Utilization estimation (util_est) for FAIR tasks

Hi, here is an update of [1], based on today's tip/sched/core [2], which mainly adds some code cleanups suggested by Peter as well as fixes compilation for !CONFIG_SMP systems.

Most notably:
a) The util_est's update flag has been renamed into UTIL_AVG_UNCHANGED, which seems to better match its usages.
b) The cpu_util_est() function has been removed to reduce cluttering by folding its code directly into cpu_util(). This last function is thus now always returning the estimated utilization of a CPU, unless this sched feature is disabled.
c) Not necessary READ_ONCE() have been removed from rq-lock protected code paths. For util_est variable, that we read/modify/write only from rq-lock protected, code we keep just the WRITE_ONCE() barriers, which are still required for synchronization with lockless readers.
The READ_ONCE() have been instead maintained in all the getter functions, like for example task_util() and cpu_util(), which can potentially be used by lockless code. e.g. schedutil or load-balancer.

Results on both x86_64 and ARM (Android) targets, which have been collected and reported in previous postings [1,3], show negligible overheads, especially compared to the corresponding power/performance benefits on mobile platforms, where this feature helps to reduce the performance gap between PELT and another other out-of-tree load tracking solution.

Best,
Patrick

.:: Changelog
=============

Changes in v6:
- remove READ_ONCE from rq-lock protected code paths
- folds cpu_util_est code into cpu_util and update its documentation
- change util_est's update flag name to better match its usage
- slightly clean up cpu_util_wake code
- add other small code cleanups as suggested by Peter
- fix compilation for !CONFIG_SMP systems
- fix documentation to match sphinx syntax
- update changelogs to better match code concepts

Changes in v5:
- rebased on today's tip/sched/core (commit 083c6eeab2cc, based on v4.16-rc2)
- update util_est only on util_avg updates
- add documentation for "struct util_est"
- always use int instead of long whenever possible (Peter)
- pass cfs_rq to util_est_{en,de}queue (Peter)
- pass task_sleep to util_est_dequeue
- use singe WRITE_ONCE at dequeue time
- add some missing {READ,WRITE}_ONCE
- add task_util_est() for code consistency

Changes in v4:
- rebased on today's tip/sched/core (commit 460e8c3340a2)
- renamed util_est's "last" into "enqueued"
- using util_est's "enqueued" for both se and cfs_rqs (Joel)
- update margin check to use more ASM friendly code (Peter)
- optimize EWMA updates (Peter)
- ensure cpu_util_wake() is cpu_capacity_orig()'s clamped (Pavan)
- simplify cpu_util_cfs() integration (Dietmar)

Changes in v3:
- rebased on today's tip/sched/core (commit 07881166a892)
- moved util_est into sched_avg (Peter)
- use {READ,WRITE}_ONCE() for EWMA updates (Peter)
- using unsigned int to fit all sched_avg into a single 64B cache line
- schedutil integration using Juri's cpu_util_cfs()
- first patch dropped since it's already queued in tip/sched/core

Changes in v2:
- rebased on top of v4.15-rc2
- tested that overhauled PELT code does not affect the util_est

.:: References
==============
[1] https://lkml.org/lkml/2018/2/22/639

    [email protected]
[2] git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git
    sched/core (commit 083c6eeab2cc)
[3] https://lkml.org/lkml/2018/1/23/645
    [email protected]

Patrick Bellasi (4):
sched/fair: add util_est on top of PELT sched/fair: use util_est in LB and WU paths sched/cpufreq_schedutil: use util_est for OPP selection sched/fair: update util_est only on util_avg updates

 include/linux/sched.h   |  29 ++++++
 kernel/sched/debug.c    |   4 +
 kernel/sched/fair.c     | 237 ++++++++++++++++++++++++++++++++++++++++++++----
 kernel/sched/features.h |   5 +
 kernel/sched/sched.h    |   9 +-
 5 files changed, 264 insertions(+), 20 deletions(-)

2.15.1

The util_avg signal computed by PELT is too variable for some use-cases. For example, a big task waking up after a long sleep period will have its utilization almost completely decayed. This introduces some latency before schedutil will be able to pick the best frequency to run a task.

