sched/eevdf: O(1) fastpath for task selection

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20231115033647.80785-4-wuyun.abel@bytedance.com

kernel/sched/fair.c

@@ -878,6 +878,7 @@ struct sched_entity *__pick_first_entity(struct cfs_rq *cfs_rq)
 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;
 
@@ -886,7 +887,7 @@ 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;
 
 	if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
 		curr = NULL;
@@ -898,9 +899,14 @@ 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;
 
 		/*
@@ -913,6 +919,8 @@ 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
@@ -925,7 +933,7 @@ 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;