Re: [PATCH 3/4] sched/eevdf: O(1) fastpath for task selection

[Abel Wu <wuyun.abel@bytedance.com>]

On 11/7/23 5:05 PM, Abel Wu Wrote:
> Since the RB-tree is now sorted by deadline, let's first try the
> leftmost entity which has the earliest virtual deadline. I've done
> some benchmarks to see its effectiveness.
> 
> All the benchmarks are done inside a normal cpu cgroup in a clean
> environment with cpu turbo disabled, on a dual-CPU Intel Xeon(R)
> Platinum 8260 with 2 NUMA nodes each of which has 24C/48T.
> 
>    hackbench: process/thread + pipe/socket + 1/2/4/8 groups
>    netperf:   TCP/UDP + STREAM/RR + 24/48/72/96/192 threads
>    tbench:    loopback 24/48/72/96/192 threads
>    schbench:  1/2/4/8 mthreads
> 
>    direct:    cfs_rq has only one entity
>    parity:    RUN_TO_PARITY
>    fast:      O(1) fastpath
>    slow:	     heap search
> 
>      (%)		direct	parity	fast	slow
>    hackbench	92.95	2.02	4.91	0.12
>    netperf	68.08	6.60	24.18	1.14
>    tbench	67.55	11.22	20.61	0.62
>    schbench	69.91	2.65	25.73	1.71
> 
> The above results indicate that this fastpath really makes task
> selection more efficient.
> 
> Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
> ---
>   kernel/sched/fair.c | 14 +++++++++++---
>   1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 459487bf8824..a1fdd0c7a051 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -896,6 +896,7 @@ int sched_update_scaling(void)
>   static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
>   {
>   	struct rb_node *node = cfs_rq->tasks_timeline.rb_root.rb_node;
> +	struct sched_entity *se = __pick_first_entity(cfs_rq);
>   	struct sched_entity *curr = cfs_rq->curr;
>   	struct sched_entity *best = NULL;
>   
> @@ -904,7 +905,7 @@ static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
>   	 * in this cfs_rq, saving some cycles.
>   	 */
>   	if (cfs_rq->nr_running == 1)
> -		return curr && curr->on_rq ? curr : __node_2_se(node);
> +		return curr && curr->on_rq ? curr : se;

Maybe we can reduce memory footprint on curr by:

		return se ? se : curr;

>   
>   	if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
>   		curr = NULL;
> @@ -916,9 +917,14 @@ static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
>   	if (sched_feat(RUN_TO_PARITY) && curr && curr->vlag == curr->deadline)
>   		return curr;
>   
> +	/* Pick the leftmost entity if it's eligible */
> +	if (se && entity_eligible(cfs_rq, se)) {
> +		best = se;
> +		goto found;
> +	}
> +
>   	/* Heap search for the EEVD entity */
>   	while (node) {
> -		struct sched_entity *se = __node_2_se(node);
>   		struct rb_node *left = node->rb_left;
>   
>   		/*
> @@ -931,6 +937,8 @@ static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
>   			continue;
>   		}
>   
> +		se = __node_2_se(node);
> +
>   		/*
>   		 * The left subtree either is empty or has no eligible
>   		 * entity, so check the current node since it is the one
> @@ -943,7 +951,7 @@ static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
>   
>   		node = node->rb_right;
>   	}
> -
> +found:
>   	if (!best || (curr && entity_before(curr, best)))
>   		best = curr;
>