[PATCH v4 09/16] sched/core: uclamp: map TG's clamp values into CPU's clamp groups

[Patrick Bellasi <patrick.bellasi@arm.com>]

Utilization clamping requires to map each different clamp value
into one of the available clamp groups used by the scheduler's fast-path
to account for RUNNABLE tasks. Thus, each time a TG's clamp value
sysfs attribute is updated via:
   cpu_util_{min,max}_write_u64()
we need to get (if possible) a reference to the new value's clamp group
and release the reference to the previous one.

Let's ensure that, whenever a task group is assigned a specific
clamp_value, this is properly translated into a unique clamp group to be
used in the fast-path (i.e. at enqueue/dequeue time).
We do that by slightly refactoring uclamp_group_get() to make the
*task_struct parameter optional. This allows to re-use the code already
available to support the per-task API.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Todd Kjos <tkjos@google.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Quentin Perret <quentin.perret@arm.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-pm@vger.kernel.org

---
Changes in v4:
 Others:
 - rebased on v4.19-rc1

Changes in v3:
 Message-ID: <CAJuCfpF6=L=0LrmNnJrTNPazT4dWKqNv+thhN0dwpKCgUzs9sg@mail.gmail.com>
 - add explicit calls to uclamp_group_find(), which is now not more
   part of uclamp_group_get()
 Others:
 - rebased on tip/sched/core
Changes in v2:
 - rebased on v4.18-rc4
 - this code has been split from a previous patch to simplify the review
---
 include/linux/sched.h | 11 +++--
 kernel/sched/core.c   | 95 +++++++++++++++++++++++++++++++++++++++----
 2 files changed, 95 insertions(+), 11 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2da130d17e70..4e5522ed57e0 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -587,17 +587,22 @@ struct sched_dl_entity {
  * The same "group_id" can be used by multiple scheduling entities, i.e.
  * either tasks or task groups, to enforce the same clamp "value" for a given
  * clamp index.
+ *
+ * Scheduling entity's specific clamp group index can be different
+ * from the effective clamp group index used at enqueue time since
+ * task groups's clamps can be restricted by their parent task group.
  */
 struct uclamp_se {
 	unsigned int value;
 	unsigned int group_id;
 	/*
-	 * Effective task (group) clamp value.
-	 * For task groups is the value (eventually) enforced by a parent task
-	 * group.
+	 * Effective task (group) clamp value and group index.
+	 * For task groups it's the value (eventually) enforced by a parent
+	 * task group.
 	 */
 	struct {
 		unsigned int value;
+		unsigned int group_id;
 	} effective;
 };
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b2d438b6484b..e617a7b18f2d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1250,24 +1250,51 @@ static inline int alloc_uclamp_sched_group(struct task_group *tg,
 					   struct task_group *parent)
 {
 	struct uclamp_se *uc_se;
+	int next_group_id;
 	int clamp_id;
 
 	for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
 		uc_se = &tg->uclamp[clamp_id];
+
 		uc_se->effective.value =
 			parent->uclamp[clamp_id].effective.value;
-		uc_se->value = parent->uclamp[clamp_id].value;
-		uc_se->group_id = parent->uclamp[clamp_id].group_id;
+		uc_se->effective.group_id =
+			parent->uclamp[clamp_id].effective.group_id;
+
+		next_group_id = parent->uclamp[clamp_id].group_id;
+		uc_se->group_id = UCLAMP_NOT_VALID;
+		uclamp_group_get(NULL, clamp_id, next_group_id, uc_se,
+				 parent->uclamp[clamp_id].value);
 	}
 
 	return 1;
 }
+
+/**
+ * release_uclamp_sched_group: release utilization clamp references of a TG
+ * @tg: the task group being removed
+ *
+ * An empty task group can be removed only when it has no more tasks or child
+ * groups. This means that we can also safely release all the reference
+ * counting to clamp groups.
+ */
+static inline void free_uclamp_sched_group(struct task_group *tg)
+{
+	struct uclamp_se *uc_se;
+	int clamp_id;
+
+	for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
+		uc_se = &tg->uclamp[clamp_id];
+		uclamp_group_put(clamp_id, uc_se->group_id);
+	}
+}
 #else /* CONFIG_UCLAMP_TASK_GROUP */
 static inline int alloc_uclamp_sched_group(struct task_group *tg,
 					   struct task_group *parent)
 {
 	return 1;
 }
+static inline void free_uclamp_sched_group(struct task_group *tg) { }
 #endif /* CONFIG_UCLAMP_TASK_GROUP */
 
 static inline int __setscheduler_uclamp(struct task_struct *p,
@@ -1417,9 +1444,18 @@ static void __init init_uclamp(void)
 #ifdef CONFIG_UCLAMP_TASK_GROUP
 		/* Init root TG's clamp group */
 		uc_se = &root_task_group.uclamp[clamp_id];
+
 		uc_se->effective.value = uclamp_none(clamp_id);
-		uc_se->value = uclamp_none(clamp_id);
-		uc_se->group_id = 0;
+		uc_se->effective.group_id = 0;
+
+		/*
+		 * The max utilization is always allowed for both clamps.
+		 * This is required to not force a null minimum utiliation on
+		 * all child groups.
+		 */
+		uc_se->group_id = UCLAMP_NOT_VALID;
+		uclamp_group_get(NULL, clamp_id, 0, uc_se,
+				 uclamp_none(UCLAMP_MAX));
 #endif
 	}
 }
@@ -1427,6 +1463,7 @@ static void __init init_uclamp(void)
 #else /* CONFIG_UCLAMP_TASK */
 static inline void uclamp_cpu_get(struct rq *rq, struct task_struct *p) { }
 static inline void uclamp_cpu_put(struct rq *rq, struct task_struct *p) { }
+static inline void free_uclamp_sched_group(struct task_group *tg) { }
 static inline int alloc_uclamp_sched_group(struct task_group *tg,
 					   struct task_group *parent)
 {
@@ -6984,6 +7021,7 @@ static DEFINE_SPINLOCK(task_group_lock);
 
