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

Patrick Bellasi
2019-03-13T16:12:29Z

On 13-Mar 14:40, Peter Zijlstra wrote:
> On Fri, Feb 08, 2019 at 10:05:40AM +0000, Patrick Bellasi wrote:
> > +static inline unsigned int uclamp_bucket_id(unsigned int clamp_value)
> > +{
> > + return clamp_value / UCLAMP_BUCKET_DELTA;
> > +}
> > +
> > +static inline unsigned int uclamp_bucket_value(unsigned int clamp_value)
> > +{
> > + return UCLAMP_BUCKET_DELTA * uclamp_bucket_id(clamp_value);
>
> return clamp_value - (clamp_value % UCLAMP_BUCKET_DELTA);
>
> might generate better code; just a single division, instead of a div and
> mult.

Wondering if compilers cannot do these optimizations... but yes, looks
cool and will do it in v8, thanks.

> > +}
> > +
> > +static inline unsigned int uclamp_none(int clamp_id)
> > +{
> > + if (clamp_id == UCLAMP_MIN)
> > + return 0;
> > + return SCHED_CAPACITY_SCALE;
> > +}
> > +
> > +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;
> > +
> > + /*
> > + * 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;
> > + do {
> > + --bucket_id;
> > + if (!rq->uclamp[clamp_id].bucket[bucket_id].tasks)
> > + continue;
> > + max_value = bucket[bucket_id].value;
> > + break;
>
> If you flip the if condition the code will be nicer.
>
> > + } while (bucket_id);
>
> But you can also use a for loop:
>
> for (i = UCLAMP_BUCKETS-1; i>=0; i--) {
> if (rq->uclamp[clamp_id].bucket[i].tasks) {
> max_value = bucket[i].value;
> break;
> }
> }

Yes, the for looks better, but perhaps like that:

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.
*/
for (; bucket_id >= 0; --bucket_id) {
if (!bucket[bucket_id].tasks)
continue;
max_value = bucket[bucket_id].value;
break;
}

... just to save a {} block.

> > + WRITE_ONCE(rq->uclamp[clamp_id].value, max_value);
> > +}

--
#include <best/regards.h>

Patrick Bellasi