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SubjectRe: [PATCH 2/2] sched/fair: Adjust the allowed NUMA imbalance when SD_NUMA spans multiple LLCs
On Fri, Dec 3, 2021 at 8:27 PM Mel Gorman <mgorman@techsingularity.net> wrote:
>
> Commit 7d2b5dd0bcc4 ("sched/numa: Allow a floating imbalance between NUMA
> nodes") allowed an imbalance between NUMA nodes such that communicating
> tasks would not be pulled apart by the load balancer. This works fine when
> there is a 1:1 relationship between LLC and node but can be suboptimal
> for multiple LLCs if independent tasks prematurely use CPUs sharing cache.
>
> Zen* has multiple LLCs per node with local memory channels and due to
> the allowed imbalance, it's far harder to tune some workloads to run
> optimally than it is on hardware that has 1 LLC per node. This patch
> adjusts the imbalance on multi-LLC machines to allow an imbalance up to
> the point where LLCs should be balanced between nodes.
>
> On a Zen3 machine running STREAM parallelised with OMP to have on instance
> per LLC the results and without binding, the results are
>
> 5.16.0-rc1 5.16.0-rc1
> vanilla sched-numaimb-v3r1
> MB/sec copy-16 166712.18 ( 0.00%) 587662.60 ( 252.50%)
> MB/sec scale-16 140109.66 ( 0.00%) 393528.14 ( 180.87%)
> MB/sec add-16 160791.18 ( 0.00%) 618622.00 ( 284.74%)
> MB/sec triad-16 160043.84 ( 0.00%) 589188.40 ( 268.14%)
>
> STREAM can use directives to force the spread if the OpenMP is new
> enough but that doesn't help if an application uses threads and
> it's not known in advance how many threads will be created.
>
> Coremark is a CPU and cache intensive benchmark parallelised with
> threads. When running with 1 thread per instance, the vanilla kernel
> allows threads to contend on cache. With the patch;
>
> 5.16.0-rc1 5.16.0-rc1
> vanilla sched-numaimb-v3r1
> Min Score-16 367816.09 ( 0.00%) 403429.15 ( 9.68%)
> Hmean Score-16 389627.78 ( 0.00%) 451015.49 * 15.76%*
> Max Score-16 416178.96 ( 0.00%) 480012.00 ( 15.34%)
> Stddev Score-16 17361.82 ( 0.00%) 32378.08 ( -86.49%)
> CoeffVar Score-16 4.45 ( 0.00%) 7.14 ( -60.57%)
>
> It can also make a big difference for semi-realistic workloads
> like specjbb which can execute arbitrary numbers of threads without
> advance knowledge of how they should be placed
>
> 5.16.0-rc1 5.16.0-rc1
> vanilla sched-numaimb-v3r1
> Hmean tput-1 73743.05 ( 0.00%) 70258.27 * -4.73%*
> Hmean tput-8 563036.51 ( 0.00%) 591187.39 ( 5.00%)
> Hmean tput-16 1016590.61 ( 0.00%) 1032311.78 ( 1.55%)
> Hmean tput-24 1418558.41 ( 0.00%) 1424005.80 ( 0.38%)
> Hmean tput-32 1608794.22 ( 0.00%) 1907855.80 * 18.59%*
> Hmean tput-40 1761338.13 ( 0.00%) 2108162.23 * 19.69%*
> Hmean tput-48 2290646.54 ( 0.00%) 2214383.47 ( -3.33%)
> Hmean tput-56 2463345.12 ( 0.00%) 2780216.58 * 12.86%*
> Hmean tput-64 2650213.53 ( 0.00%) 2598196.66 ( -1.96%)
> Hmean tput-72 2497253.28 ( 0.00%) 2998882.47 * 20.09%*
> Hmean tput-80 2820786.72 ( 0.00%) 2951655.27 ( 4.64%)
> Hmean tput-88 2813541.68 ( 0.00%) 3045450.86 * 8.24%*
> Hmean tput-96 2604158.67 ( 0.00%) 3035311.91 * 16.56%*
> Hmean tput-104 2713810.62 ( 0.00%) 2984270.04 ( 9.97%)
> Hmean tput-112 2558425.37 ( 0.00%) 2894737.46 * 13.15%*
> Hmean tput-120 2611434.93 ( 0.00%) 2781661.01 ( 6.52%)
> Hmean tput-128 2706103.22 ( 0.00%) 2811447.85 ( 3.89%)
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
> include/linux/sched/topology.h | 1 +
> kernel/sched/fair.c | 26 +++++++++++++++-----------
> kernel/sched/topology.c | 20 ++++++++++++++++++++
> 3 files changed, 36 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index c07bfa2d80f2..54f5207154d3 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -93,6 +93,7 @@ struct sched_domain {
> unsigned int busy_factor; /* less balancing by factor if busy */
> unsigned int imbalance_pct; /* No balance until over watermark */
> unsigned int cache_nice_tries; /* Leave cache hot tasks for # tries */
> + unsigned int imb_numa_nr; /* Nr imbalanced tasks allowed between nodes */
>
> int nohz_idle; /* NOHZ IDLE status */
> int flags; /* See SD_* */
