[PATCH v4 11/16] sched/core: uclamp: add system default clamps

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

Clamp values cannot be tuned at the root cgroup level. Moreover, because
of the delegation model requirements and how the parent clamps
propagation works, if we want to enable subgroups to set a non null
util.min, we need to be able to configure the root group util.min to the
allow the maximum utilization (SCHED_CAPACITY_SCALE = 1024).

Unfortunately this setup will also mean that all tasks running in the
root group, will always get a maximum util.min clamp, unless they have a
lower task specific clamp which is definitively not a desirable default
configuration.

Let's fix this by explicitly adding a system default configuration
(sysctl_sched_uclamp_util_{min,max}) which works as a restrictive clamp
for all tasks running on the root group.

This interface is available independently from cgroups, thus providing a
complete solution for system wide utilization clamping configuration.

Each task has now by default:
   task_struct::uclamp::value = UCLAMP_NOT_VALID
unless:
 - the task has been forked from a parent with a valid clamp and
   !SCHED_FLAG_RESET_ON_FORK
 - the task has got a task-specific value set via sched_setattr()

A task with a UCLAMP_NOT_VALID clamp value is refcounted considering the
system default clamps if either we do not have task group support or
they are part of the root_task_group.
Tasks without a task specific clamp value in a child task group will be
refcounted instead considering the task group clamps.

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: Paul Turner <pjt@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Todd Kjos <tkjos@google.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: Steve Muckle <smuckle@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:
 Message-ID: <20180820122728.GM2960@e110439-lin>
 - fix unwanted reset of clamp values on refcount success
 Others:
 - by default all tasks have a UCLAMP_NOT_VALID task specific clamp
 - always use:
      p->uclamp[clamp_id].effective.value
   to track the actual clamp value the task has been refcounted into.
   This matches with the usage of
      p->uclamp[clamp_id].effective.group_id
 - rebased on v4.19-rc1
---
 include/linux/sched/sysctl.h |  11 +++
 kernel/sched/core.c          | 147 +++++++++++++++++++++++++++++++++--
 kernel/sysctl.c              |  16 ++++
 3 files changed, 168 insertions(+), 6 deletions(-)

diff --git a/include/linux/sched/sysctl.h b/include/linux/sched/sysctl.h
index a9c32daeb9d8..445fb54eaeff 100644
--- a/include/linux/sched/sysctl.h
+++ b/include/linux/sched/sysctl.h
@@ -56,6 +56,11 @@ int sched_proc_update_handler(struct ctl_table *table, int write,
 extern unsigned int sysctl_sched_rt_period;
 extern int sysctl_sched_rt_runtime;
 
+#ifdef CONFIG_UCLAMP_TASK
+extern unsigned int sysctl_sched_uclamp_util_min;
+extern unsigned int sysctl_sched_uclamp_util_max;
+#endif
+
 #ifdef CONFIG_CFS_BANDWIDTH
 extern unsigned int sysctl_sched_cfs_bandwidth_slice;
 #endif
@@ -75,6 +80,12 @@ extern int sched_rt_handler(struct ctl_table *table, int write,
 		void __user *buffer, size_t *lenp,
 		loff_t *ppos);
 
+#ifdef CONFIG_UCLAMP_TASK
+extern int sched_uclamp_handler(struct ctl_table *table, int write,
+				void __user *buffer, size_t *lenp,
+				loff_t *ppos);
+#endif
+
 extern int sysctl_numa_balancing(struct ctl_table *table, int write,
 				 void __user *buffer, size_t *lenp,
 				 loff_t *ppos);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index da0b3bd41e96..fbc8d9fdfdbb 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -728,6 +728,20 @@ static void set_load_weight(struct task_struct *p, bool update_load)
  */
 static DEFINE_MUTEX(uclamp_mutex);
 
+/*
+ * Minimum utilization for tasks in the root cgroup
+ * default: 0
+ */
+unsigned int sysctl_sched_uclamp_util_min;
+
+/*
+ * Maximum utilization for tasks in the root cgroup
+ * default: 1024
+ */
+unsigned int sysctl_sched_uclamp_util_max = SCHED_CAPACITY_SCALE;
+
+static struct uclamp_se uclamp_default[UCLAMP_CNT];
+
 /**
  * uclamp_map: reference counts a utilization "clamp value"
  * @value:    the utilization "clamp value" required
@@ -961,11 +975,16 @@ static inline void uclamp_cpu_update(struct rq *rq, int clamp_id,
  *
  * This method returns the effective group index for a task, depending on its
  * status and a proper aggregation of the clamp values listed above.
+ * Moreover, it ensures that the task's effective value:
+ *    task_struct::uclamp::effective::value
+ * is updated to represent the clamp value corresponding to the taks effective
+ * group index.
  */
 static inline int uclamp_task_group_id(struct task_struct *p, int clamp_id)
 {
 	struct uclamp_se *uc_se;
 	int clamp_value;
+	bool unclamped;
 	int group_id;
 
