[PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups

[Patrick Bellasi]

Utilization clamping requires each CPU to know which clamp values are
assigned to tasks that are currently RUNNABLE on that CPU.
Multiple tasks can be assigned the same clamp value and tasks with
different clamp values can be concurrently active on the same CPU.
Thus, a proper data structure is required to support a fast and
efficient aggregation of the clamp values required by the currently
RUNNABLE tasks.

For this purpose we use a per-CPU array of reference counters,
where each slot is used to account how many tasks require a certain
clamp value are currently RUNNABLE on each CPU.
Each clamp value corresponds to a "clamp index" which identifies the
position within the array of reference counters.

                                 :
       (user-space changes)      :      (kernel space / scheduler)
                                 :
             SLOW PATH           :             FAST PATH
                                 :
    task_struct::uclamp::value   :     sched/core::enqueue/dequeue
                                 :         cpufreq_schedutil
                                 :
  +----------------+    +--------------------+     +-------------------+
  |      TASK      |    |     CLAMP GROUP    |     |    CPU CLAMPS     |
  +----------------+    +--------------------+     +-------------------+
  |                |    |   clamp_{min,max}  |     |  clamp_{min,max}  |
  | util_{min,max} |    |      se_count      |     |    tasks count    |
  +----------------+    +--------------------+     +-------------------+
                                 :
           +------------------>  :  +------------------->
    group_id = map(clamp_value)  :  ref_count(group_id)
                                 :
                                 :

Let's introduce the support to map tasks to "clamp groups".
Specifically we introduce the required functions to translate a
"clamp value" into a clamp's "group index" (group_id).

Only a limited number of (different) clamp values are supported since:
1. there are usually only few classes of workloads for which it makes
   sense to boost/limit to different frequencies,
   e.g. background vs foreground, interactive vs low-priority
2. it allows a simpler and more memory/time efficient tracking of
   the per-CPU clamp values in the fast path.

The number of possible different clamp values is currently defined at
compile time. Thus, setting a new clamp value for a task can result into
a -ENOSPC error in case this will exceed the number of maximum different
clamp values supported.

Signed-off-by: Patrick Bellasi <patrick.bellasi@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Paul Turner <pjt@xxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Todd Kjos <tkjos@xxxxxxxxxx>
Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Quentin Perret <quentin.perret@xxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Morten Rasmussen <morten.rasmussen@xxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-pm@xxxxxxxxxxxxxxx

---
Changes in v4:
 Message-ID: <20180814112509.GB2661@xxxxxxxxxxxxxx>
 - add uclamp_exit_task() to release clamp refcount from do_exit()
 Message-ID: <20180816133249.GA2964@e110439-lin>
 - keep the WARN but butify a bit that code
 Message-ID: <20180413082648.GP4043@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
 - move uclamp_enabled at the top of sched_class to keep it on the same
   cache line of other main wakeup time callbacks
 Others:
 - init uclamp for the init_task and refcount its clamp groups
 - add uclamp specific fork time code into uclamp_fork
 - add support for SCHED_FLAG_RESET_ON_FORK
   default clamps are now set for init_task and inherited/reset at
   fork time (when then flag is set for the parent)
 - enable uclamp only for FAIR tasks, RT class will be enabled only
   by a following patch which also integrate the class to schedutil
 - define uclamp_maps ____cacheline_aligned_in_smp
 - in uclamp_group_get() ensure to include uclamp_group_available() and
   uclamp_group_init() into the atomic section defined by:
      uc_map[next_group_id].se_lock
 - do not use mutex_lock(&uclamp_mutex) in uclamp_exit_task
   which is also not needed since refcounting is already guarded by
   the uc_map[group_id].se_lock spinlock
 - rebased on v4.19-rc1

