 "CLS" including cpu0-cpu3. > > This will affect load balance. For example, without this patch, while cpu0 > becomes idle, it will pull a task from cpu1-cpu15. With this patch, cpu0 > will try to pull a task from cpu1-cpu3 first. This will have much less > overhead of task migration. > > On the other hand, while doing WAKE_AFFINE, this patch will try to find > a core in the target cluster before scanning the whole llc domain. > This means it will proactively use a core which has better affinity with > target core at first. > > Though it is named "cluster", architectures or machines can define its > exact meaning of cluster as long as some cpus can share some resources > in lower level than llc. So the implementation is applicable to all > architectures. > > Different cpus might have different resource sharing like L1, L2, cache > tags, internal busses etc. > Since it is hard to know where we should start to scan, this patch adds > a SD_SHARE_CLS_RESOURCES rather than directly leveraging the existing Not sure that we need this new flag. See more about this below You should have a look at https://lkml.org/lkml/2020/12/14/560 which rework select_idle_core/cpu/smt > SD_SHARE_PKG_RESOURCES flag. Architectures or machines can decide what > is cluster and who should get SD_SHARE_CLS_RESOURCES. select_idle_cpu() > will scan from the first sched_domain with SD_SHARE_CLS_RESOURCES. > > The below is a hackbench result: > > we run the below command with different -g parameter from 1 to 10, for each > different g, we run the command 10 times and get the average time > $ numactl -N 0 hackbench -p -T -l 20000 -g $1 > > hackbench will report the time which is needed to complete a certain number > of messages transmissions between a certain number of tasks, for example: > $ numactl -N 0 hackbench -p -T -l 20000 -g 10 > Running in threaded mode with 10 groups using 40 file descriptors each > (== 400 tasks) > Each sender will pass 20000 messages of 100 bytes > Time: 8.874 > > The below is the result of hackbench w/ and w/o the patch: > g 1 2 3 4 5 6 7 8 9 10 > w/o 1.4777 2.0112 3.1919 4.2654 5.3246 6.4019 7.5939 8.7073 9.7526 10.8987 > w/ 1.4793 1.9344 2.9080 3.9267 4.8339 5.7186 6.6923 7.5088 8.3715 9.2173 > +8.9% +7.9% +9.3% +10.7% +11.8% +13.8% +14.2% +15.5% > > Tracing the kernel while g=10, it shows select_idle_cpu() has a large chance > to get cpu in the same cluster with the target while it sometimes gets cpu > outside the cluster: > target cpu > 19 -> 17 > 13 -> 15 > 23 -> 20 > 23 -> 20 > 19 -> 17 > 13 -> 15 > 16 -> 17 > 19 -> 17 > 7 -> 5 > 10 -> 11 > 23 -> 20 > *23 -> 4 > ... > > Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx> > --- > -v3: > - rebased againest 5.11-rc2 > - with respect to the comments of Valentin Schneider, Peter Zijlstra, > Vincent Guittot and Mel Gorman etc. > * moved the scheduler changes from arm64 to the common place for all > architectures. > * added SD_SHARE_CLS_RESOURCES sd_flags specifying the sched_domain > where select_idle_cpu() should begin to scan from > * removed redundant select_idle_cluster() function since all code is > in select_idle_cpu() now. it also avoided scanning cluster cpus > twice in v2 code; > * redo the hackbench in one numa after the above changes > > arch/arm64/Kconfig | 7 +++++++ > include/linux/sched/sd_flags.h | 9 +++++++++ > include/linux/sched/topology.h | 7 +++++++ > include/linux/topology.h | 7 +++++++ > kernel/sched/fair.c | 27 +++++++++++++++++++++------ > kernel/sched/topology.c | 6 ++++++ > 6 files changed, 57 insertions(+), 6 deletions(-) > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > index 05e1735..546cd61 100644 > --- a/arch/arm64/Kconfig > +++ b/arch/arm64/Kconfig > @@ -973,6 +973,13 @@ config SCHED_MC > making when dealing with multi-core CPU chips at a cost of slightly > increased overhead in some places. If unsure say N here. > > +config SCHED_CLUSTER > + bool "Cluster scheduler support" > + help > + Cluster scheduler support improves the CPU scheduler's decision > + making when dealing with machines that have clusters(sharing internal > + bus or sharing LLC cache tag). If unsure say N here. > + > config SCHED_SMT > bool "SMT scheduler support" > help > diff --git a/include/linux/sched/sd_flags.h b/include/linux/sched/sd_flags.h > index 