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SubjectRe: [PATCH v2 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path
On Wed, Jan 26, 2022 at 04:09:47PM +0800, Yicong Yang wrote:
> From: Barry Song <song.bao.hua@hisilicon.com>
>
> 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@hisilicon.com>
> Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> ---
> 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 */
> + if (!sched_smt_active() && sd) {
> + for_each_cpu_and(cpu, cpus, sched_domain_span(sd)) {
> + idle_cpu = __select_idle_cpu(cpu, p);
> + if ((unsigned int)idle_cpu < nr_cpumask_bits)
> + return idle_cpu;
> + }
> +
> + /* Don't ping-pong tasks in and out cluster frequently */
> + if (cpus_share_resources(target, prev_cpu))
> + return target;

We reach here when there aren't any idle CPUs within the
cluster. However there might be idle CPUs in the MC domain. Is a busy
@target preferable to a potentially idle CPU within the larger domain
?


--
Thanks and Regards
gautham.

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