[RFC 6/6] sched/fair: Add sched group latency support

[Vincent Guittot <vincent.guittot@linaro.org>]

Tasks can set its latency priority which is then used to decide to preempt
the current running entity of the cfs but sched group entities still have
the default latency priority.

Add a latency field in task group to set the latency priority of the group
which will be used against other entity in the parent cfs.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/core.c  | 41 +++++++++++++++++++++++++++++++++++++++++
 kernel/sched/fair.c  | 32 ++++++++++++++++++++++++++++++++
 kernel/sched/sched.h |  4 ++++
 3 files changed, 77 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 547b0da01efe..e0668652dd24 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10635,6 +10635,30 @@ static int cpu_idle_write_s64(struct cgroup_subsys_state *css,
 {
 	return sched_group_set_idle(css_tg(css), idle);
 }
+
+static s64 cpu_latency_read_s64(struct cgroup_subsys_state *css,
+			       struct cftype *cft)
+{
+	return css_tg(css)->latency_prio;
+}
+
+static int cpu_latency_write_s64(struct cgroup_subsys_state *css,
+				struct cftype *cft, s64 latency_prio)
+{
+	return sched_group_set_latency(css_tg(css), latency_prio);
+}
+
+static s64 cpu_latency_nice_read_s64(struct cgroup_subsys_state *css,
+			       struct cftype *cft)
+{
+	return LATENCY_TO_NICE(css_tg(css)->latency_prio);
+}
+
+static int cpu_latency_nice_write_s64(struct cgroup_subsys_state *css,
+				struct cftype *cft, s64 latency_nice)
+{
+	return sched_group_set_latency(css_tg(css), NICE_TO_LATENCY(latency_nice));
+}
 #endif
 
 static struct cftype cpu_legacy_files[] = {
@@ -10649,6 +10673,11 @@ static struct cftype cpu_legacy_files[] = {
 		.read_s64 = cpu_idle_read_s64,
 		.write_s64 = cpu_idle_write_s64,
 	},
+	{
+		.name = "latency",
+		.read_s64 = cpu_latency_read_s64,
+		.write_s64 = cpu_latency_write_s64,
+	},
 #endif
 #ifdef CONFIG_CFS_BANDWIDTH
 	{
@@ -10866,6 +10895,18 @@ static struct cftype cpu_files[] = {
 		.read_s64 = cpu_idle_read_s64,
 		.write_s64 = cpu_idle_write_s64,
 	},
+	{
+		.name = "latency",
+		.flags = CFTYPE_NOT_ON_ROOT,
+		.read_s64 = cpu_latency_read_s64,
+		.write_s64 = cpu_latency_write_s64,
+	},
+	{
+		.name = "latency.nice",
+		.flags = CFTYPE_NOT_ON_ROOT,
+		.read_s64 = cpu_latency_nice_read_s64,
+		.write_s64 = cpu_latency_nice_write_s64,
+	},
 #endif
 #ifdef CONFIG_CFS_BANDWIDTH
 	{
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 506c482a0e48..cbccef025089 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11496,6 +11496,7 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
 		goto err;
 
 	tg->shares = NICE_0_LOAD;
+	tg->latency_prio = DEFAULT_LATENCY_PRIO;
 
 	init_cfs_bandwidth(tg_cfs_bandwidth(tg));
 
@@ -11594,6 +11595,7 @@ void init_tg_cfs_entry(struct task_group *tg, struct cfs_rq *cfs_rq,
 	}
 
 	se->my_q = cfs_rq;
+	se->latency_weight = sched_latency_to_weight[tg->latency_prio];
 	/* guarantee group entities always have weight */
 	update_load_set(&se->load, NICE_0_LOAD);
 	se->parent = parent;
@@ -11724,6 +11726,36 @@ int sched_group_set_idle(struct task_group *tg, long idle)
 	return 0;
 }
 
+int sched_group_set_latency(struct task_group *tg, long latency_prio)
+{
+	int i;
+
+	if (tg == &root_task_group)
+		return -EINVAL;
+
+	if (latency_prio < 0 ||
+	    latency_prio > LATENCY_NICE_WIDTH)
+		return -EINVAL;
+
+	mutex_lock(&shares_mutex);
+
+	if (tg->latency_prio == latency_prio) {
+		mutex_unlock(&shares_mutex);
+		return 0;
+	}
+
+	tg->latency_prio = latency_prio;
+
+	for_each_possible_cpu(i) {
+		struct sched_entity *se = tg->se[i];
+
+		WRITE_ONCE(se->latency_weight, sched_latency_to_weight[latency_prio]);
+	}
+
+	mutex_unlock(&shares_mutex);
+	return 0;
+}
+
 #else /* CONFIG_FAIR_GROUP_SCHED */
 
 void free_fair_sched_group(struct task_group *tg) { }
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index dd92aa9c36f9..885d1c809329 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -429,6 +429,8 @@ struct task_group {
 
 	/* A positive value indicates that this is a SCHED_IDLE group. */
 	int			idle;
+	/* latency priority of the group. */
+	int			latency_prio;
 
 #ifdef	CONFIG_SMP
 	/*
@@ -542,6 +544,8 @@ extern int sched_group_set_shares(struct task_group *tg, unsigned long shares);
 
 extern int sched_group_set_idle(struct task_group *tg, long idle);
 
+extern int sched_group_set_latency(struct task_group *tg, long latency);
+
 #ifdef CONFIG_SMP
 extern void set_task_rq_fair(struct sched_entity *se,
 			     struct cfs_rq *prev, struct cfs_rq *next);
-- 
2.17.1