34b21e9..fc3c894 100644 > --- a/include/linux/sched/sd_flags.h > +++ b/include/linux/sched/sd_flags.h > @@ -100,6 +100,15 @@ SD_FLAG(SD_ASYM_CPUCAPACITY, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS) > SD_FLAG(SD_SHARE_CPUCAPACITY, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS) > > /* > + * Domain members share CPU cluster resources (i.e. llc cache tags) > + * > + * SHARED_CHILD: Set from the base domain up until spanned CPUs no longer share > + * the cluster resouces (such as llc tags and internal bus) > + * NEEDS_GROUPS: Caches are shared between groups. > + */ > +SD_FLAG(SD_SHARE_CLS_RESOURCES, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS) > + > +/* > * Domain members share CPU package resources (i.e. caches) > * > * SHARED_CHILD: Set from the base domain up until spanned CPUs no longer share > diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h > index 8f0f778..846fcac 100644 > --- a/include/linux/sched/topology.h > +++ b/include/linux/sched/topology.h > @@ -42,6 +42,13 @@ static inline int cpu_smt_flags(void) > } > #endif > > +#ifdef CONFIG_SCHED_CLUSTER > +static inline int cpu_cluster_flags(void) > +{ > + return SD_SHARE_CLS_RESOURCES | SD_SHARE_PKG_RESOURCES; > +} > +#endif > + > #ifdef CONFIG_SCHED_MC > static inline int cpu_core_flags(void) > { > diff --git a/include/linux/topology.h b/include/linux/topology.h > index bf2cc3c..81be614 100644 > --- a/include/linux/topology.h > +++ b/include/linux/topology.h > @@ -211,6 +211,13 @@ static inline const struct cpumask *cpu_smt_mask(int cpu) > } > #endif > > +#ifdef CONFIG_SCHED_CLUSTER > +static inline const struct cpumask *cpu_cluster_mask(int cpu) > +{ > + return topology_cluster_cpumask(cpu); > +} > +#endif > + > static inline const struct cpumask *cpu_cpu_mask(int cpu) > { > return cpumask_of_node(cpu_to_node(cpu)); > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 04a3ce2..c14fae6 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6145,6 +6145,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t > { > struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask); > struct sched_domain *this_sd; > + struct sched_domain *prev_ssd = NULL, *ssd; > u64 avg_cost, avg_idle; > u64 time; > int this = smp_processor_id(); > @@ -6174,15 +6175,29 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t > > time = cpu_clock(this); > > - cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr); > - > - for_each_cpu_wrap(cpu, cpus, target) { > - if (!--nr) > - return -1; > - if (available_idle_cpu(cpu) || sched_idle_cpu(cpu)) > + /* > + * we first scan those child domains who declare they are sharing > + * cluster resources such as llc tags, internal busses; then scan > + * the whole llc > + */ > + for_each_domain(target, ssd) { I don't like looping the sched domain in the fast path. Instead you should follow similar policy as for smt the rework patchset mentioned above: In the select_idle_core(), you can add a static key for cluster case and loop the cpu_cluster_mask(cpu), similarly to what is done with cpu_smt_mask but for looking for one idle cpu cpu_cluster_mask() in instead of all cpus in the case of smt > + if ((ssd->flags & SD_SHARE_CLS_RESOURCES) || (ssd == sd)) { > + cpumask_and(cpus, sched_domain_span(ssd), p->cpus_ptr); > + if (prev_ssd) > + cpumask_andnot(cpus, cpus, sched_domain_span(prev_ssd)); > + for_each_cpu_wrap(cpu, cpus, target) { > + if (!--nr) > + return -1; > + if (available_idle_cpu(cpu) || sched_idle_cpu(cpu)) > + goto done; > + } > + prev_ssd = ssd; > + } > + if (ssd == sd) > break; > } > > +done: > time = cpu_clock(this) - time; > update_avg(&this_sd->avg_scan_cost, time); > > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c > index 5d3675c..79030c9 100644 > --- a/kernel/sched/topology.c > +++ b/kernel/sched/topology.c > @@ -1361,6 +1361,7 @@ int __read_mostly node_reclaim_distance = RECLAIM_DISTANCE; > */ > #define TOPOLOGY_SD_FLAGS \ > (SD_SHARE_CPUCAPACITY | \ > + SD_SHARE_CLS_RESOURCES | \ > SD_SHARE_PKG_RESOURCES | \ > SD_NUMA | \ > SD_ASYM_PACKING) > @@ -1480,6 +1481,11 @@ static struct sched_domain_topology_level default_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 > + > #ifdef CONFIG_SCHED_MC > { cpu_coregroup_mask, cpu_core_flags, SD_INIT_NAME(MC) }, > #endif > -- > 2.7.4 >
