Re: [PATCH v6 10/16] sched/core: Add uclamp_util_with()

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jan 23, 2019 at 02:51:06PM +0000, Patrick Bellasi wrote:
> On 23-Jan 14:33, Peter Zijlstra wrote:
> > On Tue, Jan 15, 2019 at 10:15:07AM +0000, Patrick Bellasi wrote:
> > > +static __always_inline
> > > +unsigned int uclamp_util_with(struct rq *rq, unsigned int util,
> > > +			      struct task_struct *p)
> > >  {
> > >  	unsigned int min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
> > >  	unsigned int max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
> > >  
> > > +	if (p) {
> > > +		min_util = max(min_util, uclamp_effective_value(p, UCLAMP_MIN));
> > > +		max_util = max(max_util, uclamp_effective_value(p, UCLAMP_MAX));
> > > +	}
> > > +
> > 
> > Like I think you mentioned earlier; this doesn't look right at all.
> 
> What we wanna do here is to compute what _will_ be the clamp values of
> a CPU if we enqueue *p on it.
> 
> The code above starts from the current CPU clamp value and mimics what
> uclamp will do in case we move the task there... which is always a max
> aggregation.

Ah, then I misunderstood the purpose of this function.

> > Should that not be something like:
> > 
> > 	lo = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
> > 	hi = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
> > 
> > 	min_util = clamp(uclamp_effective(p, UCLAMP_MIN), lo, hi);
> > 	max_util = clamp(uclamp_effective(p, UCLAMP_MAX), lo, hi);
> 
> Here you end up with a restriction of the task clamp (effective)
> clamps values considering the CPU clamps... which is different.
> 
> Why do you think we should do that?... perhaps I'm missing something.

I was left with the impression from patch 7 that we don't compose clamps
right and throught that was what this code was supposed to do.

I'll have another look at this patch.