[PATCH v8 15/16] sched/core: uclamp: Use TG's clamps to restrict TASK's clamps

[Patrick Bellasi]

When a task specific clamp value is configured via sched_setattr(2),
this value is accounted in the corresponding clamp bucket every time the
task is {en,de}qeued. However, when cgroups are also in use, the task
specific clamp values could be restricted by the task_group (TG)
clamp values.

Update uclamp_cpu_inc() to aggregate task and TG clamp values. Every
time a task is enqueued, it's accounted in the clamp_bucket defining the
smaller clamp between the task specific value and its TG effective
value. This allows to:

1. ensure cgroup clamps are always used to restrict task specific
   requests, i.e. boosted only up to the effective granted value or
   clamped at least to a certain value

2. implement a "nice-like" policy, where tasks are still allowed to
   request less then what enforced by their current TG

This mimics what already happens for a task's CPU affinity mask when the
task is also in a cpuset, i.e. cgroup attributes are always used to
restrict per-task attributes.

Do this by exploiting the concept of "effective" clamp, which is already
used by a TG to track parent enforced restrictions.

Apply task group clamp restrictions only to tasks belonging to a child
group. While, for tasks in the root group or in an autogroup, only
system defaults are enforced.

Signed-off-by: Patrick Bellasi <patrick.bellasi@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
---
 kernel/sched/core.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index de8391bafe11..423d74ed47f6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -821,16 +821,42 @@ unsigned int uclamp_rq_max_value(struct rq *rq, unsigned int clamp_id,
 	return uclamp_idle_value(rq, clamp_id, clamp_value);
 }
 
+static inline struct uclamp_se
+uclamp_tg_restrict(struct task_struct *p, unsigned int clamp_id)
+{
+	struct uclamp_se uc_req = p->uclamp_req[clamp_id];
+#ifdef CONFIG_UCLAMP_TASK_GROUP
+	struct uclamp_se uc_max;
+
+	/*
+	 * Tasks in autogroups or root task group will be
+	 * restricted by system defaults.
+	 */
+	if (task_group_is_autogroup(task_group(p)))
+		return uc_req;
+	if (task_group(p) == &root_task_group)
+		return uc_req;
+
+	uc_max = task_group(p)->uclamp[clamp_id];
+	if (uc_req.value > uc_max.value || !uc_req.user_defined)
+		return uc_max;
+#endif
+
+	return uc_req;
+}
+
 /*
  * The effective clamp bucket index of a task depends on, by increasing
  * priority:
  * - the task specific clamp value, when explicitly requested from userspace
+ * - the task group effective clamp value, for tasks not either in the root
+ *   group or in an autogroup
  * - the system default clamp value, defined by the sysadmin
  */
 static inline struct uclamp_se
 uclamp_eff_get(struct task_struct *p, unsigned int clamp_id)
 {
-	struct uclamp_se uc_req = p->uclamp_req[clamp_id];
+	struct uclamp_se uc_req = uclamp_tg_restrict(p, clamp_id);
 	struct uclamp_se uc_max = uclamp_default[clamp_id];
 
 	/* System default restrictions always apply */
-- 
2.20.1