Changes in v3:
 Message-ID: <CAJuCfpF6=L=0LrmNnJrTNPazT4dWKqNv+thhN0dwpKCgUzs9sg@xxxxxxxxxxxxxx>
 - rename UCLAMP_NONE into UCLAMP_NOT_VALID
 - remove not necessary checks in uclamp_group_find()
 - add WARN on unlikely un-referenced decrement in uclamp_group_put()
 - make __setscheduler_uclamp() able to set just one clamp value
 - make __setscheduler_uclamp() failing if both clamps are required but
   there is no clamp groups available for one of them
 - remove uclamp_group_find() from uclamp_group_get() which now takes a
   group_id as a parameter
 Others:
 - rebased on tip/sched/core
Changes in v2:
 - rabased on v4.18-rc4
 - set UCLAMP_GROUPS_COUNT=2 by default
   which allows to fit all the hot-path CPU clamps data, partially
   intorduced also by the following patches, into a single cache line
   while still supporting up to 2 different {min,max}_utiql clamps.
---
 include/linux/sched.h      |  16 +-
 include/linux/sched/task.h |   6 +
 include/uapi/linux/sched.h |   6 +-
 init/Kconfig               |  20 ++
 init/init_task.c           |   4 -
 kernel/exit.c              |   1 +
 kernel/sched/core.c        | 395 +++++++++++++++++++++++++++++++++++--
 kernel/sched/fair.c        |   4 +
 kernel/sched/sched.h       |  28 ++-
 9 files changed, 456 insertions(+), 24 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 880a0c5c1f87..7385f0b1a7c0 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -279,6 +279,9 @@ struct vtime {
 	u64			gtime;
 };
 
+/* Clamp not valid, i.e. group not assigned or invalid value */
+#define UCLAMP_NOT_VALID -1
+
 enum uclamp_id {
 	UCLAMP_MIN = 0, /* Minimum utilization */
 	UCLAMP_MAX,     /* Maximum utilization */
@@ -575,6 +578,17 @@ struct sched_dl_entity {
 	struct hrtimer inactive_timer;
 };
 
+/**
+ * Utilization's clamp group
+ *
+ * A utilization clamp group maps a "clamp value" (value), i.e.
+ * util_{min,max}, to a "clamp group index" (group_id).
+ */
+struct uclamp_se {
+	unsigned int value;
+	unsigned int group_id;
+};
+
 union rcu_special {
 	struct {
 		u8			blocked;
@@ -659,7 +673,7 @@ struct task_struct {
 
 #ifdef CONFIG_UCLAMP_TASK
 	/* Utlization clamp values for this task */
-	int				uclamp[UCLAMP_CNT];
+	struct uclamp_se		uclamp[UCLAMP_CNT];
 #endif
 
 #ifdef CONFIG_PREEMPT_NOTIFIERS
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 108ede99e533..36c81c364112 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -68,6 +68,12 @@ static inline void exit_thread(struct task_struct *tsk)
 #endif
 extern void do_group_exit(int);
 
+#ifdef CONFIG_UCLAMP_TASK
+extern void uclamp_exit_task(struct task_struct *p);
+#else
+static inline void uclamp_exit_task(struct task_struct *p) { }
+#endif /* CONFIG_UCLAMP_TASK */
+
 extern void exit_files(struct task_struct *);
 extern void exit_itimers(struct signal_struct *);
 
diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index c27d6e81517b..ae7e12de32ca 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -50,7 +50,11 @@
 #define SCHED_FLAG_RESET_ON_FORK	0x01
 #define SCHED_FLAG_RECLAIM		0x02
 #define SCHED_FLAG_DL_OVERRUN		0x04
-#define SCHED_FLAG_UTIL_CLAMP		0x08
+
+#define SCHED_FLAG_UTIL_CLAMP_MIN	0x10
+#define SCHED_FLAG_UTIL_CLAMP_MAX	0x20
+#define SCHED_FLAG_UTIL_CLAMP	(SCHED_FLAG_UTIL_CLAMP_MIN | \
+				 SCHED_FLAG_UTIL_CLAMP_MAX)
 
 #define SCHED_FLAG_ALL	(SCHED_FLAG_RESET_ON_FORK	| \
 			 SCHED_FLAG_RECLAIM		| \
diff --git a/init/Kconfig b/init/Kconfig
index 738974c4f628..10536cb83295 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -633,7 +633,27 @@ config UCLAMP_TASK
 
 	  If in doubt, say N.
 
