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

[Patrick Bellasi <patrick.bellasi@arm.com>]

On 13-Mar 14:23, Suren Baghdasaryan wrote:
> On Wed, Mar 13, 2019 at 6:52 AM Peter Zijlstra <peterz@infradead.org> 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.

Right.

> However that's not exactly what the code does... It also resets
> rq->uclamp[clamp_id].bucket[bucket_id].value.

Right...

> 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?

... and I think you've got a point here!

The reset to nominal value line should be done unconditionally.
I'll move it outside its current block. Thanks for spotting it.

> Side note: some more explanation would be very helpful.

Will move that "bucket local max" management code into a separate
patch as suggested by Peter. Hopefully that should make the logic more
clear and allows me to add some notes in the changelog.

-- 
#include <best/regards.h>

Patrick Bellasi