[PATCH v3 04/14] sched/core: uclamp: update CPU's refcount on clamp changes

Patrick Bellasi
2018-08-06T16:40:44Z

Utilization clamp values enforced on a CPU by a task can be updated at
run-time, for example via a sched_setattr syscall, while a task is
currently RUNNABLE on that CPU. In these cases, the task can be already
refcounting a clamp group for its CPU and thus we need to update this
reference to ensure the new constraints are immediately enforced.

Since a clamp value change always implies a clamp group refcount update,
this patch hooks into the clamp group refcount getter to trigger a CPU
refcount syncup. Such a syncup is required only by currently RUNNABLE
tasks which are also referencing at least one valid clamp group.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul Turner <pjt@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Todd Kjos <tkjos@google.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-pm@vger.kernel.org

---
Changes in v3:
Message-ID: <CAJuCfpF6=L=0LrmNnJrTNPazT4dWKqNv+thhN0dwpKCgUzs9sg@mail.gmail.com>
- rename UCLAMP_NONE into UCLAMP_NOT_VALID
Other:
- rabased on tip/sched/core

Changes in v2:
Message-ID: <20180413111900.GF4082@hirez.programming.kicks-ass.net>
- get rid of the group_id back annotation
which is not requires at this stage where we have only per-task
clamping support. It will be introduce later when CGroups support is
added.
Other:
- rabased on v4.18-rc4
- this code has been split from a previous patch to simplify the review
---
kernel/sched/core.c | 61 ++++++++++++++++++++++++++++++++++++++++----
kernel/sched/sched.h | 45 ++++++++++++++++++++++++++++++++
2 files changed, 101 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f058ceb14d25..bc2beedec7bf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1065,6 +1065,52 @@ static inline void uclamp_cpu_put(struct rq *rq, struct task_struct *p)
uclamp_cpu_put_id(p, rq, clamp_id);
}

+/**
+ * uclamp_task_update_active: update the clamp group of a RUNNABLE task
+ * @p: the task which clamp groups must be updated
+ * @clamp_id: the clamp index to consider
+ * @group_id: the clamp group to update
+ *
+ * Each time the clamp value of a task group is changed, the old and new clamp
+ * groups have to be updated for each CPU containing a RUNNABLE task belonging
+ * to this tasks group. Sleeping tasks are not updated since they will be
+ * enqueued with the proper clamp group index at their next activation.
+ */
+static inline void
+uclamp_task_update_active(struct task_struct *p, int clamp_id, int group_id)
+{
+ struct rq_flags rf;
+ struct rq *rq;
+
+ /*
+ * Lock the task and the CPU where the task is (or was) queued.
+ *
+ * We might lock the (previous) RQ of a !RUNNABLE task, but that's the
+ * price to pay to safely serialize util_{min,max} updates with
+ * enqueues, dequeues and migration operations.
+ * This is the same locking schema used by __set_cpus_allowed_ptr().
+ */
+ rq = task_rq_lock(p, &rf);
+
+ /*
+ * The setting of the clamp group is serialized by task_rq_lock().
+ * Thus, if the task's task_struct is not referencing a valid group
+ * index, then that task is not yet RUNNABLE and it's going to be
+ * enqueued with the proper clamp group value.
+ */
+ if (!uclamp_task_active(p))
+ goto done;
+
+ /* Release p's currently referenced clamp group */
+ uclamp_cpu_put_id(p, rq, clamp_id);
+
+ /* Get p's new clamp group */
+ uclamp_cpu_get_id(p, rq, clamp_id);
+
+done:
+ task_rq_unlock(rq, p, &rf);
+}
+
/**
* uclamp_group_put: decrease the reference count for a clamp group
* @clamp_id: the clamp index which was affected by a task group
@@ -1100,6 +1146,7 @@ static inline void uclamp_group_put(int clamp_id, int group_id)