+config UCLAMP_GROUPS_COUNT
+	int "Number of different utilization clamp values supported"
+	range 0 32
+	default 5
+	depends on UCLAMP_TASK
+	help
+	  This defines the maximum number of different utilization clamp
+	  values which can be concurrently enforced for each utilization
+	  clamp index (i.e. minimum and maximum utilization).
+
+	  Only a limited number of clamp values are supported because:
+	    1. there are usually only few classes of workloads for which it
+	       makes sense to boost/cap for different frequencies,
+	       e.g. background vs foreground, interactive vs low-priority.
+	    2. it allows a simpler and more memory/time efficient tracking of
+	       the per-CPU clamp values.
+
+	  If in doubt, use the default value.
+
 endmenu
+
 #
 # For architectures that want to enable the support for NUMA-affine scheduler
 # balancing logic:
diff --git a/init/init_task.c b/init/init_task.c
index 5bfdcc3fb839..7f77741b6a9b 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -92,10 +92,6 @@ struct task_struct init_task
 #endif
 #ifdef CONFIG_CGROUP_SCHED
 	.sched_task_group = &root_task_group,
-#endif
-#ifdef CONFIG_UCLAMP_TASK
-	.uclamp[UCLAMP_MIN] = 0,
-	.uclamp[UCLAMP_MAX] = SCHED_CAPACITY_SCALE,
 #endif
 	.ptraced	= LIST_HEAD_INIT(init_task.ptraced),
 	.ptrace_entry	= LIST_HEAD_INIT(init_task.ptrace_entry),
diff --git a/kernel/exit.c b/kernel/exit.c
index 0e21e6d21f35..feb540558051 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -877,6 +877,7 @@ void __noreturn do_exit(long code)
 
 	sched_autogroup_exit_task(tsk);
 	cgroup_exit(tsk);
+	uclamp_exit_task(tsk);
 
 	/*
 	 * FIXME: do that only when needed, using sched_exit tracepoint
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 16d3544c7ffa..2668990b96d1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -717,25 +717,389 @@ static void set_load_weight(struct task_struct *p, bool update_load)
 }
 
