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Subject[PATCH v7 14/15] sched/core: uclamp: Use TG's clamps to restrict TASK's clamps
Date
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@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
---
kernel/sched/core.c | 42 +++++++++++++++++++++++++++++++++++++++++-
1 file changed, 41 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 35e9f06af08d..6f8f68d18d0f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -823,10 +823,44 @@ static inline void uclamp_rq_update(struct rq *rq, unsigned int clamp_id,
WRITE_ONCE(rq->uclamp[clamp_id].value, max_value);
}

+static inline bool
+uclamp_tg_restricted(struct task_struct *p, unsigned int clamp_id,
+ unsigned int *clamp_value, unsigned int *bucket_id)
+{
+#ifdef CONFIG_UCLAMP_TASK_GROUP
+ unsigned int clamp_max, bucket_max;
+ struct uclamp_se *tg_clamp;
+
+ /*
+ * Tasks in an autogroup or the root task group are restricted by
+ * system defaults.
+ */
+ if (task_group_is_autogroup(task_group(p)))
+ return false;
+ if (task_group(p) == &root_task_group)
+ return false;
+
+ tg_clamp = &task_group(p)->uclamp[clamp_id];
+ bucket_max = tg_clamp->effective.bucket_id;
+ clamp_max = tg_clamp->effective.value;
+
+ if (!p->uclamp[clamp_id].user_defined || *clamp_value > clamp_max) {
+ *clamp_value = clamp_max;
+ *bucket_id = bucket_max;
+ }
+
+ return true;
+#else
+ return false;
+#endif
+}
+
/*
* 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
*
* As a side effect, update the task's effective value:
@@ -841,7 +875,13 @@ uclamp_effective_get(struct task_struct *p, unsigned int clamp_id,
*bucket_id = p->uclamp[clamp_id].bucket_id;
*clamp_value = p->uclamp[clamp_id].value;

- /* Always apply system default restrictions */
+ /*
+ * If we have task groups and we are running in a child group, system
+ * default are already affecting the group's clamp values.
+ */
+ if (uclamp_tg_restricted(p, clamp_id, clamp_value, bucket_id))
+ return;
+
if (unlikely(*clamp_value > uclamp_default[clamp_id].value)) {
*clamp_value = uclamp_default[clamp_id].value;
*bucket_id = uclamp_default[clamp_id].bucket_id;
--
2.20.1
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