Re: [PATCH v4 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path

[Abel Wu]

On 6/9/22 8:06 PM, 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:
>                              5.19-rc1                patched
> Hmean     1        350.27 (   0.00%)      406.88 *  16.16%*
> Hmean     2        702.01 (   0.00%)      808.22 *  15.13%*
> Hmean     4       1405.14 (   0.00%)     1614.34 *  14.89%*
> Hmean     8       2830.53 (   0.00%)     3169.02 *  11.96%*
> Hmean     16      5597.95 (   0.00%)     6224.20 *  11.19%*
> Hmean     32     10537.38 (   0.00%)    10524.97 *  -0.12%*
> Hmean     64      8366.04 (   0.00%)     8437.41 *   0.85%*
> Hmean     128     7060.87 (   0.00%)     7150.25 *   1.27%*
>
> On numa 0-1:
>                              5.19-rc1                patched
> Hmean     1        346.11 (   0.00%)      408.47 *  18.02%*
> Hmean     2        693.34 (   0.00%)      805.78 *  16.22%*
> Hmean     4       1384.96 (   0.00%)     1602.49 *  15.71%*
> Hmean     8       2699.45 (   0.00%)     3069.98 *  13.73%*
> Hmean     16      5327.11 (   0.00%)     5688.19 *   6.78%*
> Hmean     32     10019.10 (   0.00%)    11862.56 *  18.40%*
> Hmean     64     13850.57 (   0.00%)    17748.54 *  28.14%*
> Hmean     128    12498.25 (   0.00%)    15541.59 *  24.35%*
> Hmean     256    11195.77 (   0.00%)    13854.06 *  23.74%*
>
> 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 | 44 +++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 41 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 77b2048a9326..6d173e196ad3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6327,6 +6327,40 @@ 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, struct cpumask *cpus,
> +			       int target, int *nr)
> +{
> +	struct sched_domain *sd = rcu_dereference(per_cpu(sd_cluster, target));
> +	int cpu, idle_cpu;
> +
> +	/* TODO: Support SMT system with cluster topology */
> +	if (!sched_smt_active() && sd) {
> +		for_each_cpu_and(cpu, cpus, sched_domain_span(sd)) {
> +			if (!--*nr)
> +				break;

return -1;
:)

> +
> +			idle_cpu = __select_idle_cpu(cpu, p);
> +			if ((unsigned int)idle_cpu < nr_cpumask_bits)
> +				return idle_cpu;
> +		}
> +
> +		cpumask_andnot(cpus, cpus, sched_domain_span(sd));
> +	}
> +
> +	return -1;
> +}
> +#else
> +static inline int scan_cluster(struct task_struct *p, struct cpumask *cpus,
> +			       int target, int *nr)
> +{
> +	return -1;
> +}
> +#endif
> +
>   /*
>    * Scan the LLC domain for idle CPUs; this is dynamically regulated by
>    * comparing the average scan cost (tracked in sd->avg_scan_cost) against the
> @@ -6375,6 +6409,10 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>   		time = cpu_clock(this);
>   	}
>   
> +	idle_cpu = scan_cluster(p, cpus, target, &nr);
> +	if ((unsigned int)idle_cpu < nr_cpumask_bits)
> +		return idle_cpu;
> +
>   	for_each_cpu_wrap(cpu, cpus, target + 1) {
>   		if (has_idle_core) {
>   			i = select_idle_core(p, cpu, cpus, &idle_cpu);
> @@ -6382,7 +6420,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>   				return i;
>   
>   		} else {
> -			if (!--nr)
> +			if (--nr <= 0)
>   				return -1;
>   			idle_cpu = __select_idle_cpu(cpu, p);
>   			if ((unsigned int)idle_cpu < nr_cpumask_bits)
> @@ -6481,7 +6519,7 @@ 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;
> @@ -6507,7 +6545,7 @@ 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)) {