Re: [RFC PATCH v4 2/3] scheduler: add scheduler level for clusters

[Vincent Guittot]

On Tue, 2 Mar 2021 at 00:08, Barry Song <song.bao.hua@xxxxxxxxxxxxx> wrote:
>
> ARM64 chip Kunpeng 920 has 6 or 8 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 help spread unrelated tasks among clusters, thus decrease the
> contention and improve the throughput, for example, stream benchmark can
> improve around 4.3%~6.3% by this patch:
>
> w/o patch:
> numactl -N 0 /usr/lib/lmbench/bin/stream -P 12 -M 1024M -N 5
> STREAM copy latency: 3.36 nanoseconds
> STREAM copy bandwidth: 57072.50 MB/sec
> STREAM scale latency: 3.40 nanoseconds
> STREAM scale bandwidth: 56542.52 MB/sec
> STREAM add latency: 5.10 nanoseconds
> STREAM add bandwidth: 56482.83 MB/sec
> STREAM triad latency: 5.14 nanoseconds
> STREAM triad bandwidth: 56069.52 MB/sec
>
> w/ patch:
> $ numactl -N 0 /usr/lib/lmbench/bin/stream -P 12 -M 1024M -N 5
> STREAM copy latency: 3.22 nanoseconds
> STREAM copy bandwidth: 59660.96 MB/sec    ->  +4.5%
> STREAM scale latency: 3.25 nanoseconds
> STREAM scale bandwidth: 59002.29 MB/sec   ->  +4.3%
> STREAM add latency: 4.80 nanoseconds
> STREAM add bandwidth: 60036.62 MB/sec     ->  +6.3%
> STREAM triad latency: 4.86 nanoseconds
> STREAM triad bandwidth: 59228.30 MB/sec   ->  +5.6%
>
> 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. So it
> helps gather related tasks within one cluster.

Could you split this patch in 2 patches ? One for adding a cluster
sched domain level and one for modifying the wake up path ?

This would ease the review and I would be curious about the impact of
each feature in the performance. In particular, I'm still not
convinced that the modification of the wakeup path is the root of the
hackbench improvement; especially with g=14 where there should not be
much idle CPUs with 14*40 tasks on at most 32 CPUs.  IIRC, there was
no obvious improvement with the changes in select_idle_cpu unless you
hack the behavior to not fall back to llc domain