 #ifdef CONFIG_UCLAMP_TASK
+/**
+ * uclamp_mutex: serializes updates of utilization clamp values
+ *
+ * A utilization clamp value update is usually triggered from a user-space
+ * process (slow-path) but it requires a synchronization with the scheduler's
+ * (fast-path) enqueue/dequeue operations.
+ * While the fast-path synchronization is protected by RQs spinlock, this
+ * mutex ensures that we sequentially serve user-space requests.
+ */
+static DEFINE_MUTEX(uclamp_mutex);
+
+/**
+ * uclamp_map: reference counts a utilization "clamp value"
+ * @value:    the utilization "clamp value" required
+ * @se_count: the number of scheduling entities requiring the "clamp value"
+ * @se_lock:  serialize reference count updates by protecting se_count
+ */
+struct uclamp_map {
+	int value;
+	int se_count;
+	raw_spinlock_t se_lock;
+};
+
+/**
+ * uclamp_maps: maps each SEs "clamp value" into a CPUs "clamp group"
+ *
+ * Since only a limited number of different "clamp values" are supported, we
+ * need to map each different clamp value into a "clamp group" (group_id) to
+ * be used by the per-CPU accounting in the fast-path, when tasks are
+ * enqueued and dequeued.
+ * We also support different kind of utilization clamping, min and max
+ * utilization for example, each representing what we call a "clamp index"
+ * (clamp_id).
+ *
+ * A matrix is thus required to map "clamp values" to "clamp groups"
+ * (group_id), for each "clamp index" (clamp_id), where:
+ * - rows are indexed by clamp_id and they collect the clamp groups for a
+ *   given clamp index
+ * - columns are indexed by group_id and they collect the clamp values which
+ *   maps to that clamp group
+ *
+ * Thus, the column index of a given (clamp_id, value) pair represents the
+ * clamp group (group_id) used by the fast-path's per-CPU accounting.
+ *
+ * NOTE: first clamp group (group_id=0) is reserved for tracking of non
+ * clamped tasks.  Thus we allocate one more slot than the value of
+ * CONFIG_UCLAMP_GROUPS_COUNT.
+ *
+ * Here is the map layout and, right below, how entries are accessed by the
+ * following code.
+ *
+ *                          uclamp_maps is a matrix of
+ *          +------- UCLAMP_CNT by CONFIG_UCLAMP_GROUPS_COUNT+1 entries
+ *          |                                |
+ *          |                /---------------+---------------\
+ *          |               +------------+       +------------+
+ *          |  / UCLAMP_MIN | value      |       | value      |
+ *          |  |            | se_count   |...... | se_count   |
+ *          |  |            +------------+       +------------+
+ *          +--+            +------------+       +------------+
+ *             |            | value      |       | value      |
+ *             \ UCLAMP_MAX | se_count   |...... | se_count   |
+ *                          +-----^------+       +----^-------+
+ *                                |                   |
+ *                      uc_map =  +                   |
+ *                     &uclamp_maps[clamp_id][0]      +
+ *                                                clamp_value =
+ *                                       uc_map[group_id].value
+ */
+static struct uclamp_map uclamp_maps[UCLAMP_CNT]
+				    [CONFIG_UCLAMP_GROUPS_COUNT + 1]
+				    ____cacheline_aligned_in_smp;
+
+#define UCLAMP_ENOSPC_FMT "Cannot allocate more than " \
+	__stringify(CONFIG_UCLAMP_GROUPS_COUNT) " UTIL_%s clamp groups\n"
+
+/**
+ * uclamp_group_available: checks if a clamp group is available
+ * @clamp_id: the utilization clamp index (i.e. min or max clamp)
+ * @group_id: the group index in the given clamp_id
+ *
+ * A clamp group is not free if there is at least one SE which is sing a clamp
+ * value mapped on the specified clamp_id. These SEs are reference counted by
+ * the se_count of a uclamp_map entry.
+ *
+ * Return: true if there are no SE's mapped on the specified clamp
+ *         index and group
+ */
+static inline bool uclamp_group_available(int clamp_id, int group_id)
+{
+	struct uclamp_map *uc_map = &uclamp_maps[clamp_id][0];
+
+	return (uc_map[group_id].value == UCLAMP_NOT_VALID);
+}
+
+/**
+ * uclamp_group_init: maps a clamp value on a specified clamp group
+ * @clamp_id: the utilization clamp index (i.e. min or max clamp)
+ * @group_id: the group index to map a given clamp_value
+ * @clamp_value: the utilization clamp value to map
+ *
+ * Initializes a clamp group to track tasks from the fast-path.
+ * Each different clamp value, for a given clamp index (i.e. min/max
+ * utilization clamp), is mapped by a clamp group which index is used by the
+ * fast-path code to keep track of RUNNABLE tasks requiring a certain clamp
+ * value.
+ *
+ */
+static inline void uclamp_group_init(int clamp_id, int group_id,
+				     unsigned int clamp_value)
+{
