[PATCH v4 5/7] sched/fair: Add trivial fair server

[Daniel Bristot de Oliveira]

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Use deadline servers to service fair tasks.

This patch adds a fair_server deadline entity which acts as a container
for fair entities and can be used to fix starvation when higher priority
(wrt fair) tasks are monopolizing CPU(s).

[ dl_server do not account for rt ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
---
 kernel/sched/core.c     |  1 +
 kernel/sched/deadline.c |  7 +++++++
 kernel/sched/fair.c     | 29 +++++++++++++++++++++++++++++
 kernel/sched/sched.h    |  4 ++++
 4 files changed, 41 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c780707e1761..4ba4f1e09a80 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10055,6 +10055,7 @@ void __init sched_init(void)
 #endif /* CONFIG_SMP */
 		hrtick_rq_init(rq);
 		atomic_set(&rq->nr_iowait, 0);
+		fair_server_init(rq);
 
 #ifdef CONFIG_SCHED_CORE
 		rq->core = rq;
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 4dac16ed1317..7844cfb73029 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1403,6 +1403,13 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64
 			resched_curr(rq);
 	}
 
+	/*
+	 * The fair server (sole dl_server) does not account for real-time
+	 * workload because it is running fair work.
+	 */
+	if (dl_server(dl_se))
+		return;
+
 	/*
 	 * Because -- for now -- we share the rt bandwidth, we need to
 	 * account our runtime there too, otherwise actual rt tasks
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5ded18e28609..580e6764a68b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6499,6 +6499,9 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	 */
 	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
@@ -6643,6 +6646,9 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 		rq->next_balance = jiffies;
 
 dequeue_throttle:
+	if (!rq->cfs.h_nr_running)
+		dl_server_stop(&rq->fair_server);
+
 	util_est_update(&rq->cfs, p, task_sleep);
 	hrtick_update(rq);
 }
@@ -8291,6 +8297,29 @@ static struct task_struct *__pick_next_task_fair(struct rq *rq)
 	return pick_next_task_fair(rq, NULL, 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 = 50 * NSEC_PER_MSEC;
+	dl_se->dl_deadline = 1000 * NSEC_PER_MSEC;
+	dl_se->dl_period = 1000 * NSEC_PER_MSEC;
+
+	dl_server_init(dl_se, rq, fair_server_has_tasks, fair_server_pick);
+}
+
 /*
  * Account for a descheduled task:
  */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index f30be4ae4c22..ac94c386741c 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -351,6 +351,8 @@ extern void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
 		    dl_server_has_tasks_f has_tasks,
 		    dl_server_pick_f pick);
 
+extern void fair_server_init(struct rq *);
+
 #ifdef CONFIG_CGROUP_SCHED
 
 struct cfs_rq;
@@ -1024,6 +1026,8 @@ 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;
-- 
2.40.1