The same issue can affect task placement. Indeed, since the task utilization is already decayed at wakeup, when the task is enqueued in a CPU, this can result in a CPU running a big task as being temporarily represented as being almost empty. This leads to a race condition where other tasks can be potentially allocated on a CPU which just started to run a big task which slept for a relatively long period.

Moreover, the PELT utilization of a task can be updated every [ms], thus making it a continuously changing value for certain longer running tasks. This means that the instantaneous PELT utilization of a RUNNING task is not really meaningful to properly support scheduler decisions.

For all these reasons, a more stable signal can do a better job of representing the expected/estimated utilization of a task/cfs_rq. Such a signal can be easily created on top of PELT by still using it as an estimator which produces values to be aggregated on meaningful events.

This patch adds a simple implementation of util_est, a new signal built on top of PELT's util_avg where:

    util_est(task) = max(task::util_avg, f(task::util_avg@dequeue))

This allows to remember how big a task has been reported by PELT in its previous activations via f(task::util_avg@dequeue), which is the new _task_util_est(struct task_struct*) function added by this patch.

If a task should change its behavior and it runs longer in a new activation, after a certain time its util_est will just track the original PELT signal (i.e. task::util_avg).

The estimated utilization of cfs_rq is defined only for root ones. That's because the only sensible consumer of this signal are the scheduler and schedutil when looking for the overall CPU utilization due to FAIR tasks.
For this reason, the estimated utilization of a root cfs_rq is simply defined as:

    util_est(cfs_rq) = max(cfs_rq::util_avg, cfs_rq::util_est::enqueued)

where:

cfs_rq::util_est::enqueued = sum(_task_util_est(task))
for each RUNNABLE task on that root cfs_rq

It's worth to note that the estimated utilization is tracked only for objects of interests, specifically:
- Tasks: to better support tasks placement decisions
- root cfs_rqs: to better support both tasks placement decisions as well as frequencies selection

---
Changes in v6:
- remove READ_ONCE from rq-lock protected code paths
- fix compilation for !CONFIG_SMP systems
- fix documentation to match sphinx syntax
- update changelog to better match code concepts

Changes in v5:
- add documentation for "struct util_est"
- always use int instead of long whenever possible (Peter)
- pass cfs_rq to util_est_{en,de}queue (Peter)
- pass task_sleep to util_est_dequeue
- use singe WRITE_ONCE at dequeue time
- add some other missing {READ,WRITE}_ONCE
- add task_util_est() for code consistency

Changes in v4:
- rebased on today's tip/sched/core (commit 460e8c3340a2)
- renamed util_est's "last" into "enqueued"
- using util_est's "enqueued" for both se and cfs_rqs (Joel)
- update margin check to use more ASM friendly code (Peter)
- optimize EWMA updates (Peter)

Changes in v3:
- rebased on today's tip/sched/core (commit 07881166a892)
- moved util_est into sched_avg (Peter)
- use {READ,WRITE}_ONCE() for EWMA updates (Peter)
- using unsigned int to fit all sched_avg into a single 64B cache line

Changes in v2:
- rebase on top of v4.15-rc2
- tested that overhauled PELT code does not affect the util_est

---
 include/linux/sched.h   |  29 ++++++++++++
 kernel/sched/debug.c    |   4 ++
 kernel/sched/fair.c     | 121 +++++++++++++++++++++++++++++++++++++++++++++---
 kernel/sched/features.h |   5 ++
 4 files changed, 153 insertions(+), 6 deletions(-)

index b161ef8a902e..82c42e0b5dd0 100644
@@ -275,6 +275,34 @@ struct load_weight {
 	u32				inv_weight;
 };
 
+/**
+ * struct util_est - Estimation utilization of FAIR tasks
+ * @enqueued: instantaneous estimated utilization of a task/cpu
+ * @ewma:     the Exponential Weighted Moving Average (EWMA)
+ *            utilization of a task
+ *
+ * Support data structure to track an Exponential Weighted Moving Average
+ * (EWMA) of a FAIR task's utilization. New samples are added to the moving
+ * average each time a task completes an activation. Sample's weight is chosen
+ * so that the EWMA will be relatively insensitive to transient changes to the
+ * task's workload.
+ *
+ * The enqueued attribute has a slightly different meaning for tasks and cpus:
+ * - task:   the task's util_avg at last task dequeue time
+ * - cfs_rq: the sum of util_est.enqueued for each RUNNABLE task on that CPU
+ * Thus, the util_est.enqueued of a task represents the contribution on the
+ * estimated utilization of the CPU where that task is currently enqueued.
+ *
+ * Only for tasks we track a moving average of the past instantaneous
+ * estimated utilization. This allows to absorb sporadic drops in utilization
+ * of an otherwise almost periodic task.
+ */
+struct util_est {
+	unsigned int			enqueued;
+	unsigned int			ewma;
+#define UTIL_EST_WEIGHT_SHIFT		2
+};
+
 /*
  * The load_avg/util_avg accumulates an infinite geometric series
  * (see __update_load_avg() in kernel/sched/fair.c).