/**
* uclamp_group_get: increase the reference count for a clamp group
+ * @p: the task which clamp value must be tracked
* @clamp_id: the clamp index affected by the task
* @next_group_id: the clamp group to refcount
* @uc_se: the utilization clamp data for the task
@@ -1110,9 +1157,10 @@ static inline void uclamp_group_put(int clamp_id, int group_id)
* this new clamp value. The corresponding clamp group index will be used by
* the task to reference count the clamp value on CPUs while enqueued.
*/
-static inline void uclamp_group_get(int clamp_id, int next_group_id,
- struct uclamp_se *uc_se,
- unsigned int clamp_value)
+static inline void uclamp_group_get(struct task_struct *p,
+ int clamp_id, int next_group_id,
+ struct uclamp_se *uc_se,
+ unsigned int clamp_value)
{
struct uclamp_map *uc_map = &uclamp_maps[clamp_id][0];
int prev_group_id = uc_se->group_id;
@@ -1129,6 +1177,9 @@ static inline void uclamp_group_get(int clamp_id, int next_group_id,
uc_map[next_group_id].se_count += 1;
raw_spin_unlock_irqrestore(&uc_map[next_group_id].se_lock, flags);

+ /* Update CPU's clamp group refcounts of RUNNABLE task */
+ uclamp_task_update_active(p, clamp_id, next_group_id);
+
/* Release the previous clamp group */
uclamp_group_put(clamp_id, prev_group_id);
}
@@ -1188,12 +1239,12 @@ static inline int __setscheduler_uclamp(struct task_struct *p,
/* Update each required clamp group */
if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
uc_se = &p->uclamp[UCLAMP_MIN];
- uclamp_group_get(UCLAMP_MIN, group_id[UCLAMP_MIN],
+ uclamp_group_get(p, UCLAMP_MIN, group_id[UCLAMP_MIN],
uc_se, attr->sched_util_min);
}
if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
uc_se = &p->uclamp[UCLAMP_MAX];
- uclamp_group_get(UCLAMP_MAX, group_id[UCLAMP_MAX],
+ uclamp_group_get(p, UCLAMP_MAX, group_id[UCLAMP_MAX],
uc_se, attr->sched_util_max);
}

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 8f96926703fc..c9df61d39a03 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2208,6 +2208,51 @@ static inline bool uclamp_group_active(struct uclamp_group *uc_grp,
{
return uc_grp[group_id].tasks > 0;
}
+
+/**
+ * uclamp_task_affects: check if a task affects a utilization clamp
+ * @p: the task to consider
+ * @clamp_id: the utilization clamp to check
+ *
+ * A task affects a clamp index if:
+ * - it's currently enqueued on a CPU
+ * - it references a valid clamp group index for the specified clamp index
+ *
+ * Return: true if p currently affects the specified clamp_id
+ */
+static inline bool uclamp_task_affects(struct task_struct *p, int clamp_id)
+{
+ return (p->uclamp[clamp_id].group_id != UCLAMP_NOT_VALID);
+}
+
+/**
+ * uclamp_task_active: check if a task is currently clamping a CPU
+ * @p: the task to check
+ *
+ * A task actively affects the utilization clamp of a CPU if:
+ * - it's currently enqueued or running on that CPU
+ * - it's refcounted in at least one clamp group of that CPU
+ *
+ * Return: true if p is currently clamping the utilization of its CPU.
+ */
+static inline bool uclamp_task_active(struct task_struct *p)
+{
+ struct rq *rq = task_rq(p);
+ int clamp_id;
+
+ lockdep_assert_held(&p->pi_lock);
+ lockdep_assert_held(&rq->lock);
+
+ if (!task_on_rq_queued(p) && !p->on_cpu)
+ return false;
+
+ for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
+ if (uclamp_task_affects(p, clamp_id))
+ return true;
+ }
+
+ return false;
+}
#endif /* CONFIG_UCLAMP_TASK */

#ifdef CONFIG_CPU_FREQ
--
2.18.0