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

[Suren Baghdasaryan]

On Wed, Mar 13, 2019 at 6:52 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> 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... ?

I agree with Peter, this part of the patch was the hardest to read.
SCHED_WARN_ON line makes sense to me. The condition that follows and
the following comment are a little baffling. Condition seems to
indicate that the code that follows should be executed only if we are
in the top-most occupied bucket (the bucket which has tasks and has
the highest uclamp value). So this bucket just lost its last task and
we should update rq->uclamp[clamp_id].value. However that's not
exactly what the code does... It also resets
rq->uclamp[clamp_id].bucket[bucket_id].value. So if I understand
correctly, unless the bucket that just lost its last task is the
top-most one its value will not be reset to nominal value. That looks
like a bug to me. Am I missing something?

Side note: some more explanation would be very helpful.