> we run the below hackbench with different -g parameter from 2 to 14, 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=    2      4     6       8      10     12      14
> w/o: 1.9596 4.0506 5.9654 8.0068 9.8147 11.4900 13.1163
> w/ : 1.9362 3.9197 5.6570 7.1376 8.5263 10.0512 11.3256
>             +3.3%  +5.2%  +10.9% +13.2%  +12.8%  +13.7%
>
> Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
> ---
> -v4:
>   * rebased to tip/sched/core with the latest unified code of select_idle_cpu
>   * also added benchmark data of spreading unrelated tasks
>   * avoided the iteration of sched_domain by moving to static_key(addressing
>     Vincent's comment
>
>  arch/arm64/Kconfig             |  7 +++++
>  include/linux/sched/cluster.h  | 19 ++++++++++++
>  include/linux/sched/sd_flags.h |  9 ++++++
>  include/linux/sched/topology.h |  7 +++++
>  include/linux/topology.h       |  7 +++++
>  kernel/sched/core.c            | 18 ++++++++++++
>  kernel/sched/fair.c            | 66 +++++++++++++++++++++++++++++++++---------
>  kernel/sched/sched.h           |  1 +
>  kernel/sched/topology.c        |  6 ++++
>  9 files changed, 126 insertions(+), 14 deletions(-)
>  create mode 100644 include/linux/sched/cluster.h
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index f39568b..158b0fa 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -971,6 +971,13 @@ 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/cluster.h b/include/linux/sched/cluster.h
> new file mode 100644
> index 0000000..ea6c475
> --- /dev/null
> +++ b/include/linux/sched/cluster.h
> @@ -0,0 +1,19 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_SCHED_CLUSTER_H
> +#define _LINUX_SCHED_CLUSTER_H
> +
> +#include <linux/static_key.h>
> +
> +#ifdef CONFIG_SCHED_CLUSTER
> +extern struct static_key_false sched_cluster_present;
> +
> +static __always_inline bool sched_cluster_active(void)
> +{
> +       return static_branch_likely(&sched_cluster_present);
> +}
> +#else
> +static inline bool sched_cluster_active(void) { return false; }
> +
> +#endif
> +
> +#endif
> 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_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 80d27d7..0b3704a 100644
> --- a/include/linux/topology.h
> +++ b/include/linux/topology.h
> @@ -212,6 +212,13 @@ static inline const struct cpumask *cpu_smt_mask(int cpu)
>  }
>  #endif
>
> +#if defined(CONFIG_SCHED_CLUSTER) && !defined(cpu_cluster_mask)
> +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/core.c b/kernel/sched/core.c
> index 88a2e2b..d805e59 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7797,6 +7797,16 @@ int sched_cpu_activate(unsigned int cpu)
>         if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
>                 static_branch_inc_cpuslocked(&sched_smt_present);
>  #endif
> +
> +#ifdef CONFIG_SCHED_CLUSTER
> +       /*
> +        * When going up, increment the number of cluster cpus with
> +        * cluster present.
> +        */
> +       if (cpumask_weight(cpu_cluster_mask(cpu)) > 1)
> +               static_branch_inc_cpuslocked(&sched_cluster_present);
> +#endif
> +
>         set_cpu_active(cpu, true);
>
>         if (sched_smp_initialized) {
> @@ -7873,6 +7883,14 @@ int sched_cpu_deactivate(unsigned int cpu)
>                 static_branch_dec_cpuslocked(&sched_smt_present);
>  #endif
>
> +#ifdef CONFIG_SCHED_CLUSTER
> +       /*
> +        * When going down, decrement the number of cpus with cluster present.
> +        */
> +       if (cpumask_weight(cpu_cluster_mask(cpu)) > 1)
> +               static_branch_dec_cpuslocked(&sched_cluster_present);
> +#endif
> +
>         if (!sched_smp_initialized)
>                 return 0;
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8a8bd7b..3db7b07 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6009,6 +6009,11 @@ static inline int __select_idle_cpu(int cpu)
>         return -1;
>  }
>
> +#ifdef CONFIG_SCHED_CLUSTER
> +DEFINE_STATIC_KEY_FALSE(sched_cluster_present);
> +EXPORT_SYMBOL_GPL(sched_cluster_present);
> +#endif
> +
>  #ifdef CONFIG_SCHED_SMT
>  DEFINE_STATIC_KEY_FALSE(sched_smt_present);
>  EXPORT_SYMBOL_GPL(sched_smt_present);
> @@ -6116,6 +6121,26 @@ static inline int select_idle_core(struct task_struct *p, int core, struct cpuma
>
>  #endif /* CONFIG_SCHED_SMT */
>
> +static inline int _select_idle_cpu(bool smt, struct task_struct *p, int target, struct cpumask *cpus, int *idle_cpu, int *nr)
> +{
> +       int cpu, i;
> +
> +       for_each_cpu_wrap(cpu, cpus, target) {
> +               if (smt) {
> +                       i = select_idle_core(p, cpu, cpus, idle_cpu);
> +               } else {
> +                       if (!--*nr)
> +                               return -1;
> +                       i = __select_idle_cpu(cpu);
> +               }
> +
> +               if ((unsigned int)i < nr_cpumask_bits)
> +                       return i;
> +       }
> +
> +       return -1;
> +}
> +
>  /*
>   * Scan the LLC domain for idle CPUs; this is dynamically regulated by
>   * comparing the average scan cost (tracked in sd->avg_scan_cost) against the
> @@ -6124,7 +6149,7 @@ static inline int select_idle_core(struct task_struct *p, int core, struct cpuma
>  static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int target)
>  {
>         struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
> -       int i, cpu, idle_cpu = -1, nr = INT_MAX;
> +       int i, idle_cpu = -1, nr = INT_MAX;
>         bool smt = test_idle_cores(target, false);
>         int this = smp_processor_id();
>         struct sched_domain *this_sd;
> @@ -6134,7 +6159,12 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>         if (!this_sd)
>                 return -1;
>
> -       cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +       if (!sched_cluster_active())
> +               cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +#ifdef CONFIG_SCHED_CLUSTER
> +       if (sched_cluster_active())
> +               cpumask_and(cpus, cpu_cluster_mask(target), p->cpus_ptr);
> +#endif
>
>         if (sched_feat(SIS_PROP) && !smt) {
>                 u64 avg_cost, avg_idle, span_avg;
> @@ -6155,24 +6185,32 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>                 time = cpu_clock(this);
>         }
>
> -       for_each_cpu_wrap(cpu, cpus, target) {
> -               if (smt) {
> -                       i = select_idle_core(p, cpu, cpus, &idle_cpu);
> -                       if ((unsigned int)i < nr_cpumask_bits)
> -                               return i;
> +       /* scan cluster before scanning the whole llc */
> +#ifdef CONFIG_SCHED_CLUSTER
> +       if (sched_cluster_active()) {
> +               i = _select_idle_cpu(smt, p, target, cpus, &idle_cpu, &nr);
> +               if ((unsigned int) i < nr_cpumask_bits) {
> +                       idle_cpu = i;
> +                       goto done;
> +               } else if (nr <= 0)
> +                       return -1;
>
> -               } else {
> -                       if (!--nr)
> -                               return -1;
> -                       idle_cpu = __select_idle_cpu(cpu);
> -                       if ((unsigned int)idle_cpu < nr_cpumask_bits)
> -                               break;
> -               }
> +               cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +               cpumask_andnot(cpus, cpus, cpu_cluster_mask(target));
>         }
> +#endif
> +
> +       i = _select_idle_cpu(smt, p, target, cpus, &idle_cpu, &nr);
> +       if ((unsigned int) i < nr_cpumask_bits) {
> +               idle_cpu = i;
> +               goto done;
> +       } else if (nr <= 0)
> +               return -1;
>
>         if (smt)
>                 set_idle_cores(this, false);
>
> +done:
>         if (sched_feat(SIS_PROP) && !smt) {
>                 time = cpu_clock(this) - time;
>                 update_avg(&this_sd->avg_scan_cost, time);
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 10a1522..48a020f 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -6,6 +6,7 @@
>
>  #include <linux/sched/autogroup.h>
>  #include <linux/sched/clock.h>
> +#include <linux/sched/cluster.h>
>  #include <linux/sched/coredump.h>
>  #include <linux/sched/cpufreq.h>
>  #include <linux/sched/cputime.h>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 09d3504..d019c25 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1361,6 +1361,7 @@ static void claim_allocations(int cpu, struct sched_domain *sd)
>   */
>  #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 void claim_allocations(int cpu, struct sched_domain *sd)
>  #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
> --
> 1.8.3.1
>
>
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