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0a969affca76..64f211879e43 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1489,6 +1489,7 @@ struct task_numa_env {
>
> int src_cpu, src_nid;
> int dst_cpu, dst_nid;
> + int imb_numa_nr;
>
> struct numa_stats src_stats, dst_stats;
>
> @@ -1885,7 +1886,7 @@ static void task_numa_find_cpu(struct task_numa_env *env,
> dst_running = env->dst_stats.nr_running + 1;
> imbalance = max(0, dst_running - src_running);
> imbalance = adjust_numa_imbalance(imbalance, dst_running,
> - env->dst_stats.weight);
> + env->imb_numa_nr);
>
> /* Use idle CPU if there is no imbalance */
> if (!imbalance) {
> @@ -1950,8 +1951,10 @@ static int task_numa_migrate(struct task_struct *p)
> */
> rcu_read_lock();
> sd = rcu_dereference(per_cpu(sd_numa, env.src_cpu));
> - if (sd)
> + if (sd) {
> env.imbalance_pct = 100 + (sd->imbalance_pct - 100) / 2;
> + env.imb_numa_nr = sd->imb_numa_nr;
> + }
> rcu_read_unlock();
>
> /*
> @@ -9046,13 +9049,14 @@ static bool update_pick_idlest(struct sched_group *idlest,
> }
>
> /*
> - * Allow a NUMA imbalance if busy CPUs is less than 25% of the domain.
> - * This is an approximation as the number of running tasks may not be
> - * related to the number of busy CPUs due to sched_setaffinity.
> + * Allow a NUMA imbalance if busy CPUs is less than the allowed
> + * imbalance. This is an approximation as the number of running
> + * tasks may not be related to the number of busy CPUs due to
> + * sched_setaffinity.
> */
> -static inline bool allow_numa_imbalance(int dst_running, int dst_weight)
> +static inline bool allow_numa_imbalance(int dst_running, int imb_numa_nr)
> {
> - return (dst_running < (dst_weight >> 2));
> + return dst_running < imb_numa_nr;
> }
>
> /*
> @@ -9191,7 +9195,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
> * a real need of migration, periodic load balance will
> * take care of it.
> */
> - if (allow_numa_imbalance(local_sgs.sum_nr_running, sd->span_weight))
> + if (allow_numa_imbalance(local_sgs.sum_nr_running, sd->imb_numa_nr))
> return NULL;
> }
>
> @@ -9283,9 +9287,9 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> #define NUMA_IMBALANCE_MIN 2
>
> static inline long adjust_numa_imbalance(int imbalance,
> - int dst_running, int dst_weight)
> + int dst_running, int imb_numa_nr)
> {
> - if (!allow_numa_imbalance(dst_running, dst_weight))
> + if (!allow_numa_imbalance(dst_running, imb_numa_nr))
> return imbalance;
>
> /*
> @@ -9397,7 +9401,7 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
> /* Consider allowing a small imbalance between NUMA groups */
> if (env->sd->flags & SD_NUMA) {
> env->imbalance = adjust_numa_imbalance(env->imbalance,
> - busiest->sum_nr_running, env->sd->span_weight);
> + busiest->sum_nr_running, env->sd->imb_numa_nr);
> }
>
> return;
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index d201a7052a29..fee2930745ab 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2242,6 +2242,26 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
> }
> }
>
> + /* Calculate allowed NUMA imbalance */
> + for_each_cpu(i, cpu_map) {
> + int imb_numa_nr = 0;
> +
> + for (sd = *per_cpu_ptr(d.sd, i); sd; sd = sd->parent) {
> + struct sched_domain *child = sd->child;
> +
> + if (!(sd->flags & SD_SHARE_PKG_RESOURCES) && child &&
> + (child->flags & SD_SHARE_PKG_RESOURCES)) {
> + int nr_groups;
> +
> + nr_groups = sd->span_weight / child->span_weight;
> + imb_numa_nr = max(1U, ((child->span_weight) >> 1) /
> + (nr_groups * num_online_nodes()));

Hi Mel, you used to have 25% * numa_weight if node has only one LLC.
for a system with 4 numa, In case sd has 2 nodes, child is 1 numa node,
then nr_groups=2, num_online_nodes()=4, imb_numa_nr will be
child->span_weight/2/2/4?

Does this patch change the behaviour for machines whose numa equals LLC?

> + }
> +
> + sd->imb_numa_nr = imb_numa_nr;
> + }
> + }
> +
> /* Calculate CPU capacity for physical packages and nodes */
> for (i = nr_cpumask_bits-1; i >= 0; i--) {
> if (!cpumask_test_cpu(i, cpu_map))
> --
> 2.31.1
>

Thanks
Barry

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