Re: [PATCH v6 09/16] sched/cpufreq: uclamp: Add utilization clamping for RT tasks

[Peter Zijlstra]

On Wed, Jan 23, 2019 at 02:40:11PM +0000, Patrick Bellasi wrote:
> On 23-Jan 11:49, Peter Zijlstra wrote:
> > On Tue, Jan 15, 2019 at 10:15:06AM +0000, Patrick Bellasi wrote:
> > > @@ -858,16 +859,23 @@ static inline void
> > >  uclamp_effective_get(struct task_struct *p, unsigned int clamp_id,
> > >  		     unsigned int *clamp_value, unsigned int *bucket_id)
> > >  {
> > > +	struct uclamp_se *default_clamp;
> > > +
> > >  	/* Task specific clamp value */
> > >  	*clamp_value = p->uclamp[clamp_id].value;
> > >  	*bucket_id = p->uclamp[clamp_id].bucket_id;
> > >  
> > > +	/* RT tasks have different default values */
> > > +	default_clamp = task_has_rt_policy(p)
> > > +		? uclamp_default_perf
> > > +		: uclamp_default;
> > > +
> > >  	/* System default restriction */
> > > -	if (unlikely(*clamp_value < uclamp_default[UCLAMP_MIN].value ||
> > > -		     *clamp_value > uclamp_default[UCLAMP_MAX].value)) {
> > > +	if (unlikely(*clamp_value < default_clamp[UCLAMP_MIN].value ||
> > > +		     *clamp_value > default_clamp[UCLAMP_MAX].value)) {
> > >  		/* Keep it simple: unconditionally enforce system defaults */
> > > -		*clamp_value = uclamp_default[clamp_id].value;
> > > -		*bucket_id = uclamp_default[clamp_id].bucket_id;
> > > +		*clamp_value = default_clamp[clamp_id].value;
> > > +		*bucket_id = default_clamp[clamp_id].bucket_id;
> > >  	}
> > >  }
> > 
> > So I still don't much like the whole effective thing;
> 
> :/
> 
> I find back-annotation useful in many cases since we have different
> sources for possible clamp values:
> 
>  1. task specific
>  2. cgroup defined
>  3. system defaults
>  4. system power default

(I'm not sure I've seen 4 happen yet, what's that?)

Anyway, once you get range composition defined; that should be something
like:

	R_p \Compose_g R_g

Where R_p is the range of task-p, and R_g is the range of the g'th
cgroup of p (where you can make an identity between the root cgroup and
the system default).

Now; as per the other email; I think the straight forward composition:

struct range compose(struct range a, struct range b)
{
	return (range){.min = clamp(a.min, b.min, b.max),
	               .max = clamp(a.max, b.min, b.max), };
}

(note that this is non-commutative, so we have to pay attention to
get the order 'right')

Works in this case; unlike the cpu/rq conposition where we resort to a
pure max function for non-interference.

> I don't think we can avoid to somehow back annotate on which bucket a
> task has been refcounted... it makes dequeue so much easier, it helps
> in ensuring that the refcouning is consistent and enable lazy updates.

So I'll have to go over the code again, but I'm wondering why you're
changing uclamp_se::bucket_id on a runnable task.

Ideally you keep bucket_id invariant between enqueue and dequeue; then
dequeue knows where we put it.

Now I suppose actually determining bucket_id is 'expensive' (it
certainly is with the whole mapping scheme, but even that integer
division is not nice), so we'd like to precompute the bucket_id.

This then leads to the problem of how to change uclamp_se::value while
runnable (since per the other thread you don't want to always update all
runnable tasks).

So far so right?

I'm thikning that if we haz a single bit, say:

  struct uclamp_se {
	...
	unsigned int	changed : 1;
  };

We can update uclamp_se::value and set uclamp_se::changed, and then the
next enqueue will (unlikely) test-and-clear changed and recompute the
bucket_id.

Would that not be simpler?