[PATCH v4 06/16] sched/cpufreq: uclamp: add utilization clamping for FAIR tasks

[Patrick Bellasi]

Each time a frequency update is required via schedutil, a frequency is
selected to (possibly) satisfy the utilization reported by the CFS
class. However, when utilization clamping is in use, the frequency
selection should consider the requirements suggested by userspace, for
example, to:

 - boost tasks which are directly affecting the user experience
   by running them at least at a minimum "required" frequency

 - cap low priority tasks not directly affecting the user experience
   by running them only up to a maximum "allowed" frequency

These constraints are meant to support a per-task based tuning of the
frequency selection thus allowing to have a fine grained definition of
performance boosting vs energy saving strategies in kernel space.

Let's add the required support to clamp the utilization generated by
FAIR tasks within the boundaries defined by their aggregated utilization
clamp constraints.
On each CPU the aggregated clamp values are obtained by considering the
maximum of the {min,max}_util values for each task. This max aggregation
responds to the goal of not penalizing, for example, high boosted (i.e.
more important for the user-experience) CFS tasks which happens to be
co-scheduled with high capped (i.e. less important for the
user-experience) CFS tasks.

For FAIR tasks both the utilization as well as the IOWait boost values
are clamped according to the CPU aggregated utilization clamp
constraints.

The default values for boosting and capping are defined to be:
 - util_min: 0
 - util_max: SCHED_CAPACITY_SCALE
which means that by default no boosting/capping is enforced on FAIR
tasks, and thus the frequency will be selected considering the actual
utilization value of each CPU.

Signed-off-by: Patrick Bellasi <patrick.bellasi@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Cc: Viresh Kumar <viresh.kumar@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: <CAKfTPtC2adLupg7wy1JU9zxKx1466Sza6fSCcr92wcawm1OYkg@xxxxxxxxxxxxxx>
 - use *rq instead of cpu for both uclamp_util() and uclamp_value()
 Message-ID: <20180816135300.GC2960@e110439-lin>
 - remove uclamp_value() which is never used outside CONFIG_UCLAMP_TASK
 Others:
 - rebased on v4.19-rc1

Changes in v3:
 Message-ID: <CAJuCfpF6=L=0LrmNnJrTNPazT4dWKqNv+thhN0dwpKCgUzs9sg@xxxxxxxxxxxxxx>
 - rename UCLAMP_NONE into UCLAMP_NOT_VALID
 Others:
 - rebased on tip/sched/core
Changes in v2:
 - rebased on v4.18-rc4
---
 kernel/sched/cpufreq_schedutil.c | 23 +++++++++++++--
 kernel/sched/sched.h             | 50 ++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 3fffad3bc8a8..949082555ee8 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -222,8 +222,13 @@ static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu)
 	 * CFS tasks and we use the same metric to track the effective
 	 * utilization (PELT windows are synchronized) we can directly add them
 	 * to obtain the CPU's actual utilization.
+	 *
+	 * CFS utilization can be boosted or capped, depending on utilization
+	 * clamp constraints configured for currently RUNNABLE tasks.
 	 */
 	util = cpu_util_cfs(rq);
+	if (util)
+		util = uclamp_util(rq, util);
 	util += cpu_util_rt(rq);
 
 	/*
@@ -307,6 +312,7 @@ static void sugov_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
 			       unsigned int flags)
 {
 	bool set_iowait_boost = flags & SCHED_CPUFREQ_IOWAIT;
+	unsigned int max_boost;
 
 	/* Reset boost if the CPU appears to have been idle enough */
 	if (sg_cpu->iowait_boost &&
@@ -322,11 +328,24 @@ static void sugov_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
 		return;
 	sg_cpu->iowait_boost_pending = true;
 
+	/*
+	 * Boost FAIR tasks only up to the CPU clamped utilization.
+	 *
+	 * Since DL tasks have a much more advanced bandwidth control, it's
+	 * safe to assume that IO boost does not apply to those tasks.
+	 * Instead, since RT tasks are not utiliation clamped, we don't want
+	 * to apply clamping on IO boost while there is blocked RT
+	 * utilization.
+	 */
+	max_boost = sg_cpu->iowait_boost_max;
+	if (!cpu_util_rt(cpu_rq(sg_cpu->cpu)))
+		max_boost = uclamp_util(cpu_rq(sg_cpu->cpu), max_boost);
+
 	/* Double the boost at each request */
 	if (sg_cpu->iowait_boost) {
 		sg_cpu->iowait_boost <<= 1;
-		if (sg_cpu->iowait_boost > sg_cpu->iowait_boost_max)
-			sg_cpu->iowait_boost = sg_cpu->iowait_boost_max;
+		if (sg_cpu->iowait_boost > max_boost)
+			sg_cpu->iowait_boost = max_boost;
 		return;
 	}
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 411635c4c09a..1b05b38b1081 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2293,6 +2293,56 @@ static inline unsigned int uclamp_none(int clamp_id)
 	return SCHED_CAPACITY_SCALE;
 }
 
+#ifdef CONFIG_UCLAMP_TASK
+/**
+ * uclamp_value: get the current CPU's utilization clamp value
+ * @rq: the CPU's RQ to consider
+ * @clamp_id: the utilization clamp index (i.e. min or max utilization)
+ *
+ * The utilization clamp value for a CPU depends on its set of currently
+ * RUNNABLE tasks and their specific util_{min,max} constraints.
+ * A max aggregated value is tracked for each CPU and returned by this
+ * function.
+ *
+ * Return: the current value for the specified CPU and clamp index
+ */
+static inline unsigned int uclamp_value(struct rq *rq, int clamp_id)
+{
+	struct uclamp_cpu *uc_cpu = &rq->uclamp;
+
+	if (uc_cpu->value[clamp_id] == UCLAMP_NOT_VALID)
+		return uclamp_none(clamp_id);
+
+	return uc_cpu->value[clamp_id];
+}
+
+/**
+ * clamp_util: clamp a utilization value for a specified CPU
+ * @rq: the CPU's RQ to get the clamp values from
+ * @util: the utilization signal to clamp
+ *
+ * Each CPU tracks util_{min,max} clamp values depending on the set of its
+ * currently RUNNABLE tasks. Given a utilization signal, i.e a signal in
+ * the [0..SCHED_CAPACITY_SCALE] range, this function returns a clamped
+ * utilization signal considering the current clamp values for the
+ * specified CPU.
+ *
+ * Return: a clamped utilization signal for a given CPU.
+ */
+static inline unsigned int uclamp_util(struct rq *rq, unsigned int util)
+{
+	unsigned int min_util = uclamp_value(rq, UCLAMP_MIN);
+	unsigned int max_util = uclamp_value(rq, UCLAMP_MAX);
+
+	return clamp(util, min_util, max_util);
+}
+#else /* CONFIG_UCLAMP_TASK */
+static inline unsigned int uclamp_util(struct rq *rq, unsigned int util)
+{
+	return util;
+}
+#endif /* CONFIG_UCLAMP_TASK */
+
 #ifdef arch_scale_freq_capacity
 # ifndef arch_scale_freq_invariant
 #  define arch_scale_freq_invariant()	true
-- 
2.18.0