@@ -336,6 +364,7 @@ struct sched_avg {
 	unsigned long			load_avg;
 	unsigned long			runnable_load_avg;
 	unsigned long			util_avg;
+	struct util_est			util_est;
 };
 
 struct sched_statistics {

index 1ca0130ed4f9..d4eb5532ea6b 100644
@@ -567,6 +567,8 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
 			cfs_rq->avg.runnable_load_avg);
 	SEQ_printf(m, "  .%-30s: %lu\n", "util_avg",
 			cfs_rq->avg.util_avg);
+	SEQ_printf(m, "  .%-30s: %u\n", "util_est_enqueued",
+			cfs_rq->avg.util_est.enqueued);
 	SEQ_printf(m, "  .%-30s: %ld\n", "removed.load_avg",
 			cfs_rq->removed.load_avg);
 	SEQ_printf(m, "  .%-30s: %ld\n", "removed.util_avg",

@@ -1018,6 +1020,8 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
 	P(se.avg.runnable_load_avg);
 	P(se.avg.util_avg);
 	P(se.avg.last_update_time);
+	P(se.avg.util_est.ewma);
+	P(se.avg.util_est.enqueued);
 #endif
 	P(policy);
 	P(prio);

index 9552fd5854bf..c459a4b61544 100644
@@ -85,3 +85,8 @@ SCHED_FEAT(ATTACH_AGE_LOAD, true)
 SCHED_FEAT(WA_IDLE, true)
 SCHED_FEAT(WA_WEIGHT, true)
 SCHED_FEAT(WA_BIAS, true)
+
+/*
+ * UtilEstimation. Use estimated CPU utilization.
+ */
+SCHED_FEAT(UTIL_EST, false)

2.15.1

When the scheduler looks at the CPU utilization, the current PELT value for a CPU is returned straight away. In certain scenarios this can have undesired side effects on task placement.

For example, since the task utilization is decayed at wakeup time, when a long sleeping big task is enqueued it does not add immediately a significant contribution to the target CPU.
As a result we generate a race condition where other tasks can be placed on the same CPU while it is still considered relatively empty.

In order to reduce this kind of race conditions, this patch introduces the required support to integrate the usage of the CPU's estimated utilization in the wakeup path, via cpu_util_wake(), as well as in the load-balance path, via cpu_util() which is used by update_sg_lb_stats().

The estimated utilization of a CPU is defined to be the maximum between its PELT's utilization and the sum of the estimated utilization (at previous dequeue time) of all the tasks currently RUNNABLE on that CPU. This allows to properly represent the spare capacity of a CPU which, for example, has just got a big task running since a long sleep period.

---
Changes in v6:
- folded cpu_util_est code into cpu_util
- updated cpu_util documentation
- slightly cleaned up cpu_util_wake code
- update changelog to better match code concepts

Changes in v5:
- always use int instead of long whenever possible (Peter)
- add missing READ_ONCE barriers (Peter)

Changes in v4:
- rebased on today's tip/sched/core (commit 460e8c3340a2)
- ensure cpu_util_wake() is cpu_capacity_orig()'s clamped (Pavan)

Changes in v3:
- rebased on today's tip/sched/core (commit 07881166a892)

Changes in v2:
- rebase on top of v4.15-rc2
- tested that overhauled PELT code does not affect the util_est

---
 kernel/sched/fair.c | 84 ++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 70 insertions(+), 14 deletions(-)

index c54560829a52..5cf4aa39a6ca 100644
@@ -6431,11 +6431,13 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	return target;
 }
 