 	/* Taks currently accounted into a clamp group */
@@ -977,13 +996,40 @@ static inline int uclamp_task_group_id(struct task_struct *p, int clamp_id)
 	clamp_value = uc_se->value;
 	group_id = uc_se->group_id;
 
+	unclamped = (clamp_value == UCLAMP_NOT_VALID);
 #ifdef CONFIG_UCLAMP_TASK_GROUP
+	/*
+	 * Tasks in the root group, which do not have a task specific clamp
+	 * value, get the system default clamp value.
+	 */
+	if (unclamped && (task_group_is_autogroup(task_group(p)) ||
+			  task_group(p) == &root_task_group)) {
+		p->uclamp[clamp_id].effective.value =
+			uclamp_default[clamp_id].value;
+
+		return uclamp_default[clamp_id].group_id;
+	}
+
 	/* Use TG's clamp value to limit task specific values */
 	uc_se = &task_group(p)->uclamp[clamp_id];
-	if (clamp_value > uc_se->effective.value)
-		group_id = uc_se->effective.group_id;
+	if (unclamped || clamp_value > uc_se->effective.value) {
+		p->uclamp[clamp_id].effective.value =
+			uc_se->effective.value;
+
+		return uc_se->effective.group_id;
+	}
+#else
+	/* By default, all tasks get the system default clamp value */
+	if (unclamped) {
+		p->uclamp[clamp_id].effective.value =
+			uclamp_default[clamp_id].value;
+
+		return uclamp_default[clamp_id].group_id;
+	}
 #endif
 
+	p->uclamp[clamp_id].effective.value = clamp_value;
+
 	return group_id;
 }
 
@@ -999,9 +1045,10 @@ static inline int uclamp_group_value(int clamp_id, int group_id)
 
 static inline int uclamp_task_value(struct task_struct *p, int clamp_id)
 {
-	int group_id = uclamp_task_group_id(p, clamp_id);
+	/* Ensure effective task's clamp value is update */
+	uclamp_task_group_id(p, clamp_id);
 
-	return uclamp_group_value(clamp_id, group_id);
+	return p->uclamp[clamp_id].effective.value;
 }
 
 /**
@@ -1047,7 +1094,7 @@ static inline void uclamp_cpu_get_id(struct task_struct *p,
 
 	/* Reset clamp holds on idle exit */
 	uc_cpu = &rq->uclamp;
-	clamp_value = p->uclamp[clamp_id].value;
+	clamp_value = p->uclamp[clamp_id].effective.value;
 	if (unlikely(uc_cpu->flags & UCLAMP_FLAG_IDLE)) {
 		/*
 		 * This function is called for both UCLAMP_MIN (before) and
@@ -1300,6 +1347,77 @@ static inline void uclamp_group_get(struct task_struct *p,
 	uclamp_group_put(clamp_id, prev_group_id);
 }
 
