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

[Peter Zijlstra]

On Fri, Feb 08, 2019 at 10:05:40AM +0000, Patrick Bellasi wrote:
> +/*
> + * 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
> + * 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;
> +
> +	rq->uclamp[clamp_id].bucket[bucket_id].tasks++;
> +
> +	/*
> +	 * Local clamping: rq's buckets always track the max "requested"
> +	 * clamp value from all RUNNABLE tasks in that bucket.
> +	 */
> +	tsk_clamp = p->uclamp[clamp_id].value;
> +	bkt_clamp = rq->uclamp[clamp_id].bucket[bucket_id].value;
> +	rq->uclamp[clamp_id].bucket[bucket_id].value = max(bkt_clamp, tsk_clamp);

So, if I read this correct:

 - here we track a max value in a bucket,

> +	rq_clamp = READ_ONCE(rq->uclamp[clamp_id].value);
> +	WRITE_ONCE(rq->uclamp[clamp_id].value, max(rq_clamp, tsk_clamp));
> +}
> +
> +/*
> + * When a task is dequeued from a rq, the clamp bucket reference counted by
> + * the task is released. If this is the last task reference counting the rq's
> + * max active clamp value, then the rq's clamp value is updated.
> + * Both the tasks reference counter and the rq's cached clamp values are
> + * expected to be always valid, if we detect they are not we skip the updates,
> + * enforce a consistent state and warn.
> + */
> +static inline void uclamp_rq_dec_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;
> +
> +	SCHED_WARN_ON(!rq->uclamp[clamp_id].bucket[bucket_id].tasks);
> +	if (likely(rq->uclamp[clamp_id].bucket[bucket_id].tasks))
> +		rq->uclamp[clamp_id].bucket[bucket_id].tasks--;
> +
> +	/*
> +	 * Keep "local clamping" simple and accept to (possibly) overboost
> +	 * still RUNNABLE tasks in the same bucket.
> +	 */
> +	if (likely(rq->uclamp[clamp_id].bucket[bucket_id].tasks))
> +		return;

(Oh man, I hope that generates semi sane code; long live CSE passes I
suppose)

But we never decrement that bkt_clamp value on dequeue.

> +	bkt_clamp = rq->uclamp[clamp_id].bucket[bucket_id].value;
> +
> +	/* The rq's clamp value is expected to always track the max */
> +	rq_clamp = READ_ONCE(rq->uclamp[clamp_id].value);
> +	SCHED_WARN_ON(bkt_clamp > rq_clamp);
> +	if (bkt_clamp >= rq_clamp) {

head hurts, this reads ==, how can this ever not be so?

> +		/*
> +		 * Reset rq's clamp bucket value to its nominal value whenever
> +		 * there are anymore RUNNABLE tasks refcounting it.

-ENOPARSE

> +		 */
> +		rq->uclamp[clamp_id].bucket[bucket_id].value =
> +			uclamp_bucket_value(rq_clamp);

But basically you decrement the bucket value to the nominal value.

> +		uclamp_rq_update(rq, clamp_id);
> +	}
> +}

Given all that, what is to stop the bucket value to climbing to
uclamp_bucket_value(+1)-1 and staying there (provided there's someone
runnable)?

Why are we doing this... ?