-/*
- * cpu_util returns the amount of capacity of a CPU that is used by CFS
- * tasks. The unit of the return value must be the one of capacity so we can
- * compare the utilization with the capacity of the CPU that is available for
- * CFS task (ie cpu_capacity).
+/**
+ * Amount of capacity of a CPU that is (estimated to be) used by CFS tasks
+ * @cpu: the CPU to get the utilization of
+ *
+ * The unit of the return value must be the one of capacity so we can compare
+ * the utilization with the capacity of the CPU that is available for CFS task
+ * (ie cpu_capacity).
  *
  * cfs_rq.avg.util_avg is the sum of running time of runnable tasks plus the
  * recent utilization of currently non-runnable tasks on a CPU. It represents

@@ -6446,6 +6448,14 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
  * current capacity (capacity_curr <= capacity_orig) of the CPU because it is
  * the running time on this CPU scaled by capacity_curr.
  *
+ * The estimated utilization of a CPU is defined to be the maximum between its
+ * cfs_rq.avg.util_avg and the sum of the estimated utilization of the tasks
+ * currently RUNNABLE on that CPU.
+ * This allows to properly represent the expected utilization of a CPU which
+ * has just got a big task running since a long sleep period. At the same time
+ * however it preserves the benefits of the "blocked utilization" in
+ * describing the potential for other tasks waking up on the same CPU.
+ *
  * Nevertheless, cfs_rq.avg.util_avg can be higher than capacity_curr or even
  * higher than capacity_orig because of unfortunate rounding in
  * cfs.avg.util_avg or just after migrating tasks and new task wakeups until

@@ -6456,13 +6466,21 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
  * available capacity. We allow utilization to overshoot capacity_curr (but not
  * capacity_orig) as it useful for predicting the capacity required after task
  * migrations (scheduler-driven DVFS).
+ *
+ * Return: the (estimated) utilization for the specified CPU
  */
-static unsigned long cpu_util(int cpu)
+static inline unsigned long cpu_util(int cpu)
 {
-	unsigned long util = cpu_rq(cpu)->cfs.avg.util_avg;
-	unsigned long capacity = capacity_orig_of(cpu);
+	struct cfs_rq *cfs_rq;
+	unsigned int util;
+
+	cfs_rq = &cpu_rq(cpu)->cfs;
+	util = READ_ONCE(cfs_rq->avg.util_avg);
+
+	if (sched_feat(UTIL_EST))
+		util = max(util, READ_ONCE(cfs_rq->avg.util_est.enqueued));
 
-	return (util >= capacity) ? capacity : util;
+	return min_t(unsigned long, util, capacity_orig_of(cpu));
 }
 
 /*

@@ -6471,16 +6489,54 @@ static unsigned long cpu_util(int cpu)
  */
 static unsigned long cpu_util_wake(int cpu, struct task_struct *p)
 {
-	unsigned long util, capacity;
+	struct cfs_rq *cfs_rq;
+	unsigned int util;
 
 	/* Task has no contribution or is new */
-	if (cpu != task_cpu(p) || !p->se.avg.last_update_time)
+	if (cpu != task_cpu(p) || !READ_ONCE(p->se.avg.last_update_time))
 		return cpu_util(cpu);
 
-	capacity = capacity_orig_of(cpu);
-	util = max_t(long, cpu_rq(cpu)->cfs.avg.util_avg - task_util(p), 0);
+	cfs_rq = &cpu_rq(cpu)->cfs;
+	util = READ_ONCE(cfs_rq->avg.util_avg);
+
+	/* Discount task's blocked util from CPU's util */
+	util -= min_t(unsigned int, util, task_util(p));
 
