[RFCv3 1/8] sched/core: Add manual jitter classification using sched_setattr syscall

[Parth Shah <parth@linux.ibm.com>]

Jitter tasks are short/bursty tasks,typically performing some housekeeping
and are less important in the overall scheme of things. In this patch we
provide a mechanism based on Patrick Bellasi's UCLAMP framework to classify
jitter tasks"

We define jitter tasks as those whose util.max in the UCLAMP framework is
the least (=0). This also provides benefit of giving the least frequency to
those jitter tasks, which is useful if all jitters are packed onto a
separate core."

UCLAMP already provides a way to set util.max for a task by using a syscall
interface. This patch uses the `sched_setattr` syscall to set
sched_util_max attribute of the task which is used to classify the task as
jitter.

Use Case with turbo_bench.c
===================
```
i=8;
./turbo_bench -t 30 -h $i -n $((2*i)) -j
```
This spawns 2*i total threads: of which i-CPU bound and i-jitter threads.

Signed-off-by: Parth Shah <parth@linux.ibm.com>
---
 include/linux/sched.h | 6 ++++++
 kernel/sched/core.c   | 9 +++++++++
 2 files changed, 15 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index e2d80e6a187d..2bd9f75a3abb 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -696,6 +696,12 @@ struct task_struct {
 	struct uclamp_se		uclamp_req[UCLAMP_CNT];
 	/* Effective clamp values used for a scheduling entity */
 	struct uclamp_se		uclamp[UCLAMP_CNT];
+	/*
+	 * Tag the task as jitter.
+	 * 0 = regular. Follows regular CFS policy for task placement.
+	 * 1 = Jitter tasks. Should be packed to reduce active core count.
+	 */
+	unsigned int			is_jitter;
 #endif
 
 #ifdef CONFIG_PREEMPT_NOTIFIERS
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ab0aa319fe60..19c7204d6351 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1189,6 +1189,15 @@ static void __setscheduler_uclamp(struct task_struct *p,
 	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
 		uclamp_se_set(&p->uclamp_req[UCLAMP_MAX],
 			      attr->sched_util_max, true);
+
+		/*
+		 * Set task to jitter class if Max util is clamped to the least
+		 * possible value
+		 */
+		if (p->uclamp_req[UCLAMP_MAX].bucket_id == 0 && !p->is_jitter)
+			p->is_jitter = 1;
+		else if (p->is_jitter)
+			p->is_jitter = 0;
 	}
 }
 
-- 
2.17.1