Re: [RFC PATCH v3 2/2] scheduler: add scheduler level for clusters

[Vincent Guittot]

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
>