[RFC][PATCH 13/13] sched/fair: Add trivial fair server

[Peter Zijlstra <peterz@infradead.org>]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/core.c     |    6 +++++-
 kernel/sched/deadline.c |    2 ++
 kernel/sched/fair.c     |   29 +++++++++++++++++++++++++++++
 kernel/sched/sched.h    |    6 +++++-
 4 files changed, 41 insertions(+), 2 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6501,6 +6504,7 @@ void __init sched_init(void)
 #endif /* CONFIG_SMP */
 		hrtick_rq_init(rq);
 		atomic_set(&rq->nr_iowait, 0);
+		fair_server_init(rq);
 	}
 
 	set_load_weight(&init_task, false);
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5239,6 +5239,9 @@ enqueue_task_fair(struct rq *rq, struct
 	 */
 	util_est_enqueue(&rq->cfs, p);
 
+	if (!rq->cfs.h_nr_running)
+		dl_server_start(&rq->fair_server);
+
 	/*
 	 * If in_iowait is set, the code below may not trigger any cpufreq
 	 * utilization updates, so do it here explicitly with the IOWAIT flag
@@ -5374,6 +5377,9 @@ static void dequeue_task_fair(struct rq
 	if (!se)
 		sub_nr_running(rq, 1);
 
+	if (!rq->cfs.h_nr_running)
+		dl_server_stop(&rq->fair_server);
+
 	util_est_dequeue(&rq->cfs, p, task_sleep);
 	hrtick_update(rq);
 }
@@ -6903,6 +6909,29 @@ done: __maybe_unused;
 	return NULL;
 }
 
+static bool fair_server_has_tasks(struct sched_dl_entity *dl_se)
+{
+	return !!dl_se->rq->cfs.nr_running;
+}
+
+static struct task_struct *fair_server_pick(struct sched_dl_entity *dl_se)
+{
+	return pick_next_task_fair(dl_se->rq, NULL, NULL);
+}
+
+void fair_server_init(struct rq *rq)
+{
+	struct sched_dl_entity *dl_se = &rq->fair_server;
+
+	init_dl_entity(dl_se);
+
+	dl_se->dl_runtime = TICK_NSEC;
+	dl_se->dl_deadline = 20 * TICK_NSEC;
+	dl_se->dl_period = 20 * TICK_NSEC;
+
+	dl_server_init(dl_se, rq, fair_server_has_tasks, fair_server_pick);
+}
+
 /*
  * Account for a descheduled task:
  */
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -345,6 +345,8 @@ extern void dl_server_init(struct sched_
 		    dl_server_has_tasks_f has_tasks,
 		    dl_server_pick_f pick);
 
+extern void fair_server_init(struct rq *);
+
 #ifdef CONFIG_CGROUP_SCHED
 
 #include <linux/cgroup.h>
@@ -905,6 +907,8 @@ struct rq {
 	struct rt_rq		rt;
 	struct dl_rq		dl;
 
+	struct sched_dl_entity	fair_server;
+
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	/* list of leaf cfs_rq on this CPU: */
 	struct list_head	leaf_cfs_rq_list;
@@ -968,7 +972,7 @@ struct rq {
 
 	/* This is used to determine avg_idle's max value */
 	u64			max_idle_balance_cost;
-#endif
+#endif /* CONFIG_SMP */
 
 #ifdef CONFIG_IRQ_TIME_ACCOUNTING
 	u64			prev_irq_time;