+	struct uclamp_map *uc_map = &uclamp_maps[clamp_id][0];
+
+	uc_map[group_id].value = clamp_value;
+	uc_map[group_id].se_count = 0;
+}
+
+/**
+ * uclamp_group_reset: resets a specified clamp group
+ * @clamp_id: the utilization clamp index (i.e. min or max clamping)
+ * @group_id: the group index to release
+ *
+ * A clamp group can be reset every time there are no more task groups using
+ * the clamp value it maps for a given clamp index.
+ */
+static inline void uclamp_group_reset(int clamp_id, int group_id)
+{
+	uclamp_group_init(clamp_id, group_id, UCLAMP_NOT_VALID);
+}
+
+/**
+ * uclamp_group_find: finds the group index of a utilization clamp group
+ * @clamp_id: the utilization clamp index (i.e. min or max clamping)
+ * @clamp_value: the utilization clamping value lookup for
+ *
+ * Verify if a group has been assigned to a certain clamp value and return
+ * its index to be used for accounting.
+ *
+ * Since only a limited number of utilization clamp groups are allowed, if no
+ * groups have been assigned for the specified value, a new group is assigned,
+ * if possible.
+ * Otherwise an error is returned, meaning that an additional clamp value is
+ * not (currently) supported.
+ */
+static int
+uclamp_group_find(int clamp_id, unsigned int clamp_value)
+{
+	struct uclamp_map *uc_map = &uclamp_maps[clamp_id][0];
+	int free_group_id = UCLAMP_NOT_VALID;
+	unsigned int group_id = 0;
+
+	for ( ; group_id <= CONFIG_UCLAMP_GROUPS_COUNT; ++group_id) {
+		/* Keep track of first free clamp group */
+		if (uclamp_group_available(clamp_id, group_id)) {
+			if (free_group_id == UCLAMP_NOT_VALID)
+				free_group_id = group_id;
+			continue;
+		}
+		/* Return index of first group with same clamp value */
+		if (uc_map[group_id].value == clamp_value)
+			return group_id;
+	}
+
+	if (likely(free_group_id != UCLAMP_NOT_VALID))
+		return free_group_id;
+
+	return -ENOSPC;
+}
+
+/**
+ * uclamp_group_put: decrease the reference count for a clamp group
+ * @clamp_id: the clamp index which was affected by a task group
+ * @uc_se: the utilization clamp data for that task group
+ *
+ * When the clamp value for a task group is changed we decrease the reference
+ * count for the clamp group mapping its current clamp value. A clamp group is
+ * released when there are no more task groups referencing its clamp value.
+ */
+static inline void uclamp_group_put(int clamp_id, int group_id)
+{
+	struct uclamp_map *uc_map = &uclamp_maps[clamp_id][0];
+	unsigned long flags;
+
+	/* Ignore SE's not yet attached */
+	if (group_id == UCLAMP_NOT_VALID)
+		return;
+
+	/* Remove SE from this clamp group */
+	raw_spin_lock_irqsave(&uc_map[group_id].se_lock, flags);
+	if (likely(uc_map[group_id].se_count))
+		uc_map[group_id].se_count -= 1;
+#ifdef SCHED_DEBUG
+	else {
+		WARN(1, "invalid SE clamp group [%d:%d] refcount\n",
+		     clamp_id, group_id);
+	}
+#endif
+	if (uc_map[group_id].se_count == 0)
+		uclamp_group_reset(clamp_id, group_id);
+	raw_spin_unlock_irqrestore(&uc_map[group_id].se_lock, flags);
+}
+
+/**
+ * uclamp_group_get: increase the reference count for a clamp group
+ * @clamp_id: the clamp index affected by the task
+ * @next_group_id: the clamp group to refcount
+ * @uc_se: the utilization clamp data for the task
+ * @clamp_value: the new clamp value for the task
+ *
+ * Each time a task changes its utilization clamp value, for a specified clamp
+ * index, we need to find an available clamp group which can be used to track
+ * this new clamp value. The corresponding clamp group index will be used by
+ * the task to reference count the clamp value on CPUs while enqueued.
+ */
+static inline void uclamp_group_get(int clamp_id, int next_group_id,
+				    struct uclamp_se *uc_se,
+				    unsigned int clamp_value)
+{
+	struct uclamp_map *uc_map = &uclamp_maps[clamp_id][0];
+	int prev_group_id = uc_se->group_id;
+	unsigned long flags;
+
+	/* Allocate new clamp group for this clamp value */
+	raw_spin_lock_irqsave(&uc_map[next_group_id].se_lock, flags);
+	if (uclamp_group_available(clamp_id, next_group_id))
+		uclamp_group_init(clamp_id, next_group_id, clamp_value);
+
+	/* Update SE's clamp values and attach it to new clamp group */
+	uc_se->value = clamp_value;
+	uc_se->group_id = next_group_id;
+	uc_map[next_group_id].se_count += 1;
+	raw_spin_unlock_irqrestore(&uc_map[next_group_id].se_lock, flags);
+
+	/* Release the previous clamp group */
+	uclamp_group_put(clamp_id, prev_group_id);
+}
+
 static inline int __setscheduler_uclamp(struct task_struct *p,
 					const struct sched_attr *attr)
 {
-	if (attr->sched_util_min > attr->sched_util_max)
-		return -EINVAL;
-	if (attr->sched_util_max > SCHED_CAPACITY_SCALE)
-		return -EINVAL;
+	int group_id[UCLAMP_CNT] = { UCLAMP_NOT_VALID };
+	int lower_bound, upper_bound;
+	struct uclamp_se *uc_se;
+	int result = 0;
 
