[PATCH v3 1/2] sched/fair: Introduce short duration task check

[Chen Yu]

Introduce short-duration task checks, as there is requirement
to leverage this attribute for better task placement.

There are several choices of metrics that could be used to
indicate if a task is a short-duration task.

At first thought the (p->se.sum_exec_runtime / p->nvcsw)
could be used to measure the task duration. However, the
history long past was factored too heavily in such a formula.
Ideally, the old activity should decay and not affect
the current status too much.

Although something based on PELT could be used, se.util_avg might
not be appropriate to describe the task duration:
1. Task p1 and task p2 are doing frequent ping-pong scheduling on
   one CPU, both p1 and p2 have a short duration, but the util_avg
   can be up to 50%.
2. Suppose a task lasting less than 4ms is regarded as a short task.
   If task p3 runs for 6ms and sleeps for 32ms, p3 should not be a
   short-duration task. However, PELT would decay p3's accumulated
   running time from 6ms to 3ms, because 32ms is the half-life in PELT.
   As a result, p3 would be incorrectly treated as a short task.

It was found that there was once a similar feature to track the
duration of a task, which is in Commit ad4b78bbcbab ("sched: Add
new wakeup preemption mode: WAKEUP_RUNNING"). Unfortunately, it
was reverted because it was an experiment. So pick the patch up
again, by recording the average duration when a task voluntarily
switches out. Introduce SIS_SHORT to control this strategy.

The threshold of short duration reuses sysctl_sched_min_granularity,
so it can be tuned by the user. Ideally there should be a dedicated
parameter for the threshold, but that might introduce complexity.

Suggested-by: Tim Chen <tim.c.chen@xxxxxxxxx>
Suggested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
 include/linux/sched.h   |  4 ++++
 kernel/sched/core.c     |  2 ++
 kernel/sched/fair.c     | 17 +++++++++++++++++
 kernel/sched/features.h |  1 +
 4 files changed, 24 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index ffb6eb55cd13..64b7acb77a11 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -558,6 +558,10 @@ struct sched_entity {
 
 	u64				nr_migrations;
 
+	u64				prev_sum_exec_runtime_vol;
+	/* average duration of a task */
+	u64				dur_avg;
+
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	int				depth;
 	struct sched_entity		*parent;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index daff72f00385..c5202f1be3f7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4348,6 +4348,8 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 	p->se.prev_sum_exec_runtime	= 0;
 	p->se.nr_migrations		= 0;
 	p->se.vruntime			= 0;
+	p->se.dur_avg			= 0;
+	p->se.prev_sum_exec_runtime_vol	= 0;
 	INIT_LIST_HEAD(&p->se.group_node);
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e4a0b8bd941c..a4b314b664f8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6200,6 +6200,16 @@ static int wake_wide(struct task_struct *p)
 	return 1;
 }
 
+/*
+ * If a task switches in and then voluntarily relinquishes the
+ * CPU quickly, it is regarded as a short duration task.
+ */
+static inline int is_short_task(struct task_struct *p)
+{
+	return sched_feat(SIS_SHORT) &&
+		(p->se.dur_avg <= sysctl_sched_min_granularity);
+}
+
 /*
  * The purpose of wake_affine() is to quickly determine on which CPU we can run
  * soonest. For the purpose of speed we only consider the waking and previous
@@ -7680,6 +7690,13 @@ static void put_prev_task_fair(struct rq *rq, struct task_struct *prev)
 	struct sched_entity *se = &prev->se;
 	struct cfs_rq *cfs_rq;
 
+	if (sched_feat(SIS_SHORT) && !prev->on_rq) {
+		u64 this_dur = se->sum_exec_runtime - se->prev_sum_exec_runtime_vol;
+
+		se->prev_sum_exec_runtime_vol = se->sum_exec_runtime;
+		update_avg(&se->dur_avg, this_dur);
+	}
+
 	for_each_sched_entity(se) {
 		cfs_rq = cfs_rq_of(se);
 		put_prev_entity(cfs_rq, se);
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index ee7f23c76bd3..efdc29c42161 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -62,6 +62,7 @@ SCHED_FEAT(TTWU_QUEUE, true)
  */
 SCHED_FEAT(SIS_PROP, false)
 SCHED_FEAT(SIS_UTIL, true)
+SCHED_FEAT(SIS_SHORT, true)
 
 /*
  * Issue a WARN when we do multiple update_rq_clock() calls
-- 
2.25.1