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SubjectRe: [PATCH v8 01/16] sched/core: uclamp: Add CPU's clamp buckets refcounting
On 06-Apr 16:51, Suren Baghdasaryan wrote:
> On Tue, Apr 2, 2019 at 3:42 AM Patrick Bellasi <patrick.bellasi@arm.com> wrote:

[...]

> > + * The first few values calculated by this routine:
> > + * bf(0) = 1
> > + * bf(1) = 1
> > + * bf(2) = 2
> > + * bf(3) = 2
> > + * bf(4) = 3
> > + * ... and so on.
> > + */
> > +#define bits_per(n) \
> > +( \
> > + __builtin_constant_p(n) ? ( \
> > + ((n) == 0 || (n) == 1) ? 1 : ( \
> > + ((n) & (n - 1)) == 0 ? \
>
> missing braces around 'n'
> - ((n) & (n - 1)) == 0 ? \
> + ((n) & ((n) - 1)) == 0 ? \
>
> > + ilog2((n) - 1) + 2 : \
> > + ilog2((n) - 1) + 1 \
>
> Isn't this "((n) & ((n) - 1)) == 0 ? ilog2((n) - 1) + 2 : ilog2((n) -
> 1) + 1" expression equivalent to a simple "ilog2(n) + 1"?

Right, since we already have n=0 and n=1 as special cases, what you
propose should work for all n>=2.

>
> > + ) \
> > + ) : \
> > + __bits_per(n) \
> > +)
> > #endif /* _LINUX_LOG2_H */

[...]

> > +static inline unsigned int uclamp_bucket_base_value(unsigned int clamp_value)
>
> Where are you using uclamp_bucket_base_value()? I would expect its
> usage somewhere inside uclamp_rq_dec_id() when the last task in the
> bucket is dequeued but I don't see it...

This behavior is not move into a dedicated patch, as per Peter
request:

Message-ID: <20190314111849.gx6bl6myfjtaan7r@e110439-lin>

This functions was left here to support a the intialization code in
init_uclamp() but... I notice know I'm doing the initialization in a
different way thus, I'll move it into the following patch.

> > +{
> > + return UCLAMP_BUCKET_DELTA * uclamp_bucket_id(clamp_value);
> > +}
> > +

[...]

> > +static inline void uclamp_rq_dec_id(struct task_struct *p, struct rq *rq,
> > + unsigned int clamp_id)
> > +{
> > + struct uclamp_rq *uc_rq = &rq->uclamp[clamp_id];
> > + struct uclamp_se *uc_se = &p->uclamp[clamp_id];
> > + struct uclamp_bucket *bucket;
> > + unsigned int rq_clamp;
> > +
> > + bucket = &uc_rq->bucket[uc_se->bucket_id];
> > + SCHED_WARN_ON(!bucket->tasks);
> > + if (likely(bucket->tasks))
> > + bucket->tasks--;
> > +
> > + if (likely(bucket->tasks))
>
> Shouldn't you adjust bucket->value if the remaining tasks in the
> bucket have a lower clamp value than the task that was just removed?

No, this is never done. As long as a bucket is not empty/idle we never
reset it to its nominal value. In this patch specifically, the value
is never changed since we moved the "local max tracking" bits into a
dedicated patch.

> > + return;
> > +
> > + rq_clamp = READ_ONCE(uc_rq->value);
> > + /*
> > + * Defensive programming: this should never happen. If it happens,
> > + * e.g. due to future modification, warn and fixup the expected value.
> > + */
> > + SCHED_WARN_ON(bucket->value > rq_clamp);
> > + if (bucket->value >= rq_clamp)
> > + WRITE_ONCE(uc_rq->value, uclamp_rq_max_value(rq, clamp_id));
> > +}

[...]

> > +static void __init init_uclamp(void)
> > +{
> > + unsigned int clamp_id;
> > + int cpu;
> > +
> > + for_each_possible_cpu(cpu) {
> > + struct uclamp_bucket *bucket;
> > + struct uclamp_rq *uc_rq;
> > + unsigned int bucket_id;
> > +
> > + memset(&cpu_rq(cpu)->uclamp, 0, sizeof(struct uclamp_rq));
> > +
> > + for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
> > + uc_rq = &cpu_rq(cpu)->uclamp[clamp_id];
> > +
> > + bucket_id = 1;
> > + while (bucket_id < UCLAMP_BUCKETS) {
> > + bucket = &uc_rq->bucket[bucket_id];
> > + bucket->value = bucket_id * UCLAMP_BUCKET_DELTA;
> > + ++bucket_id;
> > + }
> > + }
> > + }

All the initialization code above is not more required after the next
patch introducing "local max tracking".

> > +
> > + for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
> > + struct uclamp_se *uc_se = &init_task.uclamp[clamp_id];
> > +
> > + uc_se->value = uclamp_none(clamp_id);
> > + uc_se->bucket_id = uclamp_bucket_id(uc_se->value);
> > + }
> > +}

[...]

--
#include <best/regards.h>

Patrick Bellasi

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