-	p->uclamp[UCLAMP_MIN] = attr->sched_util_min;
-	p->uclamp[UCLAMP_MAX] = attr->sched_util_max;
+	mutex_lock(&uclamp_mutex);
 
-	return 0;
+	/* Find a valid group_id for each required clamp value */
+	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
+		upper_bound = (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX)
+			? attr->sched_util_max
+			: p->uclamp[UCLAMP_MAX].value;
+
+		if (upper_bound == UCLAMP_NOT_VALID)
+			upper_bound = SCHED_CAPACITY_SCALE;
+		if (attr->sched_util_min > upper_bound) {
+			result = -EINVAL;
+			goto done;
+		}
+
+		result = uclamp_group_find(UCLAMP_MIN, attr->sched_util_min);
+		if (result == -ENOSPC) {
+			pr_err(UCLAMP_ENOSPC_FMT, "MIN");
+			goto done;
+		}
+		group_id[UCLAMP_MIN] = result;
+	}
+	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
+		lower_bound = (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN)
+			? attr->sched_util_min
+			: p->uclamp[UCLAMP_MIN].value;
+
+		if (lower_bound == UCLAMP_NOT_VALID)
+			lower_bound = 0;
+		if (attr->sched_util_max < lower_bound ||
+		    attr->sched_util_max > SCHED_CAPACITY_SCALE) {
+			result = -EINVAL;
+			goto done;
+		}
+
+		result = uclamp_group_find(UCLAMP_MAX, attr->sched_util_max);
+		if (result == -ENOSPC) {
+			pr_err(UCLAMP_ENOSPC_FMT, "MAX");
+			goto done;
+		}
+		group_id[UCLAMP_MAX] = result;
+	}
+
+	/* Update each required clamp group */
+	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
+		uc_se = &p->uclamp[UCLAMP_MIN];
+		uclamp_group_get(UCLAMP_MIN, group_id[UCLAMP_MIN],
+				 uc_se, attr->sched_util_min);
+	}
+	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
+		uc_se = &p->uclamp[UCLAMP_MAX];
+		uclamp_group_get(UCLAMP_MAX, group_id[UCLAMP_MAX],
+				 uc_se, attr->sched_util_max);
+	}
+
+done:
+	mutex_unlock(&uclamp_mutex);
+
+	return result;
+}
+
+/**
+ * uclamp_exit_task: release referenced clamp groups
+ * @p: the task exiting
+ *
+ * When a task terminates, release all its (eventually) refcounted
+ * task-specific clamp groups.
+ */
+void uclamp_exit_task(struct task_struct *p)
+{
+	struct uclamp_se *uc_se;
+	int clamp_id;
+
+	for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
+		uc_se = &p->uclamp[clamp_id];
+		uclamp_group_put(clamp_id, uc_se->group_id);
+	}
+}
+
+/**
+ * uclamp_fork: refcount task-specific clamp values for a new task
+ */
+static void uclamp_fork(struct task_struct *p, bool reset)
+{
+	int clamp_id;
+
+	if (unlikely(!p->sched_class->uclamp_enabled))
+		return;
+
+	for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
+		int next_group_id = p->uclamp[clamp_id].group_id;
+		struct uclamp_se *uc_se = &p->uclamp[clamp_id];
+
+		if (unlikely(reset)) {
+			next_group_id = 0;
+			p->uclamp[clamp_id].value = uclamp_none(clamp_id);
+		}
+
+		p->uclamp[clamp_id].group_id = UCLAMP_NOT_VALID;
+		uclamp_group_get(clamp_id, next_group_id, uc_se,
+				 p->uclamp[clamp_id].value);
+	}
+}
+
+/**
+ * init_uclamp: initialize data structures required for utilization clamping
+ */
+static void __init init_uclamp(void)
+{
+	struct uclamp_se *uc_se;
+	int clamp_id;
+
+	mutex_init(&uclamp_mutex);
+
+	for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
+		struct uclamp_map *uc_map = &uclamp_maps[clamp_id][0];
+		int group_id = 0;
+
+		for ( ; group_id <= CONFIG_UCLAMP_GROUPS_COUNT; ++group_id) {
+			uc_map[group_id].value = UCLAMP_NOT_VALID;
+			raw_spin_lock_init(&uc_map[group_id].se_lock);
+		}
+
+		/* Init init_task's clamp group */
+		uc_se = &init_task.uclamp[clamp_id];
+		uc_se->group_id = UCLAMP_NOT_VALID;
+		uclamp_group_get(clamp_id, 0, uc_se, uclamp_none(clamp_id));
+	}
 }
+
 #else /* CONFIG_UCLAMP_TASK */
 static inline int __setscheduler_uclamp(struct task_struct *p,
 					const struct sched_attr *attr)
 {
 	return -EINVAL;
 }
+static inline void uclamp_fork(struct task_struct *p, bool reset) { }
+static inline void init_uclamp(void) { }
 #endif /* CONFIG_UCLAMP_TASK */
 