+int sched_uclamp_handler(struct ctl_table *table, int write,
+			 void __user *buffer, size_t *lenp,
+			 loff_t *ppos)
+{
+	int group_id[UCLAMP_CNT] = { UCLAMP_NOT_VALID };
+	struct uclamp_se *uc_se;
+	int old_min, old_max;
+	unsigned int value;
+	int result;
+
+	mutex_lock(&uclamp_mutex);
+
+	old_min = sysctl_sched_uclamp_util_min;
+	old_max = sysctl_sched_uclamp_util_max;
+
+	result = proc_dointvec(table, write, buffer, lenp, ppos);
+	if (result)
+		goto undo;
+	if (!write)
+		goto done;
+
+	result = -EINVAL;
+	if (sysctl_sched_uclamp_util_min > sysctl_sched_uclamp_util_max)
+		goto undo;
+	if (sysctl_sched_uclamp_util_max > SCHED_CAPACITY_SCALE)
+		goto undo;
+
+	/* Find a valid group_id for each required clamp value */
+	if (old_min != sysctl_sched_uclamp_util_min) {
+		value = sysctl_sched_uclamp_util_min;
+		result = uclamp_group_find(UCLAMP_MIN, value);
+		if (result == -ENOSPC) {
+			pr_err(UCLAMP_ENOSPC_FMT, "MIN");
+			goto undo;
+		}
+		group_id[UCLAMP_MIN] = result;
+	}
+	if (old_max != sysctl_sched_uclamp_util_max) {
+		value = sysctl_sched_uclamp_util_max;
+		result = uclamp_group_find(UCLAMP_MAX, value);
+		if (result == -ENOSPC) {
+			pr_err(UCLAMP_ENOSPC_FMT, "MAX");
+			goto undo;
+		}
+		group_id[UCLAMP_MAX] = result;
+	}
+	result = 0;
+
+	/* Update each required clamp group */
+	if (old_min != sysctl_sched_uclamp_util_min) {
+		uc_se = &uclamp_default[UCLAMP_MIN];
+		uclamp_group_get(NULL, UCLAMP_MIN, group_id[UCLAMP_MIN],
+				 uc_se, sysctl_sched_uclamp_util_min);
+	}
+	if (old_max != sysctl_sched_uclamp_util_max) {
+		uc_se = &uclamp_default[UCLAMP_MAX];
+		uclamp_group_get(NULL, UCLAMP_MAX, group_id[UCLAMP_MAX],
+				 uc_se, sysctl_sched_uclamp_util_max);
+	}
+	goto done;
+
+undo:
+	sysctl_sched_uclamp_util_min = old_min;
+	sysctl_sched_uclamp_util_max = old_max;
+
+done:
+	mutex_unlock(&uclamp_mutex);
+
+	return result;
+}
+
 #ifdef CONFIG_UCLAMP_TASK_GROUP
 /**
  * alloc_uclamp_sched_group: initialize a new TG's for utilization clamping
@@ -1468,6 +1586,8 @@ static void uclamp_fork(struct task_struct *p, bool reset)
 		if (unlikely(reset)) {
 			next_group_id = 0;
 			p->uclamp[clamp_id].value = uclamp_none(clamp_id);
+			p->uclamp[clamp_id].effective.value =
+				uclamp_none(clamp_id);
 		}
 		/* Forked tasks are not yet enqueued */
 		p->uclamp[clamp_id].effective.group_id = UCLAMP_NOT_VALID;
@@ -1475,6 +1595,10 @@ static void uclamp_fork(struct task_struct *p, bool reset)
 		p->uclamp[clamp_id].group_id = UCLAMP_NOT_VALID;
 		uclamp_group_get(NULL, clamp_id, next_group_id, uc_se,
 				 p->uclamp[clamp_id].value);
+
+		/* By default we do not want task-specific clamp values */
+		if (unlikely(reset))
+			p->uclamp[clamp_id].value = UCLAMP_NOT_VALID;
 	}
 }
 
@@ -1509,12 +1633,17 @@ static void __init init_uclamp(void)
 		uc_se->group_id = UCLAMP_NOT_VALID;
 		uclamp_group_get(NULL, clamp_id, 0, uc_se,
 				 uclamp_none(clamp_id));
+		/*
+		 * By default we do not want task-specific clamp values,
+		 * so that system default values apply.
+		 */
+		uc_se->value = UCLAMP_NOT_VALID;
 
 #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->effective.value = uclamp_none(UCLAMP_MAX);
 		uc_se->effective.group_id = 0;
 
 		/*
@@ -1526,6 +1655,12 @@ static void __init init_uclamp(void)
 		uclamp_group_get(NULL, clamp_id, 0, uc_se,
 				 uclamp_none(UCLAMP_MAX));
 #endif
+
+		/* Init system defaul's clamp group */
+		uc_se = &uclamp_default[clamp_id];
+		uc_se->group_id = UCLAMP_NOT_VALID;
+		uclamp_group_get(NULL, clamp_id, 0, uc_se,
+				 uclamp_none(clamp_id));
 	}
 }
 
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index cc02050fd0c4..378ea57e5fc5 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -445,6 +445,22 @@ static struct ctl_table kern_table[] = {
 		.mode		= 0644,
 		.proc_handler	= sched_rr_handler,
 	},
+#ifdef CONFIG_UCLAMP_TASK
+	{
+		.procname	= "sched_uclamp_util_min",
+		.data		= &sysctl_sched_uclamp_util_min,
+		.maxlen		= sizeof(unsigned int),
+		.mode		= 0644,
+		.proc_handler	= sched_uclamp_handler,
+	},
+	{
+		.procname	= "sched_uclamp_util_max",
+		.data		= &sysctl_sched_uclamp_util_max,
+		.maxlen		= sizeof(unsigned int),
+		.mode		= 0644,
+		.proc_handler	= sched_uclamp_handler,
+	},
+#endif
 #ifdef CONFIG_SCHED_AUTOGROUP
 	{
 		.procname	= "sched_autogroup_enabled",
-- 
2.18.0