Re: [PATCH v7 03/15] sched/core: uclamp: Add system default clamps

Peter Zijlstra
2019-03-13T14:32:40Z

On Fri, Feb 08, 2019 at 10:05:42AM +0000, Patrick Bellasi wrote:

> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 45460e7a3eee..447261cd23ba 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -584,14 +584,32 @@ struct sched_dl_entity {
> * Utilization clamp for a scheduling entity
> * @value: clamp value "requested" by a se
> * @bucket_id: clamp bucket corresponding to the "requested" value
> + * @effective: clamp value and bucket actually "assigned" to the se
> + * @active: the se is currently refcounted in a rq's bucket
> *
> + * Both bucket_id and effective::bucket_id are the index of the clamp bucket
> + * matching the corresponding clamp value which are pre-computed and stored to
> + * avoid expensive integer divisions from the fast path.
> + *
> + * The active bit is set whenever a task has got an effective::value assigned,
> + * which can be different from the user requested clamp value. This allows to
> + * know a task is actually refcounting the rq's effective::bucket_id bucket.
> */
> struct uclamp_se {
> + /* Clamp value "requested" by a scheduling entity */
> unsigned int value : bits_per(SCHED_CAPACITY_SCALE);
> unsigned int bucket_id : bits_per(UCLAMP_BUCKETS);
> + unsigned int active : 1;
> + /*
> + * Clamp value "obtained" by a scheduling entity.
> + *
> + * This cache the actual clamp value, possibly enforced by system
> + * default clamps, a task is subject to while enqueued in a rq.
> + */
> + struct {
> + unsigned int value : bits_per(SCHED_CAPACITY_SCALE);
> + unsigned int bucket_id : bits_per(UCLAMP_BUCKETS);
> + } effective;

I still think that this effective thing is backwards.

The existing code already used @value and @bucket_id as 'effective' and
you're now changing all that again. This really doesn't make sense to
me.

Also; if you don't add it inside struct uclamp_se, but add a second
instance,

> };
> #endif /* CONFIG_UCLAMP_TASK */
>

> @@ -803,6 +811,70 @@ static inline void uclamp_rq_update(struct rq *rq, unsigned int clamp_id,
> WRITE_ONCE(rq->uclamp[clamp_id].value, max_value);
> }
>
> +/*
> + * The effective clamp bucket index of a task depends on, by increasing
> + * priority:
> + * - the task specific clamp value, when explicitly requested from userspace
> + * - the system default clamp value, defined by the sysadmin
> + *
> + * As a side effect, update the task's effective value:
> + * task_struct::uclamp::effective::value
> + * to represent the clamp value of the task effective bucket index.
> + */
> +static inline void
> +uclamp_effective_get(struct task_struct *p, unsigned int clamp_id,
> + unsigned int *clamp_value, unsigned int *bucket_id)
> +{
> + /* Task specific clamp value */
> + *bucket_id = p->uclamp[clamp_id].bucket_id;
> + *clamp_value = p->uclamp[clamp_id].value;
> +
> + /* Always apply system default restrictions */
> + if (unlikely(*clamp_value > uclamp_default[clamp_id].value)) {
> + *clamp_value = uclamp_default[clamp_id].value;
> + *bucket_id = uclamp_default[clamp_id].bucket_id;
> + }
> +}

you can avoid horrors like this and simply return a struct uclamp_se by
value.