Re: [PATCH v7 01/15] sched/core: uclamp: Add CPU's clamp buckets refcounting

From: Dietmar Eggemann
Date: Tue Mar 12 2019 - 08:52:57 EST

On 2/8/19 11:05 AM, Patrick Bellasi wrote:


+ int "Number of supported utilization clamp buckets"
+ range 5 20
+ default 5
+ depends on UCLAMP_TASK
+ help
+ Defines the number of clamp buckets to use. The range of each bucket
+ number of clamp buckets the finer their granularity and the higher
+ the precision of clamping aggregation and tracking at run-time.
+ For example, with the default configuration we will have 5 clamp
+ buckets tracking 20% utilization each. A 25% boosted tasks will be
+ refcounted in the [20..39]% bucket and will set the bucket clamp
+ effective value to 25%.
+ If a second 30% boosted task should be co-scheduled on the same CPU,
+ that task will be refcounted in the same bucket of the first task and
+ it will boost the bucket clamp effective value to 30%.
+ The clamp effective value of a bucket is reset to its nominal value
+ (20% in the example above) when there are anymore tasks refcounted in

this sounds weird.


+static inline unsigned int uclamp_bucket_value(unsigned int clamp_value)
+ return UCLAMP_BUCKET_DELTA * uclamp_bucket_id(clamp_value);

Soemthing like uclamp_bucket_nominal_value() should be clearer.

+static inline void uclamp_rq_update(struct rq *rq, unsigned int clamp_id)
+ struct uclamp_bucket *bucket = rq->uclamp[clamp_id].bucket;
+ unsigned int max_value = uclamp_none(clamp_id);
+ unsigned int bucket_id;

unsigned int bucket_id = UCLAMP_BUCKETS;

+ /*
+ * Both min and max clamps are MAX aggregated, thus the topmost
+ * bucket with some tasks defines the rq's clamp value.
+ */
+ bucket_id = UCLAMP_BUCKETS;

to get rid of this line?

+ do {
+ --bucket_id;
+ if (!rq->uclamp[clamp_id].bucket[bucket_id].tasks)

if (!bucket[bucket_id].tasks)


+ * When a task is enqueued on a rq, the clamp bucket currently defined by the
+ * task's uclamp::bucket_id is reference counted on that rq. This also
+ * immediately updates the rq's clamp value if required.
+ *
+ * Since tasks know their specific value requested from user-space, we track
+ * within each bucket the maximum value for tasks refcounted in that bucket.
+ * This provide a further aggregation (local clamping) which allows to track

s/This provide/This provides

+ * within each bucket the exact "requested" clamp value whenever all tasks
+ * RUNNABLE in that bucket require the same clamp.
+ */
+static inline void uclamp_rq_inc_id(struct task_struct *p, struct rq *rq,
+ unsigned int clamp_id)
+ unsigned int bucket_id = p->uclamp[clamp_id].bucket_id;
+ unsigned int rq_clamp, bkt_clamp, tsk_clamp;

Wouldn't it be easier to have a pointer to the task's and rq's uclamp structure as well to the bucket?

- unsigned int bucket_id = p->uclamp[clamp_id].bucket_id;
+ struct uclamp_se *uc_se = &p->uclamp[clamp_id];
+ struct uclamp_rq *uc_rq = &rq->uclamp[clamp_id];
+ struct uclamp_bucket *bucket = &uc_rq->bucket[uc_se->bucket_id];

The code in uclamp_rq_inc_id() and uclamp_rq_dec_id() for example becomes much more readable.


struct sched_class {
const struct sched_class *next;
+ int uclamp_enabled;
void (*enqueue_task) (struct rq *rq, struct task_struct *p, int flags);
void (*dequeue_task) (struct rq *rq, struct task_struct *p, int flags);
- void (*yield_task) (struct rq *rq);
- bool (*yield_to_task)(struct rq *rq, struct task_struct *p, bool preempt);
void (*check_preempt_curr)(struct rq *rq, struct task_struct *p, int flags);
@@ -1685,7 +1734,6 @@ struct sched_class {
void (*set_curr_task)(struct rq *rq);
void (*task_tick)(struct rq *rq, struct task_struct *p, int queued);
void (*task_fork)(struct task_struct *p);
- void (*task_dead)(struct task_struct *p);
* The switched_from() call is allowed to drop rq->lock, therefore we
@@ -1702,12 +1750,17 @@ struct sched_class {
void (*update_curr)(struct rq *rq);
+ void (*yield_task) (struct rq *rq);
+ bool (*yield_to_task)(struct rq *rq, struct task_struct *p, bool preempt);
#define TASK_SET_GROUP 0
void (*task_change_group)(struct task_struct *p, int type);
+ void (*task_dead)(struct task_struct *p);

Why do you move yield_task, yield_to_task and task_dead here?