-	return (util >= capacity) ? capacity : util;
+	/*
+	 * Covered cases:
+	 *
+	 * a) if *p is the only task sleeping on this CPU, then:
+	 *      cpu_util (== task_util) > util_est (== 0)
+	 *    and thus we return:
+	 *      cpu_util_wake = (cpu_util - task_util) = 0
+	 *
+	 * b) if other tasks are SLEEPING on this CPU, which is now exiting
+	 *    IDLE, then:
+	 *      cpu_util >= task_util
+	 *      cpu_util > util_est (== 0)
+	 *    and thus we discount *p's blocked utilization to return:
+	 *      cpu_util_wake = (cpu_util - task_util) >= 0
+	 *
+	 * c) if other tasks are RUNNABLE on that CPU and
+	 *      util_est > cpu_util
+	 *    then we use util_est since it returns a more restrictive
+	 *    estimation of the spare capacity on that CPU, by just
+	 *    considering the expected utilization of tasks already
+	 *    runnable on that CPU.
+	 *
+	 * Cases a) and b) are covered by the above code, while case c) is
+	 * covered by the following code when estimated utilization is
+	 * enabled.
+	 */
+	if (sched_feat(UTIL_EST))
+		util = max(util, READ_ONCE(cfs_rq->avg.util_est.enqueued));
+
+	/*
+	 * Utilization (estimated) can exceed the CPU capacity, thus let's
+	 * clamp to the maximum CPU capacity to ensure consistency with
+	 * the cpu_util call.
+	 */
+	return min_t(unsigned long, util, capacity_orig_of(cpu));
 }
 
 /*

2.15.1

When schedutil looks at the CPU utilization, the current PELT value for that CPU is returned straight away. In certain scenarios this can have undesired side effects and delays on frequency selection.

For example, since the task utilization is decayed at wakeup time, a long sleeping big task newly enqueued does not add immediately a significant contribution to the target CPU. This introduces some latency before schedutil will be able to detect the best frequency required by that task.

Moreover, the PELT signal build-up time is a function of the current frequency, because of the scale invariant load tracking support. Thus, starting from a lower frequency, the utilization build-up time will increase even more and further delays the selection of the actual frequency which better serves the task requirements.

In order to reduce this kind of latencies, we integrate the usage of the CPU's estimated utilization in the sugov_get_util function. This allows to properly consider the expected utilization of a CPU which, for example, has just got a big task running after a long sleep period. Ultimately this allows to select the best frequency to run a task right after its wake-up.

---
Changes in v6:
- small code cleanup suggested by Peter
- add acked-by Viresh tag

Changes in v5:
- add missing READ_ONCE() barrieres
- add acked-by Rafael tag

Changes in v4:
- rebased on today's tip/sched/core (commit 460e8c3340a2)
- use util_est.enqueued for cfs_rq's util_est (Joel)
- simplify cpu_util_cfs() integration (Dietmar)

Changes in v3:
- rebase on today's tip/sched/core (commit 07881166a892)
- moved into Juri's cpu_util_cfs(), which should also address Rafael's suggestion to use a local variable.

Changes in v2:
- rebase on top of v4.15-rc2
- tested that overhauled PELT code does not affect the util_est

---
 kernel/sched/sched.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

index dc6c8b5a24ad..5c459479593b 100644
@@ -2122,7 +2122,14 @@ static inline unsigned long cpu_util_dl(struct rq *rq)
 
 static inline unsigned long cpu_util_cfs(struct rq *rq)
 {
-	return rq->cfs.avg.util_avg;
+	unsigned long util = READ_ONCE(rq->cfs.avg.util_avg);
+
+	if (sched_feat(UTIL_EST)) {
+		util = max_t(unsigned long, util,
+			     READ_ONCE(rq->cfs.avg.util_est.enqueued));
+	}
+
+	return util;
 }
 
 #endif

2.15.1

The estimated utilization of a task is currently updated every time the task is dequeued. However, to keep overheads under control, PELT signals are effectively updated at maximum once every 1ms.

Thus, for really short running tasks, it can happen that their util_avg value has not been updates since their last enqueue. If such tasks are also frequently running tasks (e.g. the kind of workload generated by hackbench) it can also happen that their util_avg is updated only every few activations.

This means that updating util_est at every dequeue potentially introduces not necessary overheads and it's also conceptually wrong if the util_avg signal has never been updated during a task activation.

Let's introduce a throttling mechanism on task's util_est updates to sync them with util_avg updates. To make the solution memory efficient, both in terms of space and load/store operations, we encode a synchronization flag into the LSB of util_est.enqueued.
This makes util_est an even values only metric, which is still considered good enough for its purpose.
The synchronization bit is (re)set by __update_load_avg_se() once the PELT signal of a task has been updated during its last activation.

Such a throttling mechanism allows to keep under control util_est overheads in the wakeup hot path, thus making it a suitable mechanism which can be enabled also on high-intensity workload systems. Thus, this now switches on by default the estimation utilization scheduler feature.