 static inline void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
@@ -2314,6 +2678,7 @@ static inline void init_schedstats(void) {}
 int sched_fork(unsigned long clone_flags, struct task_struct *p)
 {
 	unsigned long flags;
+	bool reset;
 
 	__sched_fork(clone_flags, p);
 	/*
@@ -2331,7 +2696,8 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 	/*
 	 * Revert to default priority/policy on fork if requested.
 	 */
-	if (unlikely(p->sched_reset_on_fork)) {
+	reset = p->sched_reset_on_fork;
+	if (unlikely(reset)) {
 		if (task_has_dl_policy(p) || task_has_rt_policy(p)) {
 			p->policy = SCHED_NORMAL;
 			p->static_prio = NICE_TO_PRIO(0);
@@ -2342,11 +2708,6 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 		p->prio = p->normal_prio = __normal_prio(p);
 		set_load_weight(p, false);
 
-#ifdef CONFIG_UCLAMP_TASK
-		p->uclamp[UCLAMP_MIN] = 0;
-		p->uclamp[UCLAMP_MAX] = SCHED_CAPACITY_SCALE;
-#endif
-
 		/*
 		 * We don't need the reset flag anymore after the fork. It has
 		 * fulfilled its duty:
@@ -2363,6 +2724,8 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 
 	init_entity_runnable_average(&p->se);
 