 static void sched_free_group(struct task_group *tg)
 {
+	free_uclamp_sched_group(tg);
 	free_fair_sched_group(tg);
 	free_rt_sched_group(tg);
 	autogroup_free(tg);
@@ -7234,6 +7272,7 @@ static void cpu_cgroup_attach(struct cgroup_taskset *tset)
  * @css: the task group to update
  * @clamp_id: the clamp index to update
  * @value: the new task group clamp value
+ * @group_id: the group index mapping the new task clamp value
  *
  * The effective clamp for a TG is expected to track the most restrictive
  * value between the TG's clamp value and it's parent effective clamp value.
@@ -7252,9 +7291,12 @@ static void cpu_cgroup_attach(struct cgroup_taskset *tset)
  * be propagated down to all the descendants. When a subgroup is found which
  * has already its effective clamp value matching its clamp value, then we can
  * safely skip all its descendants which are granted to be already in sync.
+ *
+ * The TG's group_id is also updated to ensure it tracks the effective clamp
+ * value.
  */
 static void cpu_util_update_hier(struct cgroup_subsys_state *css,
-				 int clamp_id, int value)
+				 int clamp_id, int value, int group_id)
 {
 	struct cgroup_subsys_state *top_css = css;
 	struct uclamp_se *uc_se, *uc_parent;
@@ -7282,24 +7324,30 @@ static void cpu_util_update_hier(struct cgroup_subsys_state *css,
 		}
 
 		/* Propagate the most restrictive effective value */
-		if (uc_parent->effective.value < value)
+		if (uc_parent->effective.value < value) {
 			value = uc_parent->effective.value;
+			group_id = uc_parent->effective.group_id;
+		}
 		if (uc_se->effective.value == value)
 			continue;
 
 		uc_se->effective.value = value;
+		uc_se->effective.group_id = group_id;
 	}
 }
 
 static int cpu_util_min_write_u64(struct cgroup_subsys_state *css,
 				  struct cftype *cftype, u64 min_value)
 {
+	struct uclamp_se *uc_se;
 	struct task_group *tg;
 	int ret = -EINVAL;
+	int group_id;
 
 	if (min_value > SCHED_CAPACITY_SCALE)
 		return -ERANGE;
 
+	mutex_lock(&uclamp_mutex);
 	rcu_read_lock();
 
 	tg = css_tg(css);
@@ -7310,11 +7358,25 @@ static int cpu_util_min_write_u64(struct cgroup_subsys_state *css,
 	if (tg->uclamp[UCLAMP_MAX].value < min_value)
 		goto out;
 
+	/* Find a valid group_id */
+	ret = uclamp_group_find(UCLAMP_MIN, min_value);
+	if (ret == -ENOSPC) {
+		pr_err(UCLAMP_ENOSPC_FMT, "MIN");
+		goto out;
+	}
+	group_id = ret;
+	ret = 0;
+
 	/* Update effective clamps to track the most restrictive value */
-	cpu_util_update_hier(css, UCLAMP_MIN, min_value);
+	cpu_util_update_hier(css, UCLAMP_MIN, min_value, group_id);
+
+	/* Update TG's reference count */
+	uc_se = &tg->uclamp[UCLAMP_MIN];
+	uclamp_group_get(NULL, UCLAMP_MIN, group_id, uc_se, min_value);
 
 out:
 	rcu_read_unlock();
+	mutex_unlock(&uclamp_mutex);
 
 	return ret;
 }
@@ -7322,12 +7384,15 @@ static int cpu_util_min_write_u64(struct cgroup_subsys_state *css,
 static int cpu_util_max_write_u64(struct cgroup_subsys_state *css,
 				  struct cftype *cftype, u64 max_value)
 {
+	struct uclamp_se *uc_se;
 	struct task_group *tg;
 	int ret = -EINVAL;
+	int group_id;
 
 	if (max_value > SCHED_CAPACITY_SCALE)
 		return -ERANGE;
 
+	mutex_lock(&uclamp_mutex);
 	rcu_read_lock();
 
 	tg = css_tg(css);
@@ -7338,11 +7403,25 @@ static int cpu_util_max_write_u64(struct cgroup_subsys_state *css,
 	if (tg->uclamp[UCLAMP_MIN].value > max_value)
 		goto out;
 
+	/* Find a valid group_id */
+	ret = uclamp_group_find(UCLAMP_MAX, max_value);
+	if (ret == -ENOSPC) {
+		pr_err(UCLAMP_ENOSPC_FMT, "MAX");
+		goto out;
+	}
+	group_id = ret;
+	ret = 0;
+
 	/* Update effective clamps to track the most restrictive value */
-	cpu_util_update_hier(css, UCLAMP_MAX, max_value);
+	cpu_util_update_hier(css, UCLAMP_MAX, max_value, group_id);
+
+	/* Update TG's reference count */
+	uc_se = &tg->uclamp[UCLAMP_MAX];
+	uclamp_group_get(NULL, UCLAMP_MAX, group_id, uc_se, max_value);
 
 out:
 	rcu_read_unlock();
+	mutex_unlock(&uclamp_mutex);
 
 	return ret;
 }
-- 
2.18.0