[RFCv2 1/6] sched/core: Add manual jitter classification from cgroup interface

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

Jitter tasks are usually of less important in terms of performance
and are short/bursty in characteristics. TurboSched uses this jitter
classification to pack jitters into the already running busy cores to
keep the total idle core count high.

The patch describes the use of UCLAMP mechanism to classify tasks. Patrick
Bellasi came up with a mechanism to classify tasks from the userspace
https://lore.kernel.org/lkml/20190402104153.25404-1-patrick.bellasi@arm.com/

This UCLAMP mechanism can be useful in classifying tasks as jitter. Jitters
can be classified for the cgroup by keeping util.max of the tasks as 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.

Use Case with UCLAMP
===================
To create a cgroup with all the tasks classified as jitters;

```
mkdir -p /sys/fs/cgroup/cpu/jitter
echo 0 > /proc/sys/kernel/sched_uclamp_util_min;
echo 0 > /sys/fs/cgroup/cpu/jitter/cpu.util.min;
echo 0 > /sys/fs/cgroup/cpu/jitter/cpu.util.max;
i=8;
./turbo_bench -t 30 -h $i -n $i &
./turbo_bench -t 30 -h 0 -n $i &
echo $! > /sys/fs/cgroup/cpu/jitter/cgroup.procs;
```

Signed-off-by: Parth Shah <parth@linux.ibm.com>
---
 kernel/sched/core.c  | 9 +++++++++
 kernel/sched/sched.h | 1 +
 2 files changed, 10 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d42c0f5eefa9..77aa4aee4478 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7192,6 +7192,15 @@ static int cpu_util_max_write_u64(struct cgroup_subsys_state *css,
 	tg->uclamp_req[UCLAMP_MAX].value = max_value;
 	tg->uclamp_req[UCLAMP_MAX].bucket_id = uclamp_bucket_id(max_value);
 
+	/*
+	 * Classify the tasks belonging to the last bucket of MAX UCLAMP as
+	 * jitters
+	 */
+	if (uclamp_bucket_id(max_value) == 0)
+		tg->turbo_sched_enabled = 1;
+	else if (tg->turbo_sched_enabled)
+		tg->turbo_sched_enabled = 0;
+
 	/* Update effective clamps to track the most restrictive value */
 	cpu_util_update_eff(css, UCLAMP_MAX);
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index b4019012d84b..e75ffaf3ff34 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -407,6 +407,7 @@ struct task_group {
 	struct uclamp_se	uclamp[UCLAMP_CNT];
 #endif
 
+	bool			turbo_sched_enabled;
 };
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
-- 
2.17.1