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

[Tim Chen <tim.c.chen@linux.intel.com>]

On Wed, 2022-01-26 at 16:09 +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 */

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;
>