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

[Gautham R. Shenoy]

On Wed, Jan 26, 2022 at 04:09:47PM +0800, Yicong Yang wrote:
> From: Barry Song <song.bao.hua at 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 at hisilicon.com>
> Signed-off-by: Barry Song <song.bao.hua at hisilicon.com>
> Signed-off-by: Yicong Yang <yangyicong at 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.