+	uclamp_fork(p, reset);
+
 	/*
 	 * The child is not yet in the pid-hash so no cgroup attach races,
 	 * and the cgroup is pinned to this child due to cgroup_fork()
@@ -4756,8 +5119,8 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
 		attr.sched_nice = task_nice(p);
 
 #ifdef CONFIG_UCLAMP_TASK
-	attr.sched_util_min = p->uclamp[UCLAMP_MIN];
-	attr.sched_util_max = p->uclamp[UCLAMP_MAX];
+	attr.sched_util_min = p->uclamp[UCLAMP_MIN].value;
+	attr.sched_util_max = p->uclamp[UCLAMP_MAX].value;
 #endif
 
 	rcu_read_unlock();
@@ -6107,6 +6470,8 @@ void __init sched_init(void)
 
 	init_schedstats();
 
+	init_uclamp();
+
 	scheduler_running = 1;
 }
 
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b39fb596f6c1..dab0405386c1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10055,6 +10055,10 @@ const struct sched_class fair_sched_class = {
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	.task_change_group	= task_change_group_fair,
 #endif
+
+#ifdef CONFIG_UCLAMP_TASK
+	.uclamp_enabled		= 1,
+#endif
 };
 
 #ifdef CONFIG_SCHED_DEBUG
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4a2e8cae63c4..72df2dc779bc 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1501,10 +1501,12 @@ extern const u32		sched_prio_to_wmult[40];
 struct sched_class {
 	const struct sched_class *next;
 
+#ifdef CONFIG_UCLAMP_TASK
+	int uclamp_enabled;
+#endif
+
 	void (*enqueue_task) (struct rq *rq, struct task_struct *p, int flags);
 	void (*dequeue_task) (struct rq *rq, struct task_struct *p, int flags);
-	void (*yield_task)   (struct rq *rq);
-	bool (*yield_to_task)(struct rq *rq, struct task_struct *p, bool preempt);
 
 	void (*check_preempt_curr)(struct rq *rq, struct task_struct *p, int flags);
 
@@ -1537,7 +1539,6 @@ struct sched_class {
 	void (*set_curr_task)(struct rq *rq);
 	void (*task_tick)(struct rq *rq, struct task_struct *p, int queued);
 	void (*task_fork)(struct task_struct *p);
-	void (*task_dead)(struct task_struct *p);
 
 	/*
 	 * The switched_from() call is allowed to drop rq->lock, therefore we
@@ -1554,12 +1555,17 @@ struct sched_class {
 
 	void (*update_curr)(struct rq *rq);
 
+	void (*yield_task)   (struct rq *rq);
+	bool (*yield_to_task)(struct rq *rq, struct task_struct *p, bool preempt);
+
 #define TASK_SET_GROUP		0
 #define TASK_MOVE_GROUP		1
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	void (*task_change_group)(struct task_struct *p, int type);
 #endif
+
+	void (*task_dead)(struct task_struct *p);
 };
 
 static inline void put_prev_task(struct rq *rq, struct task_struct *prev)
@@ -2177,6 +2183,22 @@ static inline void cpufreq_update_util(struct rq *rq, unsigned int flags)
 static inline void cpufreq_update_util(struct rq *rq, unsigned int flags) {}
 #endif /* CONFIG_CPU_FREQ */
 
+/**
+ * uclamp_none: default value for a clamp
+ *
+ * This returns the default value for each clamp
+ * - 0 for a min utilization clamp
+ * - SCHED_CAPACITY_SCALE for a max utilization clamp
+ *
+ * Return: the default value for a given utilization clamp
+ */
+static inline unsigned int uclamp_none(int clamp_id)
+{
+	if (clamp_id == UCLAMP_MIN)
+		return 0;
+	return SCHED_CAPACITY_SCALE;
+}
+
 #ifdef arch_scale_freq_capacity
 # ifndef arch_scale_freq_invariant
 #  define arch_scale_freq_invariant()	true
-- 
2.18.0