sched/uclamp: Add system default clamps

Tasks without a user-defined clamp value are considered not clamped
and by default their utilization can have any value in the
[0..SCHED_CAPACITY_SCALE] range.

Tasks with a user-defined clamp value are allowed to request any value
in that range, and the required clamp is unconditionally enforced.
However, a "System Management Software" could be interested in limiting
the range of clamp values allowed for all tasks.

Add a privileged interface to define a system default configuration via:

  /proc/sys/kernel/sched_uclamp_util_{min,max}

which works as an unconditional clamp range restriction for all tasks.

With the default configuration, the full SCHED_CAPACITY_SCALE range of
values is allowed for each clamp index. Otherwise, the task-specific
clamp is capped by the corresponding system default value.

Do that by tracking, for each task, the "effective" clamp value and
bucket the task has been refcounted in at enqueue time. This
allows to lazy aggregate "requested" and "system default" values at
enqueue time and simplifies refcounting updates at dequeue time.

The cached bucket ids are used to avoid (relatively) more expensive
integer divisions every time a task is enqueued.

An active flag is used to report when the "effective" value is valid and
thus the task is actually refcounted in the corresponding rq's bucket.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alessio Balsini <balsini@android.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Perret <quentin.perret@arm.com>
Cc: Rafael J . Wysocki <rafael.j.wysocki@intel.com>
Cc: Steve Muckle <smuckle@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Todd Kjos <tkjos@google.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lkml.kernel.org/r/20190621084217.8167-5-patrick.bellasi@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 80235bcd05f2..5485f411e8e1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -586,14 +586,21 @@ struct sched_dl_entity {
  * Utilization clamp for a scheduling entity
  * @value:		clamp value "assigned" to a se
  * @bucket_id:		bucket index corresponding to the "assigned" value
+ * @active:		the se is currently refcounted in a rq's bucket
  *
  * The bucket_id is the index of the clamp bucket matching the clamp value
  * which is pre-computed and stored to avoid expensive integer divisions from
  * the fast path.
+ *
+ * The active bit is set whenever a task has got an "effective" value assigned,
+ * which can be different from the clamp value "requested" from user-space.
+ * This allows to know a task is refcounted in the rq's bucket corresponding
+ * to the "effective" bucket_id.
  */
 struct uclamp_se {
 	unsigned int value		: bits_per(SCHED_CAPACITY_SCALE);
 	unsigned int bucket_id		: bits_per(UCLAMP_BUCKETS);
+	unsigned int active		: 1;
 };
 #endif /* CONFIG_UCLAMP_TASK */
 
@@ -678,6 +685,9 @@ struct task_struct {
 	struct sched_dl_entity		dl;
 
 #ifdef CONFIG_UCLAMP_TASK
+	/* Clamp values requested for a scheduling entity */
+	struct uclamp_se		uclamp_req[UCLAMP_CNT];
+	/* Effective clamp values used for a scheduling entity */
 	struct uclamp_se		uclamp[UCLAMP_CNT];
 #endif