Re: [PATCH v2 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path

From: Tim Chen
Date: Wed Jan 26 2022 - 20:14:34 EST

On Wed, 2022-01-26 at 16:09 +0800, Yicong Yang wrote:
> From: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
>
> For platforms having clusters like Kunpeng920, CPUs within the same
> cluster have lower latency when synchronizing and accessing shared
> resources like cache. Thus, this patch tries to find an idle cpu
> within the cluster of the target CPU before scanning the whole LLC
> to gain lower latency.
>
> Note neither Kunpeng920 nor x86 Jacobsville supports SMT, so this
> patch doesn't consider SMT for this moment.
>
> Testing has been done on Kunpeng920 by pinning tasks to one numa
> and two numa. On Kunpeng920, Each numa has 8 clusters and each
> cluster has 4 CPUs.
>
> With this patch, We noticed enhancement on tbench within one
> numa or cross two numa.
>
> On numa 0:
> 5.17-rc1 patched
> Hmean 1 324.73 ( 0.00%) 378.01 * 16.41%*
> Hmean 2 645.36 ( 0.00%) 754.63 * 16.93%*
> Hmean 4 1302.09 ( 0.00%) 1507.54 * 15.78%*
> Hmean 8 2612.03 ( 0.00%) 2982.57 * 14.19%*
> Hmean 16 5307.12 ( 0.00%) 5886.66 * 10.92%*
> Hmean 32 9354.22 ( 0.00%) 9908.13 * 5.92%*
> Hmean 64 7240.35 ( 0.00%) 7278.78 * 0.53%*
> Hmean 128 6186.40 ( 0.00%) 6187.85 ( 0.02%)
>
> On numa 0-1:
> 5.17-rc1 patched
> Hmean 1 320.01 ( 0.00%) 378.44 * 18.26%*
> Hmean 2 643.85 ( 0.00%) 752.52 * 16.88%*
> Hmean 4 1287.36 ( 0.00%) 1505.62 * 16.95%*
> Hmean 8 2564.60 ( 0.00%) 2955.29 * 15.23%*
> Hmean 16 5195.69 ( 0.00%) 5814.74 * 11.91%*
> Hmean 32 9769.16 ( 0.00%) 10872.63 * 11.30%*
> Hmean 64 15952.50 ( 0.00%) 17281.98 * 8.33%*
> Hmean 128 13113.77 ( 0.00%) 13895.20 * 5.96%*
> Hmean 256 10997.59 ( 0.00%) 11244.69 * 2.25%*
> Hmean 512 14623.60 ( 0.00%) 15526.25 * 6.17%*
>
> This will also help to improve the MySQL. With MySQL server
> running on numa 0 and client running on numa 1, both QPS and
> latency is imporved on read-write case:
> 5.17-rc1 patched
> QPS-16threads 143333.2633 145077.4033(+1.22%)
> QPS-24threads 195085.9367 202719.6133(+3.91%)
> QPS-32threads 241165.6867 249020.74(+3.26%)
> QPS-64threads 244586.8433 253387.7567(+3.60%)
> avg-lat-16threads 2.23 2.19(+1.19%)
> avg-lat-24threads 2.46 2.36(+3.79%)
> avg-lat-36threads 2.66 2.57(+3.26%)
> avg-lat-64threads 5.23 5.05(+3.44%)
>
> Tested-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> ---
> kernel/sched/fair.c | 46 +++++++++++++++++++++++++++++++++++++++++
> ----
> 1 file changed, 42 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5146163bfabb..2f84a933aedd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6262,12 +6262,46 @@ static inline int select_idle_smt(struct
> task_struct *p, struct sched_domain *sd
>
> #endif /* CONFIG_SCHED_SMT */
>
> +#ifdef CONFIG_SCHED_CLUSTER
> +/*
> + * Scan the cluster domain for idle CPUs and clear cluster cpumask
> after scanning
> + */
> +static inline int scan_cluster(struct task_struct *p, int prev_cpu,
> int target)
> +{
> + struct cpumask *cpus =
> this_cpu_cpumask_var_ptr(select_idle_mask);
> + struct sched_domain *sd = rcu_dereference(per_cpu(sd_cluster,
> target));
> + int cpu, idle_cpu;
> +
> + /* TODO: Support SMT case while a machine with both cluster and
> SMT born */

This is probably a clearer comment

/* TODO: Support SMT system with cluster topology */

> + if (!sched_smt_active() && sd) {
> + for_each_cpu_and(cpu, cpus, sched_domain_span(sd)) {
> + idle_cpu = __select_idle_cpu(cpu, p);
> */
> -static int select_idle_cpu(struct task_struct *p, struct
> sched_domain *sd, bool has_idle_core, int target)
> +static int select_idle_cpu(struct task_struct *p, struct
> sched_domain *sd, bool has_idle_core, int prev_cpu, int target)
> {
> struct cpumask *cpus =
> this_cpu_cpumask_var_ptr(select_idle_mask);
> int i, cpu, idle_cpu = -1, nr = INT_MAX;
> @@ -6282,6 +6316,10 @@ static int select_idle_cpu(struct task_struct
> *p, struct sched_domain *sd, bool
>
> cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
>
> + idle_cpu = scan_cluster(p, prev_cpu, target);

Shouldn't "cpus" from

cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);

be passed to scan_cluster, to make sure that the cpu returned is
in the affinity mask of the task? I don't see p->cpus_ptr
being checked in scan_cluster to make sure the cpu found is in the
affinity mask.

Tim


> + if ((unsigned int)idle_cpu < nr_cpumask_bits)
> + return idle_cpu;
> +
> if (sched_feat(SIS_PROP) && !has_idle_core) {
> u64 avg_cost, avg_idle, span_avg;
> unsigned long now = jiffies;
> @@ -6416,7 +6454,7 @@ static int select_idle_sibling(struct
> task_struct *p, int prev, int target)
> /*
> * If the previous CPU is cache affine and idle, don't be
> stupid:
> */
> - if (prev != target && cpus_share_cache(prev, target) &&
> + if (prev != target && cpus_share_resources(prev, target) &&
> (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
> asym_fits_capacity(task_util, prev))
> return prev;
> @@ -6442,7 +6480,7 @@ static int select_idle_sibling(struct
> task_struct *p, int prev, int target)
> p->recent_used_cpu = prev;
> if (recent_used_cpu != prev &&
> recent_used_cpu != target &&
> - cpus_share_cache(recent_used_cpu, target) &&
> + cpus_share_resources(recent_used_cpu, target) &&
> (available_idle_cpu(recent_used_cpu) ||
> sched_idle_cpu(recent_used_cpu)) &&
> cpumask_test_cpu(p->recent_used_cpu, p->cpus_ptr) &&
> asym_fits_capacity(task_util, recent_used_cpu)) {
> @@ -6483,7 +6521,7 @@ static int select_idle_sibling(struct
> task_struct *p, int prev, int target)
> }
> }
>
> - i = select_idle_cpu(p, sd, has_idle_core, target);
> + i = select_idle_cpu(p, sd, has_idle_core, prev, target);
> if ((unsigned)i < nr_cpumask_bits)
> return i;
>