---
Changes in v6:
- remove READ_ONCE from rq-lock protected code paths
- change flag name to better match its meaning
- fix compilation for !CONFIG_SMP systems
- add missing CCs in the changelog

Changes in v5:
- set SCHED_FEAT(UTIL_EST, true) as default (Peter)

---
 kernel/sched/fair.c     | 42 ++++++++++++++++++++++++++++++++++++++----
 kernel/sched/features.h |  2 +-
 2 files changed, 39 insertions(+), 5 deletions(-)

index 5cf4aa39a6ca..a52868b07850 100644
@@ -3257,6 +3257,32 @@ ___update_load_avg(struct sched_avg *sa, unsigned long load, unsigned long runna
 	sa->util_avg = sa->util_sum / divider;
 }
 
+/*
+ * When a task is dequeued, its estimated utilization should not be update if
+ * its util_avg has not been updated at least once.
+ * This flag is used to synchronize util_avg updates with util_est updates.
+ * We map this information into the LSB bit of the utilization saved at
+ * dequeue time (i.e. util_est.dequeued).
+ */
+#define UTIL_AVG_UNCHANGED 0x1
+
+static inline void cfs_se_util_change(struct sched_avg *avg)
+{
+	unsigned int enqueued;
+
+	if (!sched_feat(UTIL_EST))
+		return;
+
+	/* Avoid store if the flag has been already set */
+	enqueued = avg->util_est.enqueued;
+	if (!(enqueued & UTIL_AVG_UNCHANGED))
+		return;
+
+	/* Reset flag to report util_avg has been updated */
+	enqueued &= ~UTIL_AVG_UNCHANGED;
+	WRITE_ONCE(avg->util_est.enqueued, enqueued);
+}
+
 /*
  * sched_entity:
  *

@@ -3308,6 +3334,7 @@ __update_load_avg_se(u64 now, int cpu, struct cfs_rq *cfs_rq, struct sched_entit
 				cfs_rq->curr == se)) {
 
 		___update_load_avg(&se->avg, se_weight(se), se_runnable(se));
+		cfs_se_util_change(&se->avg);
 		return 1;
 	}
 

@@ -3908,7 +3935,7 @@ static inline void util_est_enqueue(struct cfs_rq *cfs_rq,
 
 	/* Update root cfs_rq's estimated utilization */
 	enqueued  = cfs_rq->avg.util_est.enqueued;
-	enqueued += _task_util_est(p);
+	enqueued += (_task_util_est(p) | UTIL_AVG_UNCHANGED);
 	WRITE_ONCE(cfs_rq->avg.util_est.enqueued, enqueued);
 }
 

@@ -3943,7 +3970,7 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
 	if (cfs_rq->nr_running) {
 		ue.enqueued  = cfs_rq->avg.util_est.enqueued;
 		ue.enqueued -= min_t(unsigned int, ue.enqueued,
-				     _task_util_est(p));
+				     (_task_util_est(p) | UTIL_AVG_UNCHANGED));
 	}
 	WRITE_ONCE(cfs_rq->avg.util_est.enqueued, ue.enqueued);
 

@@ -3954,12 +3981,19 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
 	if (!task_sleep)
 		return;
 
+	/*
+	 * If the PELT values haven't changed since enqueue time,
+	 * skip the util_est update.
+	 */
+	ue = p->se.avg.util_est;
+	if (ue.enqueued & UTIL_AVG_UNCHANGED)
+		return;
+
 	/*
 	 * Skip update of task's estimated utilization when its EWMA is
 	 * already ~1% close to its last activation value.
 	 */
-	ue = p->se.avg.util_est;
-	ue.enqueued = task_util(p);
+	ue.enqueued = (task_util(p) | UTIL_AVG_UNCHANGED);
 	last_ewma_diff = ue.enqueued - ue.ewma;
 	if (within_margin(last_ewma_diff, (SCHED_CAPACITY_SCALE / 100)))
 		return;

index c459a4b61544..85ae8488039c 100644
@@ -89,4 +89,4 @@ SCHED_FEAT(WA_BIAS, true)
 /*
  * UtilEstimation. Use estimated CPU utilization.
  */
-SCHED_FEAT(UTIL_EST, false)
+SCHED_FEAT(UTIL_EST, true)

2.15